GSM 原理

Document Sample
GSM 原理 Powered By Docstoc

   GSM Basics,
   An Introduction

QMD Field Training Event
Queensferry Microwave Division

Updated by Angela Cairns
November 1998

©1997 Hewlett-Packard Company    10001506.sam
GSM Basics, An Introduction

                                                        GSM Basics, An Introduction













     H                                                                                    Queensferry Microwave Division

The Global System for Mobile communications (GSM) is a huge,
rapidly expanding and successful technology. Less than five years
ago, there were a few 10's of companies working on GSM. Each of
these companies had a few GSM experts who brought knowledge
back from the European Telecommunications Standards Institute
(ETSI) committees designing the GSM specification. Now there
are 100's of companies working on GSM and 1000's of GSM ex-
perts. GSM is no longer state-of-the-art. It is everyday-technology,
as likely to be understood by the service technician as the ETSI
committee member.

GSM is quickly moving out of Europe and is becoming a world
standard. HP has become expert in GSM through our involvement
in Europe. With excellent internal communications, HP is in an
excellent position to help our customers, in other regions of the
world, benefit from our GSM knowledge.

©1996 Hewlett-Packard Company                                                         2                                    10001506.sam
GSM Basics, An Introduction


                   GSM Snapshot

                                  Spokane Division 10001506.pre
                H                 ©1997 Hewlett-Packard Company

I. GSM Snapshot

©1996 Hewlett-Packard Company     3                               10001506.sam
GSM Basics, An Introduction

                  GLOBAL System for Mobiles

                  Over 139 licences in 105 countries Worldwide

                  Over 32 million suscribers today

                  Approximately 25% of the total number of
                  mobile suscribers

                  Over 100 million suscribers by the year 2000

                                                       Spokane Division 10001506.pre
                H                                      ©1997 Hewlett-Packard Company

GLOBAL System for Mobiles
GSM is truly becoming the GLOBAL System for Mobile Communi-
cations. It's been clear for a long time that GSM would be used
across Europe. Now, many countries around the world, who have
been delaying their decision, have selected GSM. GSM has be-
come a Pan Asian standard and is going to be used in much of
South America.
Personal Communications Networks PCN started in the UK with
Mercury One-to-One and Hutchison Microtel (Orange) offering the
first two networks to use DCS1800.           The UK PCNs have had
overwhelming success with their competitive business tariffs and
cheap off-peak calls.
Germany's E net followed the UK PCNs. DCS1800 is becoming
more widespread with systems in Thailand, Malaysia, France,
Switzerland and Australia. Further systems are planned in Argen-
tina, Brazil, Chile, France, Hungary, Poland, Singapore and
Even the USA, which has shunned the adoption of GSM900, is
about to use the GSM based PCS1900 for it's PCS system. In the
USA, GSM will share the allocated bands with other systems based
on CDMA, NAMPS and IS-136 TDMA. The PCS1900 licenses al-
ready cover approximately half of the US population, and we are
likely to see this increase to close to full coverage as remaining li-
censes are granted.

©1996 Hewlett-Packard Company                         4                                10001506.sam
GSM Basics, An Introduction

                    Personal Communications
                    Networks     UK:                     Hungary,Poland,
       Canada:                         DCS1800           Sweden,Switzerland:
       PCS 1900                                          DCS 1800


       Brazil,                                                       Australia:
       Chile:                                                        DCS1800
                                Thailand:    Malaysia:
       DCS 1800
                                DCS1800      DCS 1800

        H                                                        Hewlett-Packard QMD Ian Reading Jan '94

How is GSM different from CT2 and DECT ?
GSM900, DCS1800 and PCS1900 are cellular systems, DECT and
CT2 are cordless systems. GSM like AMPS and TACS allows users
to make and receive calls over a wide geographic area. The sys-
tem uses a register to log the position of all mobiles, allowing calls
to be routed to the correct base station. DECT and CT2, like other
cordless systems do not include this tracking capability. They op-
erate in much the same way as a conventional domestic cordless
phone (CT0 or CT1). Calls can be received when the mobile is
within range of it's home base station, but not at other locations.
How are GSM900, DCS1800 and PCS1900 Different?
GSM900 is the original GSM system, using frequencies in the 900
MHz band and designed for wide area cellular operation. Mobiles
with output powers from 1 to 8W are typical. DCS1800 is an adap-
tation of GSM900. The term GSM can be used collectively to de-
scribe the GSM900 and DCS1800 standards. Creating DCS1800
involved widening the bands assigned to GSM and moving them up
to 1.8 GHz. The DCS1800 standard was created to allow PCN (Per-
sonal Communications Networks) to form, increasing competition
in the cellular communications industry. To avoid confusion, the
channel numbers (ARFCN) used for DCS run from 512 to 885.
GSM900 channels run from 1 to 124. With wider frequency alloca-
tion, leading to more channels, DCS1800 is able to cope with
higher user densities. DCS1800 mobiles are also designed for
lower output powers (up to 1W), so cell sizes have to be smaller,
meaning even higher densities. In all other respects, GSM900 and
©1996 Hewlett-Packard Company                            5                                                 10001506.sam
GSM Basics, An Introduction

DCS1800 are the same. The GSM phase II specifications (a revised
and re-written standard) brings the two systems even closer.
GSM900 gets additional bandwidth and channels, called E-GSM
(Extended band GSM) and lower power control levels for mobiles,
allowing micro-cell operation. These two features allow increased
user densities in GSM systems. Phase II also makes provision for
the addition of new services on GSM and DCS1800. The addition
of specific services such as data, fax and dual mode operation is
currently being defined in what is referred to as Phase II+.
In the USA, bands have been released around 2 GHz for a PCS
(Personal Communications System). Unlike Europe and the Far
East, the PCS licence holders will not be forced to use any particu-
lar radio technology. The three main contenders are GSM, CDMA
and IS-136 TDMA, all likely to have nationwide coverage. The
ready availability of GSM equipment and expertise has made GSM
at 1.9GHz very attractive for many operators. PCS1900 operators
have banded together to form the North American Interest Group
and help advance the development of GSM. The seven companies
are: American Personal Communications (APC), American Port-
able Telecom, Bell South Personal Communications, Intercel, Om-
nipoint, Pacific Bell Mobile Services and Western Wireless Co.
Many of the large GSM manufacturers are also backing PCS1900
including Nokia, Ericsson, Matra, AEG, Northern Telecom. In
technical terms PCS1900 will be identical to DCS1800 except for
frequency allocation and power levels. The first commercial PCS
system was launched by APC, under the name of Sprint Spectrum
on 15th November 1995, based on PCS1900. The majority of US
PCS licenses will become operational over the next two years.
Other systems are also on trial in the US, including DECT.
Some GSM History
Before we go into how the GSM system actually operates, let's
take a look at the past and see how we got where we are today. In
1981 analogue cellular was introduced and at about the same time
there was a joint Franco-German study looking at digital cellular
technology and the possibility of making a pan-european system.
In 1982 a special working committee, Groupe Spécial Mobile
(GSM), was formed within the CEPT to look at and continue the
Franco-German study. In 1986 the working committee was taken a
step further by the establishment of a permanent nucleus of peo-
ple to continue the work and create standards for a digital system
of the future. About a year later, the memorandum of understand-
ing, or MoU, as it is referred to, was signed by over 18 countries.
It stated that they would participate in the GSM system and get it
into operation by 1991. In 1989 GSM was moved into the ETSI
(European Telecommunications Standards Institute) organisation.

©1996 Hewlett-Packard Company               6                          10001506.sam
GSM Basics, An Introduction

Once under the control of ETSI, the GSM system had it's name
changed to Global System for Mobile communications. The com-
mittees working on the system changed from GSM to SMG (Spe-
cial Mobile Group). These changes avoided confusion between
the system name (GSM), and the people working on the specifica-
tion (SMG). It also brought the naming in line with the official
working language of ETSI (English).
In 1990 the GSM specification developed an offshoot - DCS1800.
The Original DCS1800 specifications were developed simply as ed-
ited versions of the GSM900 documents.
Interest in GSM quickly spread outside Europe. Australia was the
first non-European country to join the MoU in 1992. Since then,
many other Asian countries have adopted GSM. There's now a
Pan-Asian MoU, investigating international roaming agreements.
The Phase II specification for GSM has now been defined, merging
GSM900 and DCS1800 documents, a number of new features are
added to the system, along with many minor adjustments. The
next step is Phase II+ which will define the addition of specific
new services such as data and fax to GSM and DCS1800.

©1996 Hewlett-Packard Company             7                         10001506.sam
GSM Basics, An Introduction


                GSM Snapshot

                 The GSM Network & Air Interface

                Speech Coding & Types of Channel

                Making a Phone Call

                                                       Jonathan Dunbar
        H                                              Queensferry Microwave Division

II. The GSM Network & Air Interface

©1996 Hewlett-Packard Company                      8                                    10001506.sam
GSM Basics, An Introduction

                                                              GSM Networks
                                                                                 ADC           NMC           OSI

             GS                          1
                                                                   BTS                 OMC            OMC



                                                                   BTS     BSC
                                                         MS        BTS                          VLR
                                                                           BSS           MSC                              EIR

                                                                   BTS                                                    AUC

                                                                   BTS     BSC
                                                         MS                                                                 HLR
                                                                     BSS                        VLR
                                                                                                       Interface to other
        H                                                                                              Hewlett-Packard QMD Ian Reading Jan '94

GSM Networks
This is the GSM system. The Mobile Stations (MS), both hand held
(or portables) and traditional mobiles in a car, talk to the Base Sta-
tion System (BSS) over the RF air interface. The Base Station Sys-
tem (BSS) consists of a Base Transceiver Station (BTS), and a
Base Station Controller (BSC). It's typical for several BTS to be lo-
cated at the same site, producing 2 to 4 sectored cells around a
common antenna tower. BSC's are often connected to BTS via mi-
crowave links. The BSC to BTS link is called the Abis interface.
Typically 20 to 30 BTS will be controlled by one BSC. A number of
BSSs would then report back to the Mobile Switching Centre
(MSC) which controls the traffic among a number of different
cells. Each Mobile Switching Centre (MSC) will have a Visitors Lo-
cation Register (VLR) in which mobiles that are out of their home
cell will be listed so that the network will know where to find
them. The MSC will also be connected to the Home Location Reg-
ister (HLR), the Authentication Centre (AUC), and the Equipment
Identity Register (EIR) so the system can verify that users and
equipment are legal subscribers. This helps avoid the use of stolen
or fraud mobiles. There are also facilities within the system for
Operations and Maintenance (OMC) and Network Management
(NMC) organisations. The Mobile Switching Centre (MSC) also
has the interface to other networks such as Private Land Mobile
Networks (PLMN) and Public Switched Telephone Networks
(PSTN) and ISDN networks.

©1996 Hewlett-Packard Company                                                                    9                                               10001506.sam
GSM Basics, An Introduction

               Testing GSM
                            Air Interface   Abis Interface
                               LAP-Dm          LAP-D           SS#7          SS#7

                                        BTS            BSC

                                        BTS            BSC            MSC                 PSTN

                                        BTS             BSC

             MS             HP 8922x          HP 37900D      HP 37900D   HP 37900D
                            HP 85715/22B
                            HP 11759C
                            HP 37900D
                                                                         Jonathan Dunbar
        H                                                                Queensferry Microwave Division

Testing GSM
Taking a simpler view of the GSM system, we can see the base
transceiver station , base station controller, mobile switching cen-
tre, and public switch telephone network are tied together with
hard lines (optical fibre or microwave links). The link between the
mobiles and the base station is the air interface. Hewlett-Packard
has many measurement solutions, designed to test most areas of
the GSM system.

