GSM NETWORK BASIC by sadefkarish

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									COMPARISON
TACS Uplink 890 - 905 MHz Downlink 935 - 950 MHz Duplex Distance 45 MHz Carrier Separation 25 kHz Number of Channels 15 Mhz / 25 kHz = 600 Channels defined in Switch 1, 2, ….599, 600 Access Method FDMA GSM 900 890 - 915 MHz 935 - 960 MHz 45 MHz 200 kHz 25 MHz / 200 kHz = 124 1, 2, ….123, 124 TDMA DCS 1800 1710 - 1785 MHz 1805 - 1880 MHz 95 MHz 200 kHz 75 MHz / 200 kHz = 374 512 , 513 ….884, 885 TDMA

TACS

GSM 900/DCS 1800

LOGICAL CHANNELS
TCH CBCH

3 Broadcast Channels 1) FCCH 2) SCH 3) BCCH

3 Common Control Channels 1) PCH 2) RACH 3) AGCH

3 Dedicated Control Channels 1) SDCCH 2) SACCH 3) FACCH

TCH = TRAFFIC CHANNEL

Full rate => Used for speech at 13 Kbits/s or sending data at 9.6 Kbits/s Half rate => Used for speech at 6.5 Kbits/s or sending data at 4.8 Kbits/s Enhanced Full rate => Used for speech at 13 Kbits/s or sending data at 9.6 Kbits/s but with almost Land line quality

BROADCAST CHANNELS
FCCH = FREQUENCY CORRECTION CHANNEL => To tell the Mobile that this is the BCCH carrier => To able the Mobile to synchronize to the frequency (Downlink only) SCH = SYNCHRONISATION CHANNEL => Used for sending BSIC (Base station Identity Code) => Give TDMA frame number to the Mobile. (Downlink only) BCCH = BROADCAST CONTROL CHANNEL => Used for sending information to the mobile like CGI (Cell Global identity), LAI (Location Area Identity), BCCH carriers of the neighboring cells, maximum output power allowed in the cell and other broadcast messages like barred cell. (Downlink only)

COMMON CONTROL CHANNELS
PCH = PAGING CHANNEL => Used for paging the Mobile. (Downlink only) Reason could be an incoming call or an incoming Short Message.

RACH = RANDOM ACCESS CHANNEL => Used for responding to the paging (terminating), Location updating or to make call access (originating) by asking for a signaling channel. (Uplink only) AGCH = ACCESS GRANT CHANNEL => Used to allocate SDCCH to the mobile. (Downlink only)

DEDICATED CONTROL CHANNELS
SDCCH = STAND ALONE DEDICATED CONTROL CHANNEL => Used for allocating voice channel (TCH) to the mobile (call setup) and Location updating. => Send Short Text message to Idle Mobile (Uplink & Downlink)
SACCH = SLOW ASSOCIATED CONTROL CHANNEL => Used for sending information to the mobile like CGI (Cell Global identity), LAI (Location Area Identity), BCCH of all the neighbors and TA (Timing Advance) => Send Short Text message to Busy Mobile (Downlink => Used for sending signal strength & bit error rate measurement of the serving cell and signal strength of the BCCHs of the neighboring cells. (Uplink) FACCH = FAST ASSOCIATED CONTROL CHANNEL => Used for handover. (Uplink & Downlink)

CBCH = CELL BROADCAST CHANNEL
=> Used for sending short messages to all the mobiles within a geographic area. Typical example is Traffic congestion in a major road or a major accident in an area. Up to 93 characters can be sent. => If the mobile is in the Idle mode then the short message will be send through the CBCH. If the mobile is Busy, it will not be sent.

