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Review of lecture 3
EEC-682/782                                                Physical Layer
Computer Networks I                                                Public switched telephone network
Mobile telephone system
Lecture 4                                          Cable television
Course web page
w.zhao1@csuohio.edu
(Lecture nodes are based on materials supplied by
Textbook online:
Dr. Louise Moser at UCSB and Prentice-Hall)
http://proquest.safaribooksonline.com/0130661023
No class next week

EEC-682: Computer Networks I
Spring Semester 2005              - Wenbing Zhao                        2

Review of Lecture 3                                                            Fourier Analysis

Theoretical basis for data communication                                       Info is transmitted by varying voltage or current
Let f(t) be value of voltage or current at time t, any well-
Guided transmission media                                                      behaved periodic function g(t) with period T can be represented
as Fourier series
Wireless transmission
Communication satellites
where f=1/T, the fundamental frequency, an and bn are sine and
cosine amplitudes of nth harmonics (terms)
The amplitudes and constant are given by

EEC-682: Computer Networks I                                             EEC-682: Computer Networks I
Spring Semester 2005                    - Wenbing Zhao                     3   Spring Semester 2005              - Wenbing Zhao                        4
Bandwidth-Limited Signals
Maximum Data Rates of a Channel
A binary signal and its root-mean-square Fourier amplitudes.
(b) – (c) Successive approximations to the original signal.
Theorem (Nyquist 1924) for noiseless channels
If an arbitrary signal is run through a low-pass filter of
bandwidth H, then the filtered signal can be completely
reconstructed by making on 2H samples per second
Max data rate = 2H log2V bits/sec, where signal consists of
V discrete lines
Ex: H = 3000 Hz, V = 2 (binary)
max data rate = 2*3000*log22 = 6000 bits/sec
Ex: H = 3000 Hz, V = 64
max data rate = 2*3000*log264 = 36,000 bits/sec

EEC-682: Computer Networks I                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao               5   Spring Semester 2005               - Wenbing Zhao                        6

Maximum Data Rates of a Channel                                       Guided Transmission Data

Theorem (Shannon 1948) noisy channels                                      Magnetic Media
Amount of thermal noise = signal to noise ratio                       Twisted Pair
= signal power / noise power = S/N
Decibel (dB): 10 log10S/N
Coaxial Cable
Max data rate = H log2(1+S/N) bits/sec                                Fiber Optics
Ex: H = 3000 Hz, S/N = 30dB = 1000
max data rate = 3000*log2(1+1000) = 30,000
upper bound is hard to reach, 9600 bits/sec is good

EEC-682: Computer Networks I                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao               7   Spring Semester 2005               - Wenbing Zhao                        8
Wireless Transmission                                                                       The Electromagnetic Spectrum

The electromagnetic spectrum
Microwave transmission
Infrared and milimeter waves
Lightwave transmission

Radio, microwave, infrared, visible light
Parts of electromagnetic spectrum used to transmit info by modulating
amplitude, frequency, phase
UV, X-ray, gamma rays are hard to produce and modulate, do not
propagate well, are harmful to living things

EEC-682: Computer Networks I                                                                 EEC-682: Computer Networks I
Spring Semester 2005                   - Wenbing Zhao                                   9    Spring Semester 2005                   - Wenbing Zhao                              10

Communication Satellites                                                                     Communication Satellites
Contains one or more transponders, each
Listens to some part of spectrum
Amplifies incoming signal
Rebroadcasts at another frequency to avoid interference with
incoming signal
Downward beams can be broad, or
Narrow, covering an area only hundreds of km in diameter, this mode of
operation is known as a bent pipe
Where to place satellites
Van Allen belts – layers of highly charged particles trapped by the
earth’s magnetic field
Three regions in which satellites can be placed safely
Geostationary Satellites (GEO)
Medium-earth orbit satellites (MEO)                                                    Communication satellites and some of their properties,
Low-earth orbit satellites (LEO)
including altitude above the earth, round-trip delay time and
number of satellites needed for global coverage.