©1996 Hewlett-Packard Company                                    10                                       10001506.sam
GSM Basics, An Introduction

              A GSM Cell


                        L   IN                 K
                     UP              NL
                         DO                          BTS
                                          Traffic            Abis
                                          CHannel            Interface

                                                                         To BSC
                                                           Jonathan Dunbar
        H                                                  Queensferry Microwave Division

A GSM Cell
This is a close-up view of a typical GSM cell. Cells can be up to
35km radius for GSM900 and 2km for DCS1800 (because of lower
power DCS1800 mobiles). The most obvious part of the GSM cell
is the base station and it's antenna tower. It's common for several
cells to be sectored around a common antenna tower. The tower
will have several directional antennas, each covering a particular
area. This co-location of several BTS is sometimes called a cell-
site, or just a base station. The BTS are connected to their BSC by
the Abis interface. This sometimes a cable, or an optical fibre.
DCS1800 networks often use a microwave link for the Abis
Each BTS will be fitted with a number of TX/RX pairs or trans-
ceiver modules. The number will determine how many frequency
channels can be used in the cell, and depends on the expected
number of users.
All BTS produce a BCH (Broadcast CHannel). The BCH is like a
lighthouse or beacon. It's on all the time and allows mobile to find
the GSM network. The BCH signal strength is also used by the
network for a variety of user functions. It's a useful way of telling
which is the closest BTS to the mobile. It also has information
coded onto it, such as the identity of the network (e.g. Mannes-
mann, Detecon, or Optus), paging messages for any mobiles need-
ing to accept a phone call, and a variety of other information. The
BCH is received by all mobile's in the cell, whether they are on a
call, or not.

©1996 Hewlett-Packard Company                       11                                      10001506.sam
GSM Basics, An Introduction

The frequency channel used by the BCH is different in each cell.
Channels can be re-used by distant cells, where the risk of interfer-
ence is low.
Mobile's on a call use a TCH (Traffic CHannel). The TCH is a two
way channel used to exchange speech information between the
mobile and base-station. Information is divided into the uplink
and downlink, depending on it's direction of flow. GSM separates
out the uplink and downlink into separate frequency bands.
Within each band, the channel numbering scheme is the same. Ef-
fectively, a GSM channel consists of an uplink and a downlink.
It's interesting to note that while the TCH uses a frequency chan-
nel in both the uplink and downlink, the BCH occupies a channel
in the downlink band only. The corresponding channel in the
uplink is effectively left clear. This can be used by the mobile for
unscheduled or random access channels (RACH). When the mo-
bile wants to grab the attention of the base station (perhaps to
make a call), it can ask for attention by using this clear frequency
channel to send a RACH. Since more than one mobile may want
to grab attention at the same time, colliding RACHs are possible,
and mobiles may need to make repeated attempt to get heard.

©1996 Hewlett-Packard Company               12                          10001506.sam
GSM Basics, An Introduction

               GSM Air Interface
               Separate Bands for Uplink and Downlink

               TDMA and FDMA Multiplex
                124 Frequency Channels (ARFCN) for GSM900
                200kHz Channels
                8 Mobiles share ARFCN by TDMA

               0.3 GMSK Modulation
                270.833 kbits/sec. rate

               Variable Tx Power and Timing

                                                        Jonathan Dunbar
        H                                               Queensferry Microwave Division

GSM Air Interface

©1996 Hewlett-Packard Company                     13                                     10001506.sam
GSM Basics, An Introduction

Here are some more specifics about the air interface.

               Phase 1          Phase 2       Phase 1        Phase 2
               GSM900           GSM900        DCS1800        DCS1800       PCS1900
Uplink         890 to           880 to        1710 to        1710 to       1850 to
               915MHz           915MHz        1785MHz        1785MHz       1910MHz
Downlink       935 to           925 to        1805 to        1805 to       1930 to
               960MHz           960MHz        1880MHz        1880MHz       1990MHz
ARFCN          1 to 124         0 to 124 and 512 to 885      512 to 885    512 to 810
range                           975 to 1023
TX/RX          45MHz            45MHz         95MHz          95MHz         80MHz
TX/RX          3 Timeslots      3 Timeslots   3 Timeslots    3 Timeslots   3 Timeslots
Modulation 270.833              270.833       270.833        270.833       270.833
Data Rate kbit/s                kbit/s        kbit/s         kbit/s        kbit/s
Frame          4.615ms          4.615ms       4.615ms        4.615ms       4.615ms
Timeslot       576.9µs          576.9µs       576.9µs        576.9µs       576.9µs
Bit Period     3.692µs          3.692µs       3.692µs        3.692µs       3.692µs
Modulation 0.3GMSK              0.3GMSK       0.3GMSK        0.3GMSK       0.3GMSK
Channel        200kHz           200kHz        200kHz         200kHz        200kHz
TDMA           8                8             8              8             8
MS Max         20W (8W is 8W                  1W             4W            2W
Power          max in use)
MS Min         13dBm            5dBm          0dBm           0dBm          0dBm
MS Power       0 to 15          2 to 19       0 to 13        0 to 15       30,31,0 to 15
Voice          13kbit/s         13kbit/s,     13kbit/s       13kbit/s,     13kbit/s
Coder Bit                       5.6kBit/s                    5.6kBit/s

©1996 Hewlett-Packard Company                               14                             10001506.sam
GSM Basics, An Introduction

                 TDMA and FDMA
                 Physical Channel is           Time
                 ARFCN and
                 Timeslot                                                            6


        Amplitude                                                 4

                                                            3   Timeslot

             1     2    3    4   5     6
                                                                Jonathan Dunbar
        H                                                       Queensferry Microwave Division

GSM uses TDMA (Time Division Multiple Access) and FDMA (Fre-
quency Division Multiple Access). The frequencies available are di-
vided into two bands. The uplink is for mobile transmission, while
the downlink is for base station transmission. The slide shows
part of one of these bands. Each band is divided into 200kHz slots
called ARFCN (Absolute Radio Frequency Channel Number). As
well as slicing up frequency, we also slice up time. Each ARFCN is
shared between 8 mobiles, each using it in turn. Each mobile uses
the ARFCN for one TS (Timeslot) and then waits for its turn to
come round again. Mobiles get the use of the ARFCN once per
TDMA frame.
The slide illustrates 4 TCH (Traffic CHannels). Each one of the
TCH uses a particular ARFCN and Timeslot. Three of the TCH are
on the same ARFCN, using different timeslots. The fourth TCH is
on a different ARFCN.
The combination of a TS number and ARFCN is called a physical
There's not much space between timeslots and ARFCN's. It's im-
portant for the mobile or base-station to transmit their TDMA
bursts at exactly the right time and with exactly the right fre-
quency and amplitude. Too early or too late and a burst may col-
lide with an adjacent burst. Poorly controlled modulation
spectrum or spurious will cause interference with adjacent

©1996 Hewlett-Packard Company                          15                                        10001506.sam
GSM Basics, An Introduction

                  0.3 GMSK Modulation

      Data                              270.833kB/s

                                         +67.708 kHz

                                         -67.708 kHz Q

                                 +90deg -90deg                              I

                                                  Jonathan Dunbar
        H                                         Queensferry Microwave Division

0.3GMSK Modulation
GSM uses a digital modulation format called 0.3GMSK (Gaussian
Minimum Shift Keying). The 0.3 describes the bandwidth of the
Gaussian filter with relation to the bit rate.
GMSK is a special type of digital FM modulation. One's and zero's
are represented by shifting the RF carrier by plus or minus
67.708kHz. Modulation techniques which use two frequencies to
represent one and zero are denoted FSK (Frequency Shift Keying).
In the case of GSM, the data rate of 270.833kbit/sec is chosen to be
exactly four times the RF frequency shift. This has the effect of
minimising the modulation spectrum and improving channel effi-
ciency. FSK modulation, where the bit rate is exactly four times
the frequency shift is called MSK (Minimum Shift Keying). The
modulation spectrum is further reduced by applying a Gaussian
pre-modulation filter. This slows down the rapid frequency transi-
tions which would otherwise spread energy into adjacent
0.3GMSK is not phase modulation. Information is not conveyed by
absolute phase states, as in QPSK, for example. It's the frequency
shift, or change of phase state which conveys information. It is
sometime useful though, to try to visualise GMSK on an I/Q dia-
gram. Without the Gaussian filter, if a constant stream of 1's is
being transmitted, MSK will effectively stay 67.708kHz above the
carrier centre frequency. If the carrier centre frequency is taken as
a stationary phase reference, the +67.708kHz signal will cause a
steady increase of phase. The phase will role +360 degrees at a
©1996 Hewlett-Packard Company               16                                     10001506.sam
GSM Basics, An Introduction

rate of 67,708 revolutions per second.            In one bit period
(1/270.833kHz) the phase will get a quarter of the way round the
I/Q diagram, or 90 degrees. One's are seen as a phase increase of
90 degrees. Two one's causes a phase increase of 180 degrees,
three one's 270 degrees, and so on. Zero's cause the same phase
change in the opposite direction. Adding the Gaussian filter does
not affect this average 90 degree transition for one's and zero's.
Because the Bit rate and frequency shift are tied together by a fac-
tor of 4, filtering can not affect the average phase relationships.
The filtering does slow down the rate of change of phase velocity
(the acceleration of the phase). When Gaussian filtering is ap-
plied, the phase makes slower direction changes, but may reach
higher peak velocities to catch up again. Without Gaussian filter-
ing, the phase makes instantaneous direction changes, but moves
at a constant velocity.
The exact phase trajectory is very tightly controlled. GSM radios
need to use digital filters and I/Q or digital FM modulators to accu-
rately generate the correct trajectory. The GSM specifications al-
low no more than 5 degrees rms and 20 degrees peak deviation
from the ideal trajectory.

©1996 Hewlett-Packard Company               17                          10001506.sam
GSM Basics, An Introduction

                0.3 GMSK Modulation
                                0   90                               180

                                         Spokane Division 10001506.pre
                H                        ©1997 Hewlett-Packard Company

                0.3 GMSK Modulation
                                0   90                               180

                                         Spokane Division 10001506.pre
                H                        ©1997 Hewlett-Packard Company

©1996 Hewlett-Packard Company            18                                10001506.sam
GSM Basics, An Introduction

                0.3 GMSK Modulation
                                0   90                               180

                                         Spokane Division 10001506.pre
                H                        ©1997 Hewlett-Packard Company

                0.3 GMSK Modulation
                                0   90                               180

                                         Spokane Division 10001506.pre
                H                        ©1997 Hewlett-Packard Company

©1996 Hewlett-Packard Company            19                                10001506.sam
GSM Basics, An Introduction

                  Tx Levels

                           Tx Level Power dBm
                              5        33
                              6        31
                              7        29
                               .        .
                              14       15       Spokane Division 10001506.pre
                H                               ©1997 Hewlett-Packard Company

                              15       13

Changing Power
As the mobile moves around the cell, it's transmitter power needs
to be varied. When it's close to the base station, power levels are
set low to reduce the interference to other users. When the mobile
is further from the base station, it's power level needs to increase
to overcome the increased path loss.
All GSM mobiles are able to control their output power in 2dB
steps. The base station commands the mobile to a particular MS
Tx Level (Power level). GSM900 mobile have a maximum power of
8W (the specifications allow 20W, but so far, no 20W mobiles ex-
ist). DCS1800 mobiles have a maximum power of 1W. Conse-
quently DCS1800 cells need to be smaller.