NOT TO BE CONFUSED WITH SMS !!!!!!!! (SHORT MESSAGE SERVICE)
=> SMS messages are short TEXT messages up to 160 characters in length that you can send or receive. The messages are not sent straight to the other mobile but is sent to message centre operated by the Network provider. => If the mobile was switched off or is at outside of the coverage area, the message is stored in the Message Service Center. The message will be offered to the subscriber when the mobile is switched on again or has reentered the coverage area again. => If the mobile is in the Idle mode the short message will be send through the SDCCH. If the mobile is Busy the short message will send through the SACCH.

1 TDMA FRAME 200 kHz 4.615 ms
0.577 ms

TS 0

TS 1

TS 2

TS 3

TS 4

TS 5

TS 6

TS 7

FCCH, SCH, BCCH PCH, RACH, AGCH

SDCCH, SACCH CBCH

TCH, SACCH FACCH

TCH, SACCH FACCH

TCH, SACCH FACCH

TCH, SACCH FACCH

TCH, SACCH FACCH

TCH, SACCH FACCH

4.615 ms
TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

4.615 ms
TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

F D T T T T T T S D T T T T T T
F = FCCH S = SCH B = BCCH C = PCH or AGCH D = SDCCH A = SACCH T = TCH

R D T T T T T T R D T T T T T T
R = RACH A = SACCH T = TCH

B D B D

T T T T T T T T T T T T

R D T T T T T T R D T T T T T T
R R R R R R R R R R R R
. . .

B B C C C C F S C C C C
. . .

D D D D D D D D D D D D
. . .

T T T T T T T T A T T T
. . .

T T T T T T T T 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 T T T
. . .

T T T T T T T T A T T T
. . .

T T T T T T T T T T T T
. . .

D D D D D D D D D D D D
. . .

T T T T T T T T A T T T
. . .

T T T T T T T T I T T T
. . .

T T T T T T T T A T T T
. . .

T T T T T T T T I T T T
. . .

T T T T T T T T A T T T
. . .

T T T T T T T T I T T T
. . .

1 Frame (Downlink - BTS transmit) 1 carrier = 200 kHz

1 Frame (Uplink - Mobile transmit) 1 carrier = 200 kHz

Downlink Uplink

….

0 1 2 3 4 5 6 7

0 1

2 3 4 5

6 7 0 1 2 3 4 5 6 7

…...

…..

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 3 TIMESLOT 3 * 0.577ms = 1.73ms

…..

3 TIMESLOT 3 * 0.577ms = 1.73ms

3 TIMESLOT 3 * 0.577ms = 1.73ms

TCH UP-DOWNLINK OFFSET

This means that the mobile does not transmit and receive at the same time. Also note that : in TS 0 : All the Logical Channels will repeat itself after 51 frames in TS 1 : All the Logical Channels will repeat itself after 102 frames in TS 2 to 7 : All the Logical Channels will repeat itself after 26 frames

MOBILE STATIONS ISDN NUMBER (MSISDN) => Is the mobile number used in a GSM PLMN (Public Land Mobile Network) MSISDN = Country Code + National Destination Code + Subscriber number e.x. 63 + 0918 + 8889999 Maximum length is 15 digits.
INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI) => Is the subscriber number used over radio path for all signaling in the GSM PLMN. This number is stored in SIM (Subscriber Identity Module), HLR (Home Location Register, and VLR (Visitor Location Register). IMSI = MCC + MNC + MSIN = Mobile Country Code + Mobile Network Code + Mobile Identification Number [ 3 digit ] [ 2 digit ] [ 11 digit ] e.x. 502 + 19 + 2345451 TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI) => Is used for the subscriber's confidentiality. Since the TMSI has only local significance (within MSC/VLR) the structure of the TMSI can be chosen by the Vendor. But the size must be 1/2 of the size of IMSI. Each time a mobile request for location updating or call setup, MSC/VLR allocates to the IMSI a new TMSI, so the TMSI is used on the signaling path, protecting the IMSI identity. Plus since the TMSI is half the size of IMSI, we can page twice the amount compared to IMSI.