EEC-682: Computer Networks I                                                                 EEC-682: Computer Networks I
Spring Semester 2005                   - Wenbing Zhao                                   11   Spring Semester 2005                   - Wenbing Zhao                              12
Public Switched Telephone System                                  Structure of the Telephone System
Structure of the telephone system
The politics of telephones
The local loop: modems, ADSL and wireless
Trunks and multiplexing
Switching

(a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
EEC-682: Computer Networks I                                            EEC-682: Computer Networks I
Spring Semester 2005          - Wenbing Zhao                 13   Spring Semester 2005                - Wenbing Zhao                   14

Major Components of the Telephone System
Structure of the Telephone System

A typical circuit route for a medium-distance call.                 Local loops
Analog twisted pairs going to houses and businesses
Trunks
Digital fiber optics connecting the switching offices
Switching offices
Where calls are moved from one trunk to another

EEC-682: Computer Networks I                                            EEC-682: Computer Networks I
Spring Semester 2005          - Wenbing Zhao                 15   Spring Semester 2005                - Wenbing Zhao                   16
The Local Loop: Modems, ADSL, and Wireless
The Politics of Telephones
LATA – Local Access and Transport Areas       The use of both analog and digital transmissions for a computer to
LEC – Local Exchange Carrier
IXC – IntereXchange Carrier                     computer call. Conversion is done by the modems and codecs.
POP – Point of Presence

The relationship of LATAs, LECs, and IXCs. All the circles
are LEC switching offices. Each hexagon belongs to the                              Transmission lines suffer from three major problems
IXC whose number is on it.                                          Attenuation, delay distortion, and noise
EEC-682: Computer Networks I                                                           EEC-682: Computer Networks I
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Modems                                                                                    Modems
Modem – device used between                                                             Constellation Diagrams – legal combinations of amplitude and phase
digital computer and analog
telephone system. It converts
digital bit stream into
modulated analog signal and
vice verse
Codec – inverse of a modem.
It is a device that converts a
continuous analog signal into a
digital bit stream
Baud – number of samples per
second. During each baud, one
symbol is sent. One symbol
can carry multiple bits
(a) A binary signal
(b) Amplitude modulation                              (a) QPSK: Quadrature Phase Shift Keying
(c) Frequency modulation                              (b) QAM-16: Quadrture Amplitude Modulation
(d) Phase modulation
(c) QAM-64.
EEC-682: Computer Networks I                                                           EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                                  19   Spring Semester 2005              - Wenbing Zhao                        20
Modems                                                                     Digital Subscriber Lines

Bandwidth versus distanced over category 3 UTP for DSL.

Higher connection speed => smaller serviceable area

(a)                                           (b)
(a) V.32 for 9600 bps.
(b) V32 bis for 14,400 bps.

EEC-682: Computer Networks I                                         EEC-682: Computer Networks I
Spring Semester 2005                   - Wenbing Zhao                 21   Spring Semester 2005             - Wenbing Zhao                           22

Digital Subscriber Lines                                                   Digital Subscriber Lines

Operation of ADSL using discrete multitone modulation.              A typical ADSL equipment configuration.
NID – Network
Interface
Device
Splitter – an analog
filter that
separates the
voice band from
data

EEC-682: Computer Networks I                                         EEC-682: Computer Networks I
Spring Semester 2005                   - Wenbing Zhao                 23   Spring Semester 2005             - Wenbing Zhao                           24
Wireless Local Loops                                                                Trunks and Multiplexing
Architecture of an LMDS system.
Idea: multiplex many conversations over single
physical channel with high bandwidth
IEEE 802.16 – wireless MAN              FDM – Frequency Division Multiplexing
Frequency spectrum divided into logical channel
Each user has exclusive use of own frequency band
TDM – Time Division Multiplexing
Time divided into slots each user has time slot
Users take turns in round robin fashion

EEC-682: Computer Networks I                                                       EEC-682: Computer Networks I
Spring Semester 2005                - Wenbing Zhao                             25   Spring Semester 2005               - Wenbing Zhao              26

Frequency Division Multiplexing                                                      Wavelength Division Multiplexing