©1996 Hewlett-Packard Company                   20                              10001506.sam
GSM Basics, An Introduction

                   Timing Advance

                TDMA approach requires signals to arrive at BTS at
                the correct time. They must not overlap.


             Cell radius < 35 km
             Signal travel or time 117 usec

                                                         Jonathan Dunbar
        H                                                Queensferry Microwave Division

Timing Advance
Timing advance is required in GSM because it uses TDMA with
cells up to 35 km radius. Since a radio signal take a finite period of
time to travel from the mobile to the base-station, there must be
some way to make sure the signal arrives at the base-station at the
correct time.
Without timing advance, the transmitted burst from a user at the
edge of a cell would arrive late and overlap (and corrupt) the sig-
nal from a user right next to the base station (unless a guard time,
between timeslots, greater than the longest signal travel time was
used). By advancing the timing of the mobiles, their transmissions
arrive at the base station at the correct time. As a mobile (MS)
moves, the Base Station (BTS) will signal the MS to reduce its
timing advance as it gets closer to the centre of the cell, and in-
crease its timing advance as it away from the centre of the cell.
Mobile's in idle mode (not on a call, but still camped to the net-
work) receive and decode the BCH (Broadcast CHannel) from the
base station. One element of the BCH, the SCH (Synchronisation
CHannel) allows the mobile to adjust it's internal timing. When
the mobile is receiving the SCH, it doesn't know how far it is from
the base station. A distance of 30km will cause the mobile to set
it's internal timing 100µs behind the base-station. When the mo-
bile sends it's first RACH burst, it will leave 100µs late, after a
100µs transit delay, it will arrive 200µs late, colliding with the
bursts from mobile's closer to the base station. For this reason,
the RACH, and other types of access burst are shorter than
©1996 Hewlett-Packard Company                       21                                    10001506.sam
GSM Basics, An Introduction

normal. The mobile only sends normal length bursts once it's re-
ceived timing advance information from the base-station. The mo-
bile in our example would need to advance it's timing by 200µs.
We'll see later how the base station commands the mobile to
change it's timing advance or transmitter power using the SACCH
(Slow Associated Control CHannel)

©1996 Hewlett-Packard Company            22                        10001506.sam
GSM Basics, An Introduction

                  GSM TDMA Pow e r B u r s t

       Power                      +4 dB
                                                  +1.0 dB
                                                  -1.0 dB                               -6 dB
                    -6 dB

             -30 dB                                                                             -30 dB
                                           147 "Useful" Bits
         -70 dB                                                                                           -70 dB
                  10µs 8µs 10µs                                                    10µs 8µs 10µs
                                  3   57     1     26       1        57        3

                                       148 "Active" Bits, 546.42µs

    H                                                                                       Hewlett-Packard QMD Ian Reading Jan '94

GSM TDMA Power Burst
Since GSM is a TDMA system and there are 8 users on a frequency
pair, each user must only turn his transmitter on at the allowed
time and, have his transmitter off in time so that he does not inter-
fere with other users in the adjacent timeslots. Because of this
need, GSM has specified an amplitude envelope for the RF burst of
the timeslots. There's also a demanding flatness specification over
the active part of the useful bits in the timeslot. The amplitude en-
velope has greater than 70dB of dynamic range yet needs to meas-
ure less than +/-1dB flatness over the active part of the timeslot.
All of this is happening over the 577µs period of a timeslot.

©1996 Hewlett-Packard Company                                             23                                                          10001506.sam
GSM Basics, An Introduction

                 F r a m e s a n d M u ltiframes

                                                                             51 Multiframes

                                                                             26 Frames
     Frame                                                                   8 Timeslots

                                            576.92 us
     Timeslot          3
                                57            26
                                                      1   57 3 8.25
                                        1                                    156.25 Bits
                                bits          bits        bits  bits
    (normal burst)
                     Tail            Control    Control    Tail
                     bits       Data   bit         bit Data bit Guard
    H                                      Midamble                 Period     Hewlett-Packard QMD Ian Reading Jan '94

Frames and Multiframes
The GSM system is a time division multiplex system. The smallest
unit within that system is the individual data bits. Each date bit is
3.69µs long. A timeslot, the amount of time each mobile has to
transmit or receive information, has a time period equal to 156.25
of these data bits. Since there are 8 users on each frequency, there
are 8 timeslots per frame. The pattern repeats giving the users an-
other timeslot each frame. The frame period is 4.615 ms. Frames
are grouped into larger structures called multiframes. There are
two sizes of multiframes, 26 frame multiframes and 51 frame mul-
tiframes. TCH use 26 frame multiframes, while BCH use pairs of 51
frame multiframes stacked end-to-end to make a 102 frame se-
quence. A superframe consists 51 or 26 multiframes and a hyper-
frame is made up of superframes
These multiframe structures are necessary to allow the partition-
ing of physical channels (an ARFCN and a timeslot) into logical
channels. A logical channel is simply an end-to-end conduit for in-
formation. In later slides, we will see how the TCH is mainly used
for carrying speech data. Once per multiframe, one of the TCH's
physical channel timeslots is used to carry control information.
This logical control channel which shares the same physical chan-
nel as the TCH is called an SACCH. There are long repeat patterns
on the BCH too. Times are set aside for different types of logical
channels to coexist on the same physical channel.
The midamble or training sequence in the centre of the burst is a
known pattern. It allows the equaliser in the mobile or base

©1996 Hewlett-Packard Company                                      24                                                    10001506.sam
GSM Basics, An Introduction

station to analyse the RF path characteristics before decoding the
other useful data. Midambles have a few allowed patterns or col-
our codes. On either side of the midamble there are control bits
called steeling flags. Sometimes the TCH has to be interrupted
with urgent control information in an FACCH (Fast Associated
Control CHannel). The FACCH is used to tell the MS to change
ARFCN or TS, for example, and results in some lost TCH data. The
steeling flags allow the TCH and FACCH to be distinguished. The
remainder of the burst carries data (speech for example) and
tail/guard bits to fill the gaps between bursts.
It's easy to get confused about the number of bits in a timeslot, are
there 148 bits in a timeslot or 147 bits in a timeslot? There are 148
ACTIVE bits in a timeslot, consisting of the mid-amble, the con-
trol bits, the data and the tail-bits. There are 147 USEFUL bits
from the middle of the first bit to the middle of the last. Effectively
1/2 a bit off each end is lost.

©1996 Hewlett-Packard Company                 25                          10001506.sam
GSM Basics, An Introduction

                D o w n lin k a n d U p link
     DOWNLINK               2 3 4 5 6 7 012 3 4 5 6 7 01

    45 MHz

                01 2 3 4 5 6 701
                    Timeslots                  Frame
                 Uplink Lags Downlink by 3 Timeslot periods
                 Uplink and Downlink use same Timeslot Number
                 Uplink and Downlink use same Channel Number (ARFCN)
                 Uplink and Downlink use different bands (45MHz apart for GSM900)

    H                                                                 Hewlett-Packard QMD Ian Reading Jan '94

Downlink and Uplink
To see how information is transmitted let's look at an example. We
have been assigned timeslot 2 and we're in a traffic mode, receiv-
ing and transmitting information to the base station. The down-
link, on which we receive information, will be in the frequency
range of 935 to 960MHz. The uplink, the frequency which the mo-
bile will transmit information to the base station, will be in the fre-
quency range of 890 to 915MHz. The uplink and the downlink
make up a frequency pair, which for GSM900, is always separated
by 45MHz. We can see that the timeslots are offset by 3 between
the downlink and the uplink. We receive information in timeslot
two in the downlink we have two timeslots in which to switch to
the uplink frequency and be ready to transmit information. Then,
we have to get ready to receive our next time slot of information
in the next frame.

©1996 Hewlett-Packard Company                              26                                                   10001506.sam
GSM Basics, An Introduction

               Measuring Adjacent Cell BCH Power



    H                                                Hewlett-Packard QMD Ian Reading Jan '94

Measuring Adjacent Cell BCH Power
Besides receiving and transmitting information, the mobile must
switch frequency and get ready to receive and measure the level of
the adjacent cell's broadcast channels. It then reports this (RXLev)
information to its own base station in order to establish when a
handover is appropriate between cells. Again, information is re-
ceived on timeslot 2, we switch 45MHz to transmit information
and then, need to switch back 45MHz +/- a few MHz to monitor
and measure the level of the adjacent cell's broadcast channels.
This information will be reported back to the base station at least
every 30 seconds so that the base station can determine the appro-
priate time to do a handoff. The RxLev information is reported
back to the base-station on the uplink SACCH (Slow Associated
Control CHannel).
The mobile uses a list of ARFCN in the BA (Base Allocation) table
to know which BCH frequencies to go out and measure. The BA
table is coded onto the BCH, and also the downlink SACCH.
This is the primary (or non-hopped) mode of operation in the GSM
system. If there is an area which has bad multipath, such as urban
areas with lots of reflections from buildings, the cell may need to
be defined as a hopping cell.

©1996 Hewlett-Packard Company               27                                                 10001506.sam
GSM Basics, An Introduction

                H o p p ing Traffic Channel
      C1 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

     C2    0123456701234567012345670123456701234567

      C3   0123456701234567012345670123456701234567

      C1         0123456701234567012345670123456701234567

     C2          0123456701234567012345670123456701234567

     C3          0123456701234567012345670123456701234567


    H                                                                              Hewlett-Packard QMD Ian Reading Jan '94

Hopping Traffic Channel
All mobiles must have the capability of hopping. However, not all
cells will be hopping cells. Only those cells which have bad multi-
path problems will be defined as hopping cells. In this example,
there are three frequencies pairs to hop among. The mobile still
needs to go out and measure the adjacent cells' broadcast channel
(BCH). In the first frame, the mobile receives information on chan-
nel 1 downlink, then switches to the uplink for channel 1 (45MHz
away), transmits it's information, and finally monitors one of the
adjacent cells to measure its level. The mobile must move to the
downlink for channel 2 and receive information in timeslot 2,
switch 45MHz, and transmit on the uplink for channel 2. Then it
monitors another cell's broadcast channel and measures its level.
This continues through the sequence of frequencies that have been
assigned to the cell. The hopping sequence is defined by the CA
(Cell Allocation) and MA (Mobile Allocation) tables. The CA table
is a master list of all the hop frequencies available in a particular
cell. It's sent to the mobile on the BCH and also the downlink
SACCH. The MA table is an index into the CA table, and gives a
hopping sequence for a particular mobile. The MA table is sent to
the mobile as part of the handover or channels assignment

©1996 Hewlett-Packard Company                                       28                                                       10001506.sam
GSM Basics, An Introduction


                 GSM Snapshot

                 The GSM Network & Air Interface

                   Speech Coding & Types of Channel
                 Making a Phone Call

    H                                                   Hewlett-Packard QMD Ian Reading Jan '94

III. Speech Coding & Types of Channel

©1996 Hewlett-Packard Company                      29                                             10001506.sam
GSM Basics, An Introduction

               Speech Coder

                                             20 ms Blocks

                                                            RELP and LTP Coder
                                   Speech Coder

                                       260 Bits             Coverts Speech to Low Data Rate

                                    Bits Ordered            20ms Speech makes 260 Bits

                                       260 Bits
                                                            Output 13 kbit/s

                  50              132                78
        Very Important Bits     Important Bits      Other Bits
    H                                                                          Hewlett-Packard QMD Ian Reading Jan '94