LOCATION AREA IDENTITY (LAI) => Is used to uniquely identify each location area in the GSM PLMN. When the system receives an incoming call it knows in which location area it should page the mobile and does not page the entire network. LAI = MCC + MNC + LAC Mobile Country Code + Mobile Network Code + Location Area Code [ 3 digit ] [ 2 digit ] [ 1 to 65 536 ] e.x. = 502 + 20 + 60001 CELL GLOBAL IDENTITY (CGI) => Is used for cell identification within the GSM network. LAI = MCC + MNC + LAC + CI Mobile Country Code + Mobile Network Code + Location Area Code + Cell Identity [ 3 digit ] [ 2 digit ] [ 1 to 65 536 ] [ 1 to 65 536 ] e.x. = 502 + 20 + 60001 + 50001

BASE STATION IDENTITY CODE (BSIC) => Is used to distinguish co channel Frequency used in the neighboring cell. BSIC = NCC + BCC Network Color Code + Base Station Color Code [ 1 to 7 ] [ 1 to 7 ]

SUBSCRIBER IDENTITY MODULE (SIM)
SIM is used to provide storage on subscriber related information as following : • IMSI (International Mobile Subscriber Identity). • Temporary network data like TMSI, LAI, Location update status. • Subscriber Authentication Key (Ki) and Ciphering Key (Kc) which are used for security purposes. • BCCH information : List of carrier frequencies to be used for cell selection. • Forbidden PLMN. • Language preference. • PIN number (Personal Identification Number) and PIN error counter. • PUK number (Personal Unlock Key) and PUK error counter.

PIN management The PIN number consist of 4 to 8 digit and it is loaded by the service activator an subscription time. Afterwards the PIN number can be changed as many times an user wishes including the length of the PIN number. The user can disable the PIN function but again can be inhibited at subscription time by a authorized person. If an incorrect PIN is entered, an indication is given to the user. After 3 consecutive entries the SIM is blocked, even if if the SIM is removed or the mobile is switch off and on. If the SIM card is blocked the user cannot access the network. The unblocking of the SIM card can only be done by keying in the PUK (Personal Unlock Key). PUK is 8 digit and is given to the user at subscription time. If an incorrect PUK is entered more than 10 times then the PUK will not work anymore and the SIM card will continue to be blocked until taken to the mobile vendor service center.
Two physical types of SIM are specified : ID - 1 SIM - Looks like a Credit card Plug in SIM - Look like a small chip is installed semi permanent in the mobile equipment.

When the mobile is in idle mode it must always be camped to a BCCH carrier. Why ???
There are 3 reasons: 1) For the PLMN to know in which location area the mobile is so that it can page the mobile when an incoming call or Short Text Message is received. 2) The Mobile can initiate a call by accessing the network on Random Access Channel (RACH) of the cell which it camped on. 3) To receive system information from the PLMN like Traffic congestion and major Accidents.

AN IDLE MOBILE DOES 4 TASKS:
• PLMN SELECTION • CELL SELECTION • CELL RESELECTION
• LOCATION UPDATING

PLMN SELECTION
When the mobile is switched on it will select the registered PLMN in the mobile if there exist one. If there is no registered PLMN or the registered PLMN is not available (no coverage) then the mobile will try to select another PLMN either automatically or manually depending on it mode.

AUTOMATIC MODE The automatic mode uses a list of PLMNS in an order of priority. The priority will be : 1)The last network on which you were registered. 2) Home PLMN. 3) Each PLMN stored in the SIM card in priority order. 4) Other PLMN with signal level above -85 dBm in random order. 5) All other PLMN in decreasing signal strength.

MANUAL MODE In the manual mode the mobile will try to connect to the Home PLMN first. If it is unsuccessful then it will provide a list of available PLMN and ask the user to choose one. If the second chosen PLMN is not successful then the mobile will make an indication to the user to select another PLMN. Until the users selects another network a message “No access” will be displayed. If there is no GSM or DCS coverage at all then a message “No Network” will be displayed.