(a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel.
EEC-682: Computer Networks I                                                       EEC-682: Computer Networks I
Spring Semester 2005                - Wenbing Zhao                             27   Spring Semester 2005               - Wenbing Zhao              28
Time Division Multiplexing
T1 Carrier (1.544 Mbps)
Time Division Multiplexing
24 voice channels multiplexed together, each channel inserts 8 bits (7 data,
1 control) into output stream in round-robin fashion
Encoding systems for digitizing analog signals – use
Why 125 microsecond?
Telephone channel bandwidth - 4000 Hz                                                 statistical techniques to reduce number of
Nyquist thm – codecs make 8000 samples/sec, enough to capture all info                bits/channel (signal changes slowly compared to
This technique is called PCM (Pulse Code Modulation)
sampling frequency)
Differential pulse code modulation – output difference
between current value and previous value rather than
digitized amplitude
Predictive encoding – Extrapolate previous few values to
predict next value. Encode difference between actual signal
and predicted one
Delta modulation – Requires each sampled value to differ
from its predecessor by +/-1

EEC-682: Computer Networks I                                                            EEC-682: Computer Networks I
Spring Semester 2005                     - Wenbing Zhao                                  29   Spring Semester 2005               - Wenbing Zhao                        30

Time Division Multiplexing                                                                    Time Division Multiplexing

Delta modulation.                                                       Multiplexing T1 streams into higher carriers.

EEC-682: Computer Networks I                                                            EEC-682: Computer Networks I
Spring Semester 2005                     - Wenbing Zhao                                  31   Spring Semester 2005               - Wenbing Zhao                        32
Time Division Multiplexing
Time Division Multiplexing
SONET – Synchronous Optical NETwork
Standard TDM system at physical layer                                       SONET and SDH (Synchronous Digital Hierarchy) multiplex rates
(started 1984, standardized 1989)
Two back-to-back SONET frames.

EEC-682: Computer Networks I                                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                           33   Spring Semester 2005                  - Wenbing Zhao                34

Switching                                                                          Circuit Switching

Two types of switching
Circuit switching – physical path set up from source to
destination before any data transmitted, e.g., phone system
Adv: no congestion problem, only delay is propagation time
Disadv: unused bandwidth on allocated circuit is wasted
Packet switching – store-and-forward, one hop at a time,
uses pipelining, each packet has limited size
Adv: low overhead – no setup required, high utilization
Disadv: packets may be received out of order, packets may be
lost due to buffer overflow

(a) Circuit switching.
(b) Packet switching.
EEC-682: Computer Networks I                                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                           35   Spring Semester 2005                  - Wenbing Zhao                36
Message Switching
Circuit Switching vs. Packet Switching

(a) Circuit switching (b) Message switching (c) Packet switching
EEC-682: Computer Networks I                                           EEC-682: Computer Networks I
Spring Semester 2005                  - Wenbing Zhao               37   Spring Semester 2005                 - Wenbing Zhao                             38

Advanced Mobile Phone System (AMPS)
The Mobile Telephone System
Transmitter/receiver on different frequency levels
First-Generation Mobile Phones                                         Region divided into cells (10-20km diameter), each using some set
of frequencies
Analog Voice
Frequencies reused in non-adjacent cells
Second-Generation Mobile Phones                                        Small cells - Less power, small cheap devices
Digital Voice                                                      Reuse – multiple simultaneous calls on each frequency in widely
separated cells
Third-Generation Mobile Phones
Digital Voice and Data

(a) Frequencies are
not reused in
(b) To add more
users, smaller cells
can be used.
EEC-682: Computer Networks I                                           EEC-682: Computer Networks I
Spring Semester 2005                  - Wenbing Zhao               39   Spring Semester 2005                 - Wenbing Zhao                             40
Advanced Mobile Phone System
Advanced Mobile Phone System
Base station – at the center of each cell, to which all phones in
the cell transmit                                                                    Uses FDM
Consists of computer and transmitter/receiver connected to an
antenna                                                                               832 transmission channels, 30kHz wide
Connected to Mobile Telephone Switching Office (MTSO), or                             832 receive