Speech Coder
Most modern digital communications systems use some sort of
voice compression. GSM is no exception. It uses a voice coder to
model the tone and noise generation in the human throat and the
acoustic filtering of the mouth and tongue. These characteristics
are used to produce coefficients which are sent via the TCH.
The speech coder is based on a residually excited linear predictive
coder (RELP), this is enhanced by including a long term predictor
(LTP). The LTP improves speech quality by removing the struc-
ture from vowel sounds prior to coding the residual data. The
coder outputs 260 bits for each 20 ms block of speech. This yields
a 13kbit per second rate. Output bits are ordered, depending on
their importance, into groups of 182 and 78bits. The most impor-
tant 182 bits get further subdivided, with the 50 very important bits
being separated out
The data rate of 13kbit/sec is considerably lower than for direct
speech digitising as in PCM. In the future, more advanced voice
coders will cut this to 6.5kbit/sec (half rate coding)

©1996 Hewlett-Packard Company                                     30                                                     10001506.sam
GSM Basics, An Introduction

               Error Correction
                   Type Ia 50          Type Ib 132 Type II                           262 Bits
                                                                      78             in
                     Block Code
                   Type Ia      CRC          Type Ib             Type II
                       50        3            132                     78

           Type Ia   Type Ib    CRC Type Ib         Type Ia   Tail         Type II
              25        66      3       66             25        4         78

                   Half rate convolutional code                                               456 Bits
                                                                           Type II            Out
                                 378                                         78

                                     456     Bits from 20ms of
                                             Speech                             Jonathan Dunbar
        H                                                                       Queensferry Microwave Division

Error Correction
The nature of the GSM air interface means that some bit errors
will be introduced. The bits are handled in such a way that errors
are more likely where they matter least. The sound quality is af-
fected more by the most significant coefficient bits than the least
significant. The least important or type II bits have no error cor-
rection or detection. The premier type Ia bits have error detection
CRC bits added. Both type Ia and the medium importance type Ib
have convolutional error correction bits added.
It's sometimes interesting to think of GSM bits as aircraft passen-
gers! There are three classes, Ia, Ib and II. The most important
bits get first-class treatment, they get surrounded by lot's of error
correction, and in the case of Ia bits, error detection as well.
These extra bits take up space in the TCH bursts. The second
class, type II bits, take up the least space on the TCH, just like first
and second class passengers on an aeroplane.
We will see in the next slide how the final 456bits are sent over the
TCH. To minimise the effects of a whole lost frame, the bits are re-
ordered before convolutional error correction coding.

©1996 Hewlett-Packard Company                                        31                                          10001506.sam
GSM Basics, An Introduction

                 Diagonal Interleaving

       456       Bits from 20ms of Speech
                                                  456       Bits from 20ms of Speech

      57    57   57   57   57   57   57   57     57    57   57   57    57   57   57         57                 57

                           57 57     57 57     57 57    57 57     57 57      57 57           57 57

           Traffic Channel (TCH) Bursts Carry Two 57 Bit Blocks (114) Each
           120ms of Speech = 456x6 = 2736 bits
           2736 / 114 = 24 bursts i.e. 24 frames (mobile transmits once per frame)
           Multiframe has 26 frames in 120ms
           There are 2 spare frames ......One SACCH, One Idle
    H                                                                            Hewlett-Packard QMD Ian Reading Jan '94

Diagonal Interleaving
Just as important groups of individuals, like a company board of
directors, generally don't travel together (in case the plane crashes
and wipes out the whole management team), GSM bits spread
themselves over several TCH bursts. If a burst is lost due to inter-
ference, enough bits will still get through to allow the error correc-
tion algorithms to work, maintaining reasonable speech quality.
The 456 bits of speech data are sliced up into 8 blocks of 57. Each
TCH frame carries two 57 bit blocks of data from two different
20ms 456bit speech segments.
From the arithmetic on the slide, notice that in the period taken
up by 1 frame (120ms), six 20ms blocks of speech are processed
by the speech coder. Each of these blocks results in 456 bits. A
120ms segment of speech will produce 2736 bits. Each TCH burst
has a pair of 57 bit data sections on either side of the midamble.
Effectively, each TCH burst carries 114 bits. It takes 24 of these
TCH bursts to ship the 2736 bits from 120ms of speech. In an ear-
lier slide, we saw how the TCH frame structures has 26 frames in a
multiframe, lasting 120ms. Since the mobile or base station trans-
mits one burst per frame, there are two more bursts available in
120ms than are actually needed to transmit the voice data. One of
these spare bursts is used for an SACCH, the other is an idle burst.

©1996 Hewlett-Packard Company                                     32                                                       10001506.sam
GSM Basics, An Introduction

              TCH Multiframe

                      26 Frames - 120 ms
                 24 Carry Speech, 1 Idle, 1 SACCH

       0   1 2   3   4 5   6    7 8   9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

       T   T T   T T   T   T T    T T    T T   A T T   T   T T     T T   T   T T T                T -

                               SACCH                                              Idle

    H                                                                        Hewlett-Packard QMD Ian Reading Jan '94

This is how the SACCH and Idle bursts fit in with the other TCH
frames. The Idle burst is used by the mobile to make more de-
tailed measurements on the adjacent cell BCH. It stays tuned to
the adjacent BCH ARFCN long enough to decode the midamble.
The colour code, encoded in the midamble, allows the mobile to
get a positive identification of the signal being measured.

©1996 Hewlett-Packard Company                                 33                                                       10001506.sam
GSM Basics, An Introduction

                   Broadcast CHannel

               One ARFCN, On all the time, in every cell

               BCH Information carried in Timeslot 0
                other Timeslots can be used for TCH

               Allows Mobile to Synchronise

               Identifies Network

               Carries Paging Messages and other Control

    H                                                           Hewlett-Packard QMD Ian Reading Jan '94

The concept of a BCH is very simple, but the details can get a little
complicated. In simple terms, the BCH acts like a beacon, or light-
house. It's on all the time and is the first thing the mobile looks
for when it's trying to find service. The BCH ARFCN has to be ac-
tive in all timeslots to allow mobiles synchronised to other cells to
measure it's power. The useful BCH information is always carried
in timeslot 0. The other timeslots are filled with dummy bursts, or
are available for TCH. There are a number of interesting parts to
the BCH:
   The FCH (Frequency correction CHannel) uses a special burst
   which repeats on the BCH, it has a special fixed bit sequence to
   allow the mobile to tune it's internal frequency reference when
   it first turns on.
   The SCH (Synchronisation CHannel) has a burst with extended
   midamble. It's used by the mobile after the FCH to adjust it's
   internal timing and get synchronised to the multiframe
   The BCCH (Broadcast Control CHannel) has information
   encoded on it which identifies the network. It also carries lists
   of the channels in use in the cell (BA and CA tables)
   The CCCH (Common Control CHannel) is like a message board.
   Just like the FCH, SCH and BCCH, it can be received by any
   mobile. Sub-channels like PCH (Paging CHannel) are posted on
   the CCCH. When the mobile sees its number on the PCH it

©1996 Hewlett-Packard Company                              34                                             10001506.sam
GSM Basics, An Introduction

   recognises that it should respond by requesting service with a
   Another CCCH sub-channel is the AGCH (Access Grant
   CHannel). Once a mobile has sent a RACH, the base station
   responds by putting an AGCH on the CCCH, bearing the
   mobiles random number (read from the RACH). The AGCH
   instructs the mobile to go to an SDCCH or TCH.
There are a variety of different configurations for all these
channels on the BCH. The selection depends on the number of
users expected in the cell. If a large number of users are expected,
a large CCCH capacity is needed, which when added to the SCH,
FCH and BCCH, fills the BCH completely. In other situations,
spare capacity on the BCH can be used for an SDCCH (Stand-alone
Dedicated Control CHannel).

©1996 Hewlett-Packard Company               35                         10001506.sam
GSM Basics, An Introduction

                   Random Access CHannel


         Mobile doesn't know path delay
          So RACH has to be a special SHORT BURST
          Mobile sends normal burst only after getting Timing Advance
          on Downlink SACCH

    H                                                                   Hewlett-Packard QMD Ian Reading Jan '94

When the mobile has become synchronised to the frequency and
frame timing of the cell, and looked at the other information on
the BCH it is ready to make and receive calls. Once the mobile is
in this state it is 'camped' to the base station. If the mobile is near
the base station their timing will be closely aligned. If the mobile
is on the edge of the cell, maybe 30km from the base station, the
SCH will have a propagation delay of 100µs. The mobile's timing
will be 100µs in error. When the mobile sends out a RACH, to start
a call, the RACH is transmitted 100µs late, with another 100µs
transit time to the base station, it arrives 200µs late. To avoid
collisions with bursts in adjacent TS, RACH busts are shorter than
The RACH is not the only type of short access burst. When a mo-
bile is handed over to another cell, there will be a short period of
time before it receives timing advance information on the down-
link SACCH from the new cell. During this period, there's a risk of
the mobiles bursts colliding with bursts in the new cell. Until it
gets timing advance information from the new cell it sends short
access bursts.

©1996 Hewlett-Packard Company                             36                                                      10001506.sam
GSM Basics, An Introduction

              S t a n d -a l o n e D e d i c a t e d C o n t r o l C H a n n e l

                 USED DURING CALL SET-UP

                 Stepping Stone between BCH and TCH

                 Used for Authentication Etc.

    H                                                                         Hewlett-Packard QMD Ian Reading Jan '94

The SDCCH is sometimes configured as a logical channel on the
BCH, and sometimes on it's own physical channel. The SDCCH
has a different multiframe structure to the TCH. SDCCH bursts
repeat less frequently than once per frame. For this reason, more
than 8 SDCCH can share a physical channel. As a consequence,
the data rate on the SDCCH is lower than on the TCH.
The SDCCH is used like a stepping stone. During the call set-up
process, there can be a lot of time between the mobile sending a
RACH and getting service, to the start of conversation. Time is
taken up while the phone is ringing and waiting to be answered.
During this period, there's a need to exchange control information
between the mobile and base station. Alerting messages are sent,
and authentication takes place, but there's no need to send speech
information. The SDCCH, by using less of the cells resource of
physical channels, improves efficiency, and provides a useful
holding channel for the mobile until speech data needs to be
exchanged. Just like the TCH, the SDCCH has an SACCH
associated with it.

©1996 Hewlett-Packard Company                                   37                                                      10001506.sam
GSM Basics, An Introduction

               Fast Associated Control CHannel


           Rapid message exchange for handovers

           Control Bits either side of midamble:
            Indicate TCH or FACCH

                                                        Jonathan Dunbar
        H                                               Queensferry Microwave Division

When the SACCH reports coming back to the base station indicate
that another cell would offer the mobile better signal quality, a
handover is necessary. The SACCH just doesn't have the band-
width to transfer all the information associated with a handover
(like the new ARFCN and timeslot, or the MA table). For a short
period of time, the TCH is replaced by an FACCH. The FACCH
uses consecutive bursts, so has a much higher data rate that the
SACCH, which uses only one burst in 26. The frame stealing flags
(the control bits on either side of the midamble) are set to indicate
that the data being sent is an FACCH, not the TCH. In other re-
spects, the FACCH looks just the same as the TCH. It uses the
same physical channel (ARFCN and timeslot). When the FACCH
steals bursts from the TCH, speech data is lost. It's often possible
to hear a small speech drop-out when handovers take place.