CELL SELECTION
Once the mobile is switched on and the registered home PLMN was selected (e.x. SMART), it will next search for a BCCH frequency list, stored in its memory or in its SIM card. The list can have up to 32 BCCH frequencies for the mobile to scan. This reduces the time of cell selection, compared to scanning the whole frequency band. If this feature is turned off at the switch then the mobile has to scan the entire frequency band for the strongest BCCH carrier. The BCCH frequency list is called BA (BCCH Allocation) list and there are 2 types, Active and Idle. Idle is a list of BCCH used for scanning when the mobile is in an idle mode and Active is a list of BCCH used during mobile busy mode. Why 2 List ??? When the mobile is in idle mode it may want to scan a longer list of BCCH and tune to the strongest whereas when in Active mode the list of BCCH should be shorter (correspond to defined neighbors) so that the mobile will scan the short list and get a more accurate signal strength measurements to achieve better handover performance. It is also to reduce the time spend by the mobile to decode the BSIC. Recommendation : ACTIVE MODE LIST SHOULD NOT BE MORE THAN 15 BCCH FREQUENCIES.
If there is no BA list stored in the Mobile or SIM card then the mobile will scan all the 124 GSM channels and 374 DCS channel and arrange the frequencies in a DESCENDING order of signal strength. It will take the mobile 3 to 5 seconds to scan the whole band. After which it will tune to the strongest frequency. The mobile will check if this is a BCCH carrier by looking out for the frequency correction burst send by the FCCH (Frequency Correction Channel). If it is the BCCH carrier than mobile tunes to this carrier to read the SCH (Synchronization Channel) for the BSIC parameter. Next it will read the BCCH for system information like CGI (Cell Global identity), LAI (Location Area Identity), BCCH carriers of the neighboring cells (BA List), maximum output power allowed in the cell and other broadcast messages like barred cell. [Continues …]

CELL SELECTION
Next the mobile will compare if the selected cell belongs to a forbidden PLMN stored in its SIM card. It will look at the 2 digit Network Mobile Code transmitted by the BCCH on the LAI (Location Area Identity). If those 2 digits ware registered as forbidden in the SIM card then the mobile will not select this cell. The mobile then will tune to the second strongest BCCH carrier and subsequently does the same process over and over again until it finds the right cell. Once it finds the right cell it will start using the BA (BCCH allocation) list transmitted by the BCCH carrier for cell reselection, will be discussed later.

Lets say the chosen PLMN is correct, able to read the FCH, SCH and BCCH and the chosen Cell is accessible (no cell barring), DOES THIS MEAN THAT THE MOBILE NOW CAN CAMP ON THIS SITE ???

NO !!!!!!!!!!!

THERE IS ONE LAST CRITERIA CALLED C1 CELL SELECTION CRITERION WHICH MUST BE CALCULATED BY THE MOBILE AND IF THE C1 VALUE IS GREATER THAN 0 THEN THE MOBILE CAN CAMP ON THIS CELL OR ELSE THE NEXT CELL WITH C1 > 0 WILL BE SELECTED

C1-CELL SELECTION CRITERION
C1 = A - Max (B,0) and C1 > 0 for the mobile to camp on this BTS where: A = RxLev - RxLevAccMin B = MsTxPwrMaxCCH - P
RxLev = Signal strength received by the mobile

RxLevAccMin = Minimum Signal level to be received by the mobile from BTS (BCCH) before it could access the BTS
MsTxPwrMaxCCH = Maximum Transmit Power allowed to access the BTS (using RACH) P = Mobile Class power

C1 = (RxLev - RxLevAccMin) - Max(MsTxPwrMaxCCH - P, 0) Ex : C1 = ( -80 - (-100) ) - Max (33 - 33, 0) = -80 + 100 - Max (0, 0) = 20 => C1 > 0 so mobile will camp on this site