©1996 Hewlett-Packard Company                      38                                    10001506.sam
GSM Basics, An Introduction

                 Slow Associated Control
                   Mobile Tx Power Commands
                   Mobile Timing Advance
                   Cell's Channel Configuration

                   Received signal quality report (RXQual)
                   Received signal level report (RXLev)
                   Adjacent BCH power measurements
                   Mobile's status

                                                     Spokane Division 10001506.pre
                H                                    ©1997 Hewlett-Packard Company

One of the two spare frames every 12 TCH frames is used for the
SACCH (Slow Associated Control Channel). On the down-link, the
SACCH is used to send slowly but regularly changing control infor-
mation to the mobile. Examples are instructing the mobile to
change its transmitter power (MS TX Lev) and burst timing ad-
vance (to compensate for RF transit time) as it moves around the
cell. It also carries the BA and CA tables.

The up-link SACCH carries information about received signal
strength (RXLev) and quality (RXQual) of the TCH and the adja-
cent cell BCH measurement results (also RXLev).

©1996 Hewlett-Packard Company                        39                              10001506.sam
GSM Basics, An Introduction

                   Typical BCH Organization

                                   0   1      2    3                  0         1

                     FCH        Frames




        H                                              Jonathan Dunbar
                                                       Queensferry Microwave Division

BCH Organization
The base station generates the BCH, always on timeslot zero. The
broadcast channel can take different formats, the above diagram
depicts the first 26 frames of the BCCH + CCCH + 4 SDCCH/4

©1996 Hewlett-Packard Company                 40                                        10001506.sam
GSM Basics, An Introduction

                   Lo g i c a l a n d P h y s i c a l C h a n n e l s

                     Physical channels - can be described in frequency
                                         and time domains
                        Timeslot number

                     Logical channels - are mapped onto physical channels

                        Traffic channels
                        Control and signalling

    H                                                                   Hewlett-Packard QMD Ian Reading Jan '94

Logical and Physical Channels
What's the difference between logical and physical channels?
Physical channels can be described in terms of the frequency do-
main and time domain. They are the actual frequencies and/or the
timeslot the MS or BS are transmitting or receiving on. The logical
channels are mapped onto these physical channels. At any particu-
lar instant a frequency/timeslot may be either a traffic channel or
some control or signalling channel. A logical channel describes the
function of a physical channel is at that point in time.

©1996 Hewlett-Packard Company                                   41                                                10001506.sam
GSM Basics, An Introduction

                  Subscriber ID Module
                  Plugs into Every GSM Mobile
                   Two Sizes - Standard (credit card) and Micro
                   (postage stamp)

                  Holds All Unique Subscriber Information
                   IMSI (International Mobile Subscriber)
                   Lists of Networks Allowed For User

                  Stores Information on Last Location

                  Stores User Information                                   Test SIM
                   Speed Dial Lists, Memories Etc.

                                                      Spokane Division 10001506.pre
                H                                     ©1997 Hewlett-Packard Company

The SIM card comes in two sizes: standard (credit card size) and
micro (postage stamp size). SIMs (subscriber Identification Mod-
ules) plug into the GSM mobile. The SIM holds all the information
related to a subscriber. For example:
   His unique subscriber number or IMSI (International Mobile
   Subscriber Identification)
   The networks and countries where he is entitled to service
   (MCC and MNC)
   Any other user specific information like speed dial numbers and
Without a SIM installed, all GSM mobiles are identical. It's the SIM
card which gives a mobile it's identity. If a user (Fred) takes his
SIM on a business trip and plugs it into the GSM mobile fitted to
his rental car, the car's phone takes on the SIM's identity. Fred's
network access rights, his speed-dial memories and any other
saved features, are transferred to the rental car phone. The really
nice feature of SIM's is that they also carry your phone number. If
Fred's office want to call him, they simply dial his normal mobile
number. The network knows the location of the phone with
Fred's SIM in it and so routes the call directly to the rental car.
For test purposes, there are special Test-SIMs. Test SIMs allow
mobiles to enter a special loop-back mode for receiver BER test.

©1996 Hewlett-Packard Company                        42                                10001506.sam
GSM Basics, An Introduction

              Phase II Features

                   Bringing GSM and DCS together


                   Dual Band

                   Enhanced Full Rate Speech

                   Half Rate Speech

                   Fast BER Measurements

                   Data Applications
     H                                                  Queensferry Microwave Division

In 1995 the Phase II GSM specifications were released. This revi-
sion brought the GSM and DCS specifications even closer      to-
gether. There weremodifications and extensions to current
specifications and new features were added.

Extended GSM was introduced which gives an extra 10MHz of fre-
quency spectrum at 890MHz. Lower power levels were also added.
The combination of these two features allow increase user density
in GSM.

The new features and specifications which go under the banner of
'PhaseII' are constantly being added to. We will now go on and
look at some of the features that exist now. This paper will be up-
dated as new features are introduced.

©1996 Hewlett-Packard Company                      43                                    10001506.sam
GSM Basics, An Introduction

                                                      Dual Band

                                                        What is Dual Band?

                                                        GSM900 and DCS1800

                                                        Market Drivers

                                                        How does it work?
                                                         dual band handover
                                                         multi band measurement reporting










     H                                                                                           Queensferry Microwave Division

What is dual band? Dual band phones have to be able to operate
in two bands, GSM900 and DCS1800 and are able to seemlessly
handover between the two bands.

Dual band phones combine GSM900 and DSC1800 technologies in
a single product. There were two main drivers in the market push-
ing ETSI and phone manufacturers for multiband GSM phones.
The first is that existing DCS1800 operators wanted to offer roam-
ing outside of the UK and Germany. To compete with GSM900 op-
erators who have virtual worldwide roaming. They then require
phones which are GSM900/DCS1800 capable.

The second was that many GSM operators also had spectrum allo-
cated at 1800MHz. Dual band phones allow these operators to in-
crease capacity in already overcrowded urban areas by treating
DCS1800 channels as an extension of their existing GSM 900

How does it work? There are two main requirements on a dual
band phone. One as we have mentioned having the capability of
performing GSM900 to DCS1800 seamless handover. The mobile
front end now has to work over a wider frequency range therefore
it is important to test the ability of the mobile to successfully han-
dover between frequency bands. Secondly mulitband
©1996 Hewlett-Packard Company                                                               44                                    10001506.sam
GSM Basics, An Introduction

measurement reporting. While on a call, mobiles are required to
measure the signal strength of adjacent cell BCH. This information
is reported back to the base station and is used to determine when
a handover should be performed. Dual band mobiles have to moni-
tor adjacent call BCH in both frequency bands. This puts a very
tight on the receiver of the mobile. Mobile manufacturers will
want to test that the receiver can successfully perform adjacent
band BCH measurement reporting and still maintain the call.
We will look in some more detail not at how dual band actually

©1996 Hewlett-Packard Company              45                        10001506.sam
GSM Basics, An Introduction

                                                               How Does Dual Band Work?

                                                                 Set up a call
                                                                  in GSM or DCS

                                                                 Intra band handover

                                                                 Inter cell handover













      H                                                                                        Queensferry Microwave Division

 How do dual band phones actually work? A call can be set up in
either GSM900 or DCS1800 band. A call can go ahead as normal in
one of these two bands. However it may be necessary to handover
the call in some way.
It may be the case where there is a GSM network and a DCS
 network existing on top of each other. There may be capacity
problems in the GSM network and the call could be maintained by
handing the TCH only over to the DCS band. This is called an intra
cell handover.
It may also be that the user has left a GSM network and entered a
DCS network in which case the BCH and the TCH will be handed
over from GSM to DCS. This is called an inter cell handover.
These handovers will take place as required and the phone will be
instructed by the base station when one of these operations
should take place.
to this a higher quality algorithm was developed. This slide is a
pictorial representation of an intra band handover.
This slide shows that a call has been set up in the GSM900 band.
At some point in time the traffic channel (TCH) is moved over to
the DCS band. Now the conversation is taking place in the DCS
band will the mobile is still referencing the BCH in GSM.

©1996 Hewlett-Packard Company                                                             46                                    10001506.sam
GSM Basics, An Introduction


             Dual Band Assignment                                      #





              GSM band                            DCS band
                                Use Intra-cell channel assignment procedure.
                                BCH stays in GSM900 band on same ARFCN
                                TCH moves to DCS1800 band
                 BCH TCH

   H             BCH                                   TCH             QMD Marketing
                                                                       September 1997
                                                                       Rev 1.0

This slides shows an intra cell dual band handover.

©1996 Hewlett-Packard Company                          47                                                                    10001506.sam
GSM Basics, An Introduction

                  Enhanced Full Rate Speech

                                                                                                       20 ms Blocks
                                             20 ms Blocks
                                                                                              Speech Coder

                                    Speech Coder                                                 244 Bits
                                                                                            Channel Coding
                                       260 Bits
                                                                                                260 Bits
                                    Bits Ordered
                                                                                              Bits Ordered
                                      260 Bits                                                   260 Bits

                   50              132                    78                50               132                    78
          Very Important         Important            Other        Very Important          Important            Other
          Bits                   Bits                 Bits         Bits                    Bits                 Bits

                     (type Ia)                (type Ib)                        (type Ia)                (type Ib)
                                                               (type II)                                                 (type II)

                           Full Rate Speech                                  Enhanced Full Rate Speech
       H                                                                                                                      Queensferry Microwave Division

The GSM standards introduced a new type of voice codec which
provides superior voice quality over that currently offered by
GSM900 and DCS 1800 MHz networks. The new codec was based
on that developed for use in the PCS1900MHz GSM networks in
the US cellular market and has become an option on many GSM
phones. It was developed to address concerns that the US had
over the voice quality of GSM. The perceived quality was worse
than that of the established analogue cellular, and as a conse-
quence doubted whether GSM would be successful in the US. In
Following the introduction of EFS to the US market, the benefits
of higher voice quality could clearly be applied to the GSM market
as a whole. GSM900 and DCS1800 networks operators have now
 started offering EFS calls, and most of the latest GSM handsets
support EFS.

©1996 Hewlett-Packard Company                                                                    48                                                            10001506.sam
GSM Basics, An Introduction

              Speech and Channel Coding Process
               Speech Coder
               Full Rate Speech Frame              Enhanced Full Rate Speech Frame
                        260 bits                               244 bits

               Channel Coder
                                                           EFS Preliminary Coding
                     1 Speech Frame - 260 bits
                    Class I (182 bits)                    ClassII (78 bits)
                                 189 bits
                    ClassIa (50bits) CRC(3bits) ClassIb(132bits) Tail(4bits)

                                                1/2 rate convolutional code
                                     378 bits                             78 bits
      Reordering and diagonal
      interleaving over 8 bits

                                  1 burst=2x57=114 data bits      data bits
     H                                                                               Queensferry Microwave Division

 As you can see in this slide, the size of an individual EFS speech
frame is different to a full rate speech frame (244 bits compared to
260 bits). However the same channel coding process it used for
both the EFS and FS. Before the enhanced full rate speech frame
is passed through the channel coding process, it is passed through
a preliminary coding stage.

The EFS coding process adds a further level of error protection to
the EFS speech data bits. The preliminary coding involves the cal-
culation of an 8 bit CRC and the addition of 8 repetition bits.

This is shown in the next slide.

©1996 Hewlett-Packard Company                                                  49                                     10001506.sam
GSM Basics, An Introduction

              EFS Preliminary Coding Stage

       EFS Speech Frame                        EFD Preliminary Coded
       244 bits                                Speech Frame 260 bits

                          CRC calculated for
                                                                             (50 bits)
                          65 most important
          244 data                                 8 bit CRC
                          bits                                              ClassIb
          bits                                                              (132 bits)
                                                                            Includes 8 bit
                          4 bits are each                                   CRC
                          repeated twice
                                                 8 repition bits
                                                                              (78 bits)
                                                                              Includes 8
                                                                              repition bits

     H                                                             Queensferry Microwave Division

©1996 Hewlett-Packard Company                       50                                              10001506.sam
GSM Basics, An Introduction

                             EFS Loopback Path

                                             Channel Coder

                                                                         Exisiting Channel Coder
                         EFS Codec
      Speech Interface

                                             EFS Pre-                                                   TX


                                     FS Codec                                                           RX

                                     A Loopback as EFS frame, 244 bits
     H                               B Loopback as normal speech frame, 260 bits                         Queensferry Microwave Division

This slide shows the new loopback path for EFS and the tradi-
tional FS path. There was some discussion over which path would
be used for EFS, would it be necessary to have a new loopback
path? After further analysis ETSI decided that the 244 bits after
preliminary coding must be looped for TCH/EFS. This means that
the preliminary coding for enhanced full rate is part of the channel
coding. Frames failing the CRC check of the preliminary coding
are erased frames.

©1996 Hewlett-Packard Company                                                                      51                                     10001506.sam
GSM Basics, An Introduction

              Fast BER

              1 Speech Frame - 260 bits

             Class I (182 bits)                          ClassII (78 bits)

                          189 bits

             ClassIa (50bits) CRC(3bits) ClassIb(132bits) Tail(4bits)

                                             1/2 rate convolutional code

                                  378 bits                                   78 bits

                            1 burst=2x57=114 data bits            data bits
     H                                                                                 Queensferry Microwave Division

We will now discuss at the last Phase II measurement that we will
look at today. Fast BER. All BER measurements are performed on
the TCH using the speech frame. This slide shows the format of
the speech frame. We have already looked at the error protection
of the ClassIa/Ib and II bits. As a consequence of this protection
BER is measured separately for the three classes. For certain
receiver sensitivity measurements ClassII BER is used, the reason
for this is due to the fact that the error protection given to ClassI
bits by the channel coding process masks the sensitivity
characteristics of the receiver.

©1996 Hewlett-Packard Company                                                     52                                    10001506.sam
GSM Basics, An Introduction

              Mobile TCH Test Loops

                 Demod-         Decryp-   Channel         Speech
                 ulator         tion      Decoder         Decode

                                          New Test Loop   Loop A/B
                                          Loop C

                 Modul-         Encryp-   Channel         Speech
                 ator           tion      Encoder         Encode

     H                                                               Queensferry Microwave Division

This slide shows the TCH test loops that have been defined for
mobiles. Three TCH test loops have been defined.
*Loop A is used to measure RBER and FER for ClassIa,Ib and II. In
residual loopback mode the mobile erases any frames that have a
bad CRC or contain errors above a certain threshold. A frame is
erased by setting all data bits to zero. The erased frames are not
included in the BER measurement, only the remaining, or residual
frames are used.
*LoopB is used to measure the bit error ratio for ClassIa,Ib and II
*LoopC is the new test loop that has been defined for Burst-by-
Burst (or Fast) BER.
This new loop allows the 114 data bits from each received TCH
burst to be passed directly to the modulator without going through
the Channel Encoding/Decoding process. The Channel Coder has
the effect of correcting Class Ia and Ib errors, and recoding the
ClassIa bits with a new CRC value. As you will recall ClassII bits
do not have any error protection, and pass through the Channel
Coding process unchanged. This characteristic makes ClassII BER
an extremely useful measurement. The new loopback path C, by-
passes the Channel Coder, which means that every data bit will
have the characteristic of a ClassII bit. The BER calculated using
loop , is equivalent to ClassII BER. Since every data bit in a Burst-
by-Burst BER measurement is now an equivalent ClassII bit,
©1996 Hewlett-Packard Company                             53                                          10001506.sam
GSM Basics, An Introduction


           i) Class II BER using loops A or B

             There are 78 Class II bits in each channel coded speech frame
             Number of speech frames required = 129
             Actual number of bits tested = 10062 (nearest frame mulitiple to 10000)
             Actual time taken = 2.6 secs

           ii) Burst-by-Burst BER using loop C (equivalent to ClassII BER)

             There are 456 equivalent ClassII bits in each channel coded speech frame
             Number of speech frames required = 22
             Actual number of bits tested = 10032 (nearest frame mulitiple to 10000)
             Actual time taken = 0.5secs

     H                                                                       Queensferry Microwave Division

ClassII BER can be calculated 5 times faster using loop C than the
conventional methods using loops A or B.
This is explained in the following example.

This example shows the time taken to perform a ClassII BER
measurement over 10,000 bits using
i) the conventional method using loops A/B and
ii) the Burst-by-burst method using loop C.
The burst-by-burst loopback path defines a path that allows each
received burst to be passed directly to the modulator without
passing through the Channel Coder. This implies that the burst can
be treated as an individual data block containing 114 random bits,
and the BER measurement can be performed on an individual
burst-by-burst basis.
As can be seen the conventional ClassII BER measurement takes
just over 5 times longer to perform than the Fast BER measure-
ment. Typically manufacturing test would perform anything be-
tween 10,000 and 30,000 bits on several different channels. This
©1996 Hewlett-Packard Company                                 54                                              10001506.sam
GSM Basics, An Introduction

              Mobile Turn-On

                Mobile Searches for Broadcast Channels

                Synchronizes Frequency and Timing

                Decodes BCH sub-channels (BCCH)

                Checks if Network Allowed by SIM

                Location Update


     H                                                    Queensferry Microwave Division

means that the Fast BER measurement could save on average 10
seconds per phone test.

©1996 Hewlett-Packard Company                        55                                    10001506.sam
GSM Basics, An Introduction

IV. Making a Phone Call
Mobile Turn-On
When a mobile first turns on, it searches all 124 channels in the
downlink for signals. It will then order the channels by received
signal strengths and check to determine if the channel was a BCH
(Broadcast CHannel). Once the MS finds a BCH, it adjusts internal
frequency and timing from the FCH and SCH, then checks to de-
termine if the BCH is from its PLMN (Public Land Mobile Net-
work). This involves comparing the allowed network and country
codes stored on the SIM card with the information encoded on the
BCCH. The mobile repeats this cycle until a good broadcast chan-
nel is found. If the mobile recognises that it's in a different cell
from the last time it was used, it needs to tell the network where it
is. The network has to keep track of where every mobile is so that
it can route calls to the correct cell for any particular mobile. This
process of telling the network "here I am" is called a location up-
date. The mobile sends a RACH, gets assigned to an SDCCH, ex-
changes control information, then ends the call. The user will
typically not be aware that this process is taking place.
Some networks have IMSI attach enabled. This forces the mobile
to do a location update every time it turns on, even if it has not
moved to different location.

©1996 Hewlett-Packard Company                56                          10001506.sam
GSM Basics, An Introduction

                   Mobile Call Origination

                   Mobile Sends RACH

                   Channel Assignment Posted on BCH (AGCH)

                   Mobile and Base Station communicate on SDCCH


                   Mobile Assigned to Traffic Channel (TCH)

                   Speech Data sent and received

    H                                                         Hewlett-Packard QMD Ian Reading Jan '94

Mobile Call Origination
Once the mobile has synchronised to the BCH, determined that it's
allowed to use the network (PLMN) and if necessary done a loca-
tion update, it's camped. Once camped, the mobile is ready to
send or receive calls.
When a user dials a number, and presses the send button on the
mobile, call origination takes place. The mobile transmits a short
RACH burst on the uplink, using the same ARFCN as the BCH is
using on the downlink. The base station responds to the RACH by
posting an AGCH (Access Grant CHannel) on the CCCH. These
are logical channels on the BCH physical channel. The mobile lis-
tens on the BCH for the AGCH, when it receives it and decodes the
instructions, it re-tunes to another ARFCN and/or timeslot and be-
gins a two-way dialogue with the base station on an SDCCH. One
of the first things that the mobile will receive is the SACCH associ-
ated with the SDCCH. Once it receives the SACCH, it will get tim-
ing advance and transmitter power information from the base
station. The base station will have calculated the correct timing
advance from the arrival time of the RACH. Once the mobile gets
timing advance information, it can send normal length bursts. The
SDCCH is used to send messages back and forth, taking care of
alerting (making the mobile ring) and authentication (verifying
that this mobile is allowed to use the network). After a short pe-
riod of time (1 to 2 seconds), the mobile is commanded over the
SDCCH to re-tune to a TCH. Once on the TCH, speech data is
transferred on the uplink and downlink.

©1996 Hewlett-Packard Company                      57                                                   10001506.sam
GSM Basics, An Introduction

The process for base station originated calls is very similar. The
base-station posts a PCH (paging CHannel) on the CCCH part of
the BCH. When the mobile receives the PCH, it responds by send-
ing a RACH. The remainder of the process is identical to the mo-
bile originated case.
If you can find a way to translate the GSM bursts into audio tones
(AM demodulate), it's interesting to hear the difference between
the channel types as a call is set up. A good way to do this is to
use a GSM phone near an old TV set or a conventional wired
phone. The interference created in these devices amounts to AM
The RACH burst can be heard as a single 'Tick' sound. It's quickly
followed by the SDCCH 'Tat, Tat-tat-tat, tat-tat-tat ...'. After a few
seconds, the TCH is connected 'Buzzzzzzzzz'
Like any technical subject, GSM can be confusing at first. You
may need to read through the hand-out several times to under-
stand the system.

©1996 Hewlett-Packard Company                58                          10001506.sam
GSM Basics, An Introduction

More GSM Topics

                   DTX and DRX

                Idle mode to save battery power

                MS's divided into paging groups based on IMSI
                - Paging requests transmitted by
                   network at predefined intervals

                Saves battery power
                Inserts comfort noise
    H                                                  Hewlett-Packard QMD Ian Reading Jan '94

Discontinuous Reception or DRx and Discontinuous Transmission
or DTx are modes used by the mobile to save battery power. Mo-
biles are divided into paging groups (depending on their subscriber
identity number). Because paging groups are only paged or called
at pre-defined times, the mobile only needs to listen to see if the
network has any messages or calls for it at these times. In DRx the
mobile "goes to sleep" (conserving battery power), wakes up when
it is supposed to listen for pages (dependent on its paging group)
and then go back to sleep.
Discontinuous Transmission occurs if the user is just listening and
not talking. In order to conserve battery power, the radio will not
transmit a burst (transmitting is the biggest power drain) until
there is information to be sent. When DTx occurs the system will
insert "comfort noise" so that the caller on the other end will know
that a link is still established.

©1996 Hewlett-Packard Company                 59                                                 10001506.sam
GSM Basics, An Introduction


                     BS controls whether On or Off

                     Happens after interleaving, before burst building

                     Algorithms very tightly controlled

                     Similar to techniques used by DOD
                     and Intelligence Agencies

    H                                                               Hewlett-Packard QMD Ian Reading Jan '94

One of the key features of the GSM system is security. This comes
about because of the use of encryption or ciphering. The base
station controls whether ciphering is on or off. The encryption of
the data occurs after the data is interleaved and formed into the
eight data blocks. (before the actual bursts are built). The
encryption algorithms are very tightly controlled. They are very
similar to the techniques used by many of the top intelligence
agencies around the world. The security of this is enhanced with
the fact that the encryption algorithms change from call to call
(even if it is deciphered for one call, the encryption on the next
call will be different).