ShortCut : If RxLevel > RxLevAccMin then Mobile can camp on this site

ACCMIN (Ericsson) RxLevAccessMin (Nokia) SSACC (TACS) (Uplink)

Minimum Signal level that must be received by the mobile from BTS (BCCH) before it could access the BTS

= -102 (GSM900)

= -100 (DCS1800)

General rule : The signal received by the mobile should be 2 dB higher than the mobile sensitivity

What is the accurate way of setting the RxLevAccessMin parameter ?
RxLevAccessMin = Mobile Sensitivity + Body loss + Multipath loss + Interference Margin
Mobile Sensitivity = -104 for GSM900 and -102 for DCS1800 Body loss = 3 dB recommended by ETSI and 5 dB recommended by Ericsson for GSM 900 = 3 dB recommended by ETSI and 3 dB recommended by Ericsson for DCS 1800

Multipath loss = Signal loss from base station due to reflection by buildings, etc before reaching mobile. Normally the Multipath loss is around 3 dB but can be overcome by Antenna Diversity which has gain around 3 dB too. (Space diversity = 3 dB, 90 degrees polarized diversity = 3 dB, 45 degrees slant polarized diversity = 4.5 dB)
Interference Margin = Margin allocated to overcome C/I and C/N, the recommended value is 2 dB RxLevAccessMin = Mobile Sensitivity + Body loss + Multipath loss + Interference Margin RxLevAccessMin = -104 + 3 + 0 + 2 (for GSM 900 with ETSI standard) = - 99 dBm RxLevAccessMin = -104 + 5 + 0 + 2 (for GSM 900 with Ericsson standard) = - 97 dBm RxLevAccessMin = -104 + 3 + 0 + 2 (for DCS 1800) = - 99 dBm

Class Class Class Class Class

1 2 3 4 5

TACS 10 Watt (40dBm) 4 Watt (36dBm) 1 Watt (30dBm) 0.6 Watt (28dBm) *

GSM 900 20 Watt (43dBm) 8 Watt (39dBm) 5 Watt (37dBm) 2 Watt (33dBm) 0.8 Watt (29dBm)

DCS 1800 1 Watt (30dBm) 0.25 Watt (24dBm) 4 Watt (36dBm) * *

Mobile Sensitivity BTS Sensitivity

TACS -113 dBm -116 dBm

GSM 900 -104 dBm -107 dBm

DCS 1800 -102 dBm -106 dBm

CCHPWR (Ericsson) MsTxPwrMaxCCH(Nokia) PLC (TACS)

Maximum Transmit Power allowed to access the BTS (using RACH) - Mobile is Idle

= 33 dBm (GSM900)

= 30 dBm (DCS1800)

= 0 (28 dBm) (TACS)

MsTxPwr (Ericsson) MsTxPwrMax(Nokia)

Maximum Transmit Power allowed to use in a BTS during busy status (Using TCH) - Mobile is Busy

PLVM (TACS) = 33 dBm (GSM900) = 30 dBm (DCS1800) = 0 (28 dBm) (TACS)

MsTxPwrMin(Nokia)

Minimum Transmit Power allowed to use in a BTS during busy status (Using TCH) - Mobile is Busy

= 13 dBm (GSM900)

= 13 dBm (DCS1800)

Switch on the Mobile For 2 to 3 seconds the Mobile will scan all the 124 channels in GSM900 and 374 channels in DCS1800

Mobile will compare the signal strength of the 124 channels and tune to the strongest

Tune to the second strongest channel

No

Mobile will check if it is a BCCH carrier ?
Yes The mobile will synchronize to this carrier and read the BCCH info like LAI, CGI

No

Does the BCCH belong to the wanted PLMN, E.x: Smart, Globe, Islacom ? Yes

Yes

Is the Cell Barred from accessing ? No

No

Is C1 >0 ? Yes Camp on this site !!!