©1996 Hewlett-Packard Company                             60                                                  10001506.sam
GSM Basics, An Introduction

              Multi-path and Equalisation

                  Unwanted                                   Doppler

                Wanted                               Reflections

                                    Reference Colour Code

                                    Actual Received Signal

    H                                                         Hewlett-Packard QMD Ian Reading Jan '94

Multipath and Equalization
This is a simple example of a base station, a mobile, a direct
transmission path, one large reflection from a mountain or a
building and some slight frequency shift due to the movement of
the mobile. The mobile knows it should be receiving a particular
colour code for the base station. By calculating the characteristics
of the RF path from the disturbance it caused to the midamble, the
mobile's equaliser can more effectively reconstruct the other parts
of the burst, reducing the chance of detecting a bit wrong.

©1996 Hewlett-Packard Company                  61                                                       10001506.sam
GSM Basics, An Introduction

                   Mid a m b le o r Tr a ining Bits

           Timeslot               57               26
                                                            1    57   3 8.25
                            3               1
                                  bits             bits          bits    bits
     (normal burst)        Tail          Control          Control   Tail
                           bits   Data     bit              bit Data bit Guard
          8 Midamble Patterns (Colour Codes) of 26 bits
          RACH and SCH have Longer 41 and 64 bit Midambles

          Equaliser Estimates Channel Impulse Response From Midamble
          Mathematically Construct Inverse Filter
          Uses Inverse to Decode Data Bits

    H                                                                                 Hewlett-Packard QMD Ian Reading Jan '94

Midamble and Training Bits
In the timeslot structures we saw, there was 26 bits referred to as
either a mid-amble or a training sequence. For a normal burst this
mid-amble will consist of 8 base station colour codes and these are
numbered 0 through 7. They are 26 bits long. Another mid-amble
or training sequence is used in the random access channel (RACH)
and it is 41 bits long. There is also a 64 bit long sequence that is
used on the SCH or synchronisation channel. Mid-ambles are
placed in the centre of the burst to minimise the time difference
from to any bit in the burst. The mid-amble has a number of
different uses, the most important is equalisation to improve bit
error rate. The mobile knows the mid-amble it should be receiving
(part of the information the MS gets when assigned to a BS). This
is a pre-defined sequence is 26 bits in the case of a traffic channel.
It receives the mid-amble and compares the it to what it should
have been. From the difference it can estimate the impulse
response of the transmission path at that instant in time. Once it
knows the impulse response it can mathematically calculate an
inverse filter, it can apply this filter to the data bits on each side of
the mid-amble and clean them up, reducing the chance of
detecting a bit wrong. This is referred to as equalisation or the
equaliser within the radio. Equaliser mechanisms are a closely
guarded design feature of most mobiles. It's a key area of
competition between mobile manufacturers.

©1996 Hewlett-Packard Company                                           62                                                      10001506.sam
GSM Basics, An Introduction

                    Signalling Layers 1, 2, 3
                 Layer 1:   Functions required to transfer bit streams over
                            physical TDMA/FDMA channels
                               Setting of timing advance & TX power
                               Channel coding
                               Mapping of logical channels to physical channel
                            Example - HPIB - Layer 1 is the actual bits
                Layer 2:    Provides a reliable signalling link between MS and the
                            network. Protocol is based on LAPD of ISDN (LAPDm ).
                            Example - HPIB - Layer 2 is the handshaking
                Layer 3:    Takes care of the main control of MS-network
                            procedures. It is split into 3 sublayers:
                               Radio Resource Management
                               Mobility Management
                               Connection Management
                            Example - HPIB - Layer 3 is the mnemonics
    H                                                                                Hewlett-Packard QMD Ian Reading Jan '94

Signalling Layers 1, 2, 3
Some other terms you will hear thrown about in the GSM world
are the different layers of signalling (layer 1, layer 2, layer 3).
These layers are from the OSI (Open Systems Interconnection)
seven layer model. The GSM system uses the first 3 layers from the
OSI model. Layer 1 can be thought of as the functions required to
transfer bit streams over physical TDMA and FDMA channels. This
includes things like ciphering, the setting of timing advance, and
transmit power, the channel coding, and the mapping of logical
channels onto physical channels. An example: In HPIB, layer 1 is
the actual bits or voltage levels on the bus. Layer 2 provides a reli-
able signalling link (protocol) between the mobile station and the
network. The protocol is based on LAP-D of ISDN or LAP-Dm. An
example for layer 2 (in the HPIB example) is the handshaking be-
tween the listener and the talker. Layer 3 takes care of the main
control on the MS - network procedures and it is really split into 3
sub-layers. These include radio resource management, mobility
management, and connection management. Our HPIB example of
layer 3 is the mnemonics or higher level control we have over the

©1996 Hewlett-Packard Company                                       63                                                         10001506.sam
GSM Basics, An Introduction

              Mobile Call Origination

                               MS      BTS   BSC   MSC   AUC

            Channel Request                                    PSTN
            Immediate Assign to DCCH
            Asynchronous Mode
            Unnumbered Acknowledge
            Authentication Request
            Authentication Response
            Cipher Mode
            Cipher Mode Complete
            Call Set Up
            Assign Command
            Assign Complete
            Alert (Ringing)
            Call Connect

    H                                                                 Hewlett-Packard QMD Ian Reading Jan '94

Mobile Call Origination
To originate a call, the mobile sends out a channel request to the
base station. The mobile is immediately assigned to a SDCCH
(Stand-alone Dedicated Control Channel, or sometimes just
DCCH) and responds to the authentication request to ensure that
it is legal. Again, the Cipher mode will turn off or on ciphering and
when completed the MS will then do a call set-up. The system will
assign the MS to a frequency and timeslot. Once the connection is
made, the MS is in a traffic mode and the information goes back
and forth as we saw earlier. (receives, transmits, measures the ad-
jacent BCH and then repeats for the next frame).

©1996 Hewlett-Packard Company                             64                                                    10001506.sam
GSM Basics, An Introduction

               Logical and Physical Channels

                        Physical channels - can be described in frequency
                                            and time domains
                              Timeslot number

                        Logical channels - are mapped onto physical channels

                             Traffic channels
                             Control and signaling

                 Lo c a t i o n U p d a t e

                                                 VLR   EIR
          MS        BTS         BSC      MSC           AJC

          Location Update Request                            PSTN   MSC             AUC
          Identity Request
          Identity Response

          Authentication Request
         Authentication Response
         Cipher Mode Command
         Cipher Complete
         Location Updating Complete
          Relocation Complete

    H                                                                         Hewlett-Packard QMD Ian Reading Jan '94

Location Update
The process begins when the mobile sends a location update re-
quest to the system. The system asks for the mobile's identity and
Home Location Register (HLR), and authenticates the mobile,
making sure it is a legal mobile. The BS will then turn ciphering on
or off. The new MSC lists the mobile in its Visitors Location
©1996 Hewlett-Packard Company                                65                                                         10001506.sam
GSM Basics, An Introduction


                       DOWNLINK               UPLINK

              * Timing               * Adjacent BCH

              * Power Ctrl.          * Channel Power

              * Call configuration   * Rx Level

              * Hopping frequency    * Rx Qual.

     H                                                 Queensferry Microwave Division

Register (VLR) and notifies the MS's HLR of the fact that the MS is
in the new location and will be serviced by the new MSC.

These last two slides are for your reference and to help you re-
member some key points

©1996 Hewlett-Packard Company                     66                                    10001506.sam
GSM Basics, An Introduction

©1996 Hewlett-Packard Company   67   10001506.sam
GSM Basics, An Introduction

Abbreviations and Acronyms                                       BS-FREQ-N Frequency of the RF carrier on which
                                                                 CELL      the BCCH of an adjacent cell is
A               See MS-PWR-CLASS                                           transmitted
AB              Access Burst                                     BSI          Base Station Interface
AC              Administration Centre                            BSIC         Base Station Identity Code
ACCH            Associated Control Channel                       BSIC-NCEL BSIC of an adjacent cell
ACM             Address Complete Message                         L
ACS             Adjacent Channel/Carrier Suppression             BS-PA-MFR The number of multiframes between
ACU             Antenna Combinary Unit                           MS        two transmissions of the same paging
                                                                           message to MSs of the same paging
AEF             Additional Elementary Functions                            group
AFC             Automatic Frequency Control                      BSS          Base Station System
AGC             Automatic Gain Control                           BSSAP        Base Station Application Part
AGCH            Access Grant Channel                             BSSMAP       BSS Management Application Part
ARFCN           Absolute Radio Frequency Channel                 BSSOMAP      BSS Operation and Maintenance
                Number                                                        Application Part
ARQ             Automatic Request for Retransmission             BSSTE        Base Station Test Equipment
AUC             Authentication Centre                            BTS          Bast Transceiver Station
AUT             Authentication
                                                                 C            Conditional
BCC             Base station Colour Code                         CA           Cell Allocation
BCCH            Broadcast Control Channel                        CA-CN        Cell Allocation RF Channel Number
BCD             Binary Coded Decimal                             CA-NO        RF Channel Number of BCCH in a
BCF             Base Control Function                                         Particular Cell Allocation
BCH             Broadcast Channel                                CBCH         Cell Broadcast Channel
BER             Bit Error Rate                                   CC           Country Code
BFI             Bad Frame Indication                             CCBS         Completion of Calls to Busy
Bm              Traffic channel for full rate voice coder
                                                                 CCCH         Common control channel
BN              Bit Number
                                                                 CCCH         Group of MS in Idle Mode
BNHO            Barring all outgoing calls except those
                to Home PLMN
                                                                 CCPE         Control Channel Protocol Entity
BS              Base Station
                                                                 CELL-BAR- Cell Access Barred
BS-AG-BLK The number of blocks on each common
S-RES     control channel reserved for access
          grant messages                                         CELL-RESE RXLEV Hysteresis required for Cell
                                                                 LECT-HYST reselection
BS-BCCH-S Combination of dedicated and
DCCH-COM associated control channels on the
B         same physical channel                                  CI           Cell Identify
BSC             Base station Controller                          CM           Connection Management
BS-CC-CHA The number of basic physical channels                  CMD          Command
NS        supportingcommon control channels                      CNIP         Calling Number Identification
BSCU            Base Station Controller Unit                                  Presentation

©1997 Hewlett-Packard Company                               68                                             10001506.sam
                                                                                                              Rev. 10/97
GSM Basics, An Introduction

CNIR            Calling Number Identification                EA         External Alarms
                Restriction                                  Ec/No      Ratio of energy per modulating bit to
COM             Complete                                                the noise spectral density
CONI            Connect Number Identify                      EIR        Equipment Identify Register
CONN            Connect                                      ERR        ERRor
CONP            Connect Number Identification
                Presentation                                 FA         Full Association
CRC             Cyclic Redundancy Check                      FB         Frequency correction Burst
CRE             Call RE-establishment procedure              FACCH      Fast ACCH
CSPDN           Circuit Switched Public Data Networks        FACCH/F    Full-rate FACCH
CU              Central Unit of a MS                         FACCH/H    Half-rate FACCH
CT              Channel Tester                               FCH        Frequency Correction Channel
CUG             Closed User Group                            FDMA       Frequency Division Multiple Access
C/I             Carrier to Interference Ratio                FEC        Forward Error Correction
                                                             FER        Frame Erasure Rate
D               Downlink                                     FFS        No Further Study
DB              Dummy Burst                                  FN         Frame Number
DCF             Data Communications Function                 FN-MAX     Maximum TDMA Frame Number
DCCH            Dedicated Control Channel                    FS         Further Study
DCN             Data Communication Network
DCPE            Data Connection Physical Endpoint            GB         Guard Bits
DCS             Digital Communication System                 GMSC       Gateway Mobile Services Switching
DET             Detach                                                  Centre
DISC            DISConnect                                   GMSK       Gaussian Minimum Shift Keying
DL              Data Link (layer)                            GSA        GSM System Area
DLCI            Data Link Connection Identifier              GSM        Global Syatem for Mobile
DLD             Data Link Discriminator                                 communications