CELL RESELECTION
1) Perform Cell reselection measurement first
After the cell has been successfully selected, the mobile now will start reselection tasks. It will continuously make measurements on its neighboring cells (as indicated by the BA list) to initiate cell reselection if necessary. At least 5 measurement sample per neighboring cell is needed. A running average of the received signal level will be maintained for each carrier in the BA list.

All system information messages sent on the current BCCH on the serving cell must be read by the mobile every 30 seconds to monitor changes in cell parameters (ex: MsTxPwrMax). The mobile also has to read the 6 strongest BCCH every 5 minutes to receive its cell parameters (ex: MsTxPwrMax). The 6 strongest can be seen from the BA list which has the updated measurement of the 32 BCCH carrier. The neighboring list for the best 6 neighbors is updated every 60 seconds, which means the mobile has to measure each neighbor by 10 seconds. The mobile also has to read the BSIC of the 6 strongest BCCH every 30 seconds to confirm that it is still monitoring the same cells. If a new BSIC is detected, then the BCCH of this BSIC will be read to receive the cell parameters.

BSIC Serving cell Six neighbors Every 30 secs

BCCH Every 30 secs Every 5 minute
[Continues …]

CELL RESELECTION
2) Cell reselection Criteria :
The mobile will reselect and camp on another cell if any of the following criteria is satisfied : a) The serving cell is barred. b) C1 value in the current cell is below 0 for 5 seconds which indicates that the path loss is high and the mobile needs to change cell. c) The Mobile has unsuccessfully tried to access the network as defined by the MAXRET (Ericsson) parameter or MaxNumberRetransmissions (Nokia). MAXRET is the maximum number of retransmission a mobile can do when it is accessing the system It is defined per cell.

Assuming that one of the criteria above was satisfied then the mobile will select a cell with a better C1. However if the cell belongs to a different location area then the C1 for that cell has to exceed a reselection hysterisis parameter called CRH (Ericsson) or CellReselectHyseteris (Nokia) for the reselection to happen !!
[Continues …]

CELL RESELECTION
If the mobile is moving in a border area between location areas, it might repeatedly change between location areas. Each change requires location updating and cause heavy signaling load and risk paging message being lost. To prevent this, a cell reselect hysteresis parameter CRH is used. The cell in a different location area will only be selected if the C1 of that cell is higher than the C1 of the current serving cell by the value of the Reselect Hysteresis.
Since the Value of CRH maybe different for each cell, the CRH used for comparison will be the one broadcast by the serving cell. If the value is set very low then the mobile will Ping-Pong between location areas which will increase signaling load. If the value is set very high the mobile may camp in the wrong cell too long.

Recommendations : Set the value to 6
LA 1
C1 = 1 CRH = 4

LA 2
C1 = 6

Serving cell

Target cell

BASIC COMPARISON
MOBILE IDLE
When the mobile is idle, it listens to the best cell to camp. Mobile decides to choose the cell by itself without the help of BSC. This is done by comparing signal strength of each BCCH frequency and if found the strongest then it will camp once the C1 > 0 (Cell Selection Criterion). If after camping to this cell, it finds that a neighboring cell is much better then it will change to that neighboring cell. If the new cell is in same location area the mobile does not have to inform the BSC about its new cell but if the new cell is from a different different location area then the mobile will perform a location updating to inform the BSC.

MOBILE BUSY
A mobile is considered busy when there is a call going on (speech, data or fax) or it is in the middle of a call setup. At this stage the mobile cannot decide by itself whether it is necessary for the mobile to handover to a better cell. Only BSC can determine if a mobile has to change to another cell other than the serving cell. BSC makes the decision based on measurement reports sent by both Mobile and BTS. This decision making is called locating. In a busy state, mobiles can receive Short Text Message (SMS) but cannot receive Cell Broadcast Messages.


								
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