Dm              Control Channel (ISDN terminology            GSM PLMN GSM Public Land Mobile Network
                applied to mobile service)
DMR             Digital Mobile Radio                         HANDO      Handover
DP              Dialled Pulse                                HDLC       High Level Data Link Control
DRM             Discontinuous Reception Mechanisms           HLR        Home Location Register
DTAP            Direct Transfer Application Part             HMSC       Home Mobile-services Switching
DTE             Data Terminal Equipment                                 Centre

DTMF            Dual Tone Multi-Frequency                    HO-MARGI SDL Message name for Handover
                (signalling)                                 N        Margin

DRX             Discontinuous Reception                      HPLMN      Home PLMN

DTX             Discontinuous Transmission                   HPU        Hand Portable Unit
                Mechanism                                    HSN        Hop Sequence Number

©1997 Hewlett-Packard Company                           69                                           10001506.sam
                                                                                                        Rev. 10/97
GSM Basics, An Introduction

I               Information (frames)                          MAIO        Mobile Allocation Index Offset
IAM             Initial Address Message                       MAP         Mobile Application Part
ICB             Incoming Calls Barred                         MCC         Mobile Country Code
ID              Identification                                MCI         Malicious Call Identification
IDN             Integrated Digital Network                    MD          Mediation Device
IE              Signalling Information Element                MDL         (mobile) Management (entity) - Data
IF              Intermediate Frequency                                    Link (layer)

IMEI            International Mobile station                  ME          Maintenance Entity
                Equipment Identity                            MEF         Maintenance Entity Function
IMSI            International Mobile Subscriber               MIC         Mobile Interface Controller
                Identity                                      MM          Man Machine
INU             Interworking Unit                             MME         Mobile Management Entity
ISDN            Integrated Services Digital Network           MMI         Man Machine Interface
IWF             Inter Working Function                        MNC         Mobile Network Code
I/Q             In-phase and Quadrature                       MPH         (mobile) Management (entity) -
                                                                          PHysical (layer) [primitive]
K               Constraint Length of the Convolutional        MS          Mobile Station
                Code                                          MSC         Mobile-services Switching Centre
Kc              Cipher Key                                    MSCU        Mobile Station Control Unit
Ki              Key used to calculate SRES                    MS ISDN     Mobile Station ISND Number
Kl              Location Key                                  MSL         Main Signalling Link
Ks              Session Key                                   MSRN        Mobile Station Roaming Number
                                                              MS-RANGE- Mobile Station Range Maximum
LAC             Location Area Code                            MAX
LAI             Location Area Identify                        MS-RXLEV- Lower Receive Level
LAN             Local Area Network                            L

LAP-Dm          Link Access Protocol on Dm Channel            MS-TXPWR- MS Transmitted RF Power
                                                              CONF      Confirmation
L2R             Layer 2 Relay
                                                              MS-TXPWR- Maximum Allowed Transmitted RF
LCN             Local Communication Network                   MAX-CCH Power for MSs to Access the System
LE              Local Exchange                                MS-TXPWR- MS Transmitted RF Power Request.
Lm              Traffic channel with capacity lower           REQUEST   Parameter sent by the BS that
                than Bm.                                                commands the required MS RF Power
LPC             Linear Prediction Coding (Voice
                Codec)                                        MT          Message Transfer Part
LR              Location Register                             MT          Mobile Termination
                                                              MTP         Message Transfer Part
M               Mandatory                                     MUMS        Multi User Mobile Station
MA              Mobile Allocation
MACN            Mobile Allocation Channel Number              NB          Normal Burst
MAF             Mobile Additional Function                    NBIN        A parameter in the hopping sequence

©1997 Hewlett-Packard Company                            70                                             10001506.sam
                                                                                                           Rev. 10/97
GSM Basics, An Introduction

NCELL           Neighbouring (adjacent) Cell                  PSPDN        Public Switched Public Data Network
NDC             National Destination Code                     PSTN         Public Switched Telephone Network
NE              Network Element                               PTO          Public Telecommunications Operators
NEF             Network Element Function
NER             Normal Error Rates                            QA           Q (Interface) - Adapter
NF              Network Function                              QAF          Q - Adapter Function
NM              Network Management                            QOS          Quality of Service
NMC             Network Management Centre
NMSI            National Mobile Station identification        R            Value of Reduction of the MS
                number                                                     Transmitted RF Power relative to the
NMT             Nordic Mobile Telephone                                    maximum allowed output power of the
                                                                           highest power class of MS (A)
NSAP            Network Service Access Point
                                                              RA           Random Mode Request information
NT              Network Termination                                        field
N/W             Network                                       RAB          Random Access Burst
                                                              RACH         Random Access Channel
O               Optional                                      RADIO-LIN The timeout period for radio link
OACSU           Off-Air-Call-Set-Up                           K-TIMOUT failure
OD              Optional for operators to implement           RAND         RANDom Number (authentication)
                for their aim                                 RBER         Residual Bit Error Rate (BER after
O&M             Operations & Maintenance                                   errored frames removed)
OCB             Outgoing Calls Barred                         REC          RECommendation
OMC             Operations & Maintenance Centre               REL          RELease
OS              Operating System                              REQ          REQuest
OSI             Open System Interconnection                   RES          RESponse (authentication)
OSI RM          OSI Reference Model                           RF           Radio Frequency
                                                              RFC          Radio Frequency Channel
PAD             Packet Assembly/Disassembly facility          RFCH         Radio Frequency Channel
PCH             Paging Channel                                RFN          Reduced TDMA Frame Number
PD              Public Data                                   RLP          Radio Link Protocol
PCS             Personal Communications System                RNTABLE      Table of 128 integers in the hopping
PDN             Public Data Networks
                                                              RPE          Regular Pulse Excitation (Voice
PH              Physical (layer)
PI              Presentation Indicator
                                                              RR           Radio Resource
PIN             Personal Identification Number
                                                              RSE          Radio System Entity
PLMN            Public Land Mobile Network
                                                              RX           Receiver
PLMN-PER        PLMN Permitted for handover
MITTED          purposes                                      RXLEV        Received Signal Level
                                                              RXLEV-MIN The minimum received signal level at
PPE             Primitive Procedure Entity
                                                                        a MS from an adjacent cell for
PRBS            Pseudo Random Binary Sequence                           handover into that cell to be permitted
Ps              Location Probability

©1997 Hewlett-Packard Company                            71                                            10001506.sam
                                                                                                          Rev. 10/97
GSM Basics, An Introduction

RXLEV-AC The minimum received signal level at             SID        Silence Descriptor
CESS-MIN a MS for access to a cell                        SIM        Subscriber Identification Module
RXLEV-NC Received signal level of neighbouring            SLTM       Signalling Link Test Message
ELL      (adjacent) cell
                                                          SMG        Special Mobile Group
RXLEV-NC The received signal level in adjacent
ELL-[1-N] cell                                            SMS        Short Message Service Support
RXLEV-SER The received signal level in the serving        SMSCB      Short Message Service Cell Broadcast
VING CELL cell                                            SN         Subscriber Number
RXQUAL          Received Signal Quality                   SP         Signalling Point
RXQUAL-SE Received signal quality of serving cell         SRES       Signal RESponse (authentication)
                                                          SS         Supplementary Resource Support
                                                          STP        Signalling Transfer Point
                                                          S/W        Software
SABM            Set Asynchronous Balanced Mode
SACCH           Slow Associated Control Channel
                                                          TA         Terminal Adapter
SACCH/C4        Slow, SDDCCH/4 Associated, Control
                Channel                                   TAC        Type Approval Code
SACCH/C8        Slow, SDDCCH/8 Associated, Control        TACS       Total Access Communication System
                Channel                                   TAF        Terminal Adaptation Function
DACCH/T         Slow, TCH-Associated, Control             TB         Tail Bits
                                                          TC         Transaction Capabilities
SACCH/TF        Slow, TCH/F-Associated, Control
                                                          TCAP       Transaction Capabilities Application
SACCH/TH        Slow, TCH/H-Associated, Control
                                                          TCH        Traffic CHannel
                                                          TCH/F      A Full rate TCH
SAP             Service Access Points
                                                          TCH/H      A Half rate TCH
SAPI            Service Access Point Indicator
                                                          TCH/FS     A Full rate Speech TCH
SB              Synchronisation Burst
                                                          TCH/HS     A Half rate Speech TCH
SCCP            Signalling Connection Control Part
                                                          TCH/F2.4   A Full rate data TCH (<2.4kbit/s)
SCH             Synchronisation Channel
                                                          TCH/F4.8   A Full rate data TCH (4.8kbit/s)
SCN             Sub-Channel Number
                                                          TCH/F9.6   A Full rate data TCH (9.6kbit/s)
SDCCH           Stand alone Dedicated Control
                CHannel                                   TCH/H4.8   A Half rate data TCH (4.8kbit/s)
SDCCH/4         Stand alone Dedicated Control             TCI        Transceiver Control Interface
                CHannel/4                                 TDMA       Time Division Multiple Access
SDCCH/8         Stand alone Dedicated Control             TE         Terminal Equipment
                                                          TFA        Transfer Allowed
SDL             Specification Description Language
                                                          TFP        Transfer Prohibited
SE              Support Entity
                                                          TMN        Telecommunication Management
SEF             Support Entity Function                              Network
SEG             Security Experts Group                    TMSI       Temporary Mobile Subscriber Identity
SFH             Slow Frequency Hop                        TN         Timeslot Number
SI              Service Interworking                      TPS        Three Part Service

©1997 Hewlett-Packard Company                        72                                          10001506.sam
                                                                                                    Rev. 10/97
GSM Basics, An Introduction

TRX             Transceiver
TS              Time Slot
TSC             Training Sequence Code
TSDI            Transceiver Speech & Data Interface
TX              Transmitter
TXPWR           TX power level in the
                MS-TXPWR-REQUEST and
                MS-TXPWR-CONF parameters

U               Uplink
UA              Unnumbered Acknowledge
UI              Unnumbered Information (Frame)
UPD             UP to Date
VAD             Voice Activity Detection
VLR             Visitor Location Register
VPLMN           Visited PLMN

WS              Work Station

©1997 Hewlett-Packard Company                         73   10001506.sam
                                                              Rev. 10/97
GSM Basics, An Introduction

Further Reading
GSM Measurement Basics
Hewlett-Packard Symposium Paper

Selecting GSM Measurements for Your
Hewlett-Packard Symposium Paper

Repairing GSM Mobiles
Hewlett-Packard Symposium Paper

GSM Mobile Service
Hewlett-Packard Symposium Paper

GSM900 & DCS1800 Base Station Installation
& Maintenance
Hewlett-Packard Symposium Paper

Key Test Concerns in GSM Mobile Phone
Hewlett-Packard Symposium Paper

Test and measurement solutions for wireless
Hewlett-Packard Literature No. 5091-7273E

GSM Mobile Service Solutions
Hewlett-Packard Literature No. 5963-0037E

GSM Mobile Manufacturing Solutions
Hewlett-Packard Literature No. 5962-0197E

GSM 11.10 Specification &
GSM 05.05 Specification
European Telecommunications Standards

The GSM System for Mobile Communications
M. Mouly and M. B. Pautet
ISBN 2-9507190-0-7
Order direct from M. Mouly
Phone: +33 1 69 31 03 18
Fax: +33 1 69 31 03 38

©1997 Hewlett-Packard Company                 74   10001506.sam
                                                      Rev. 10/97

Shared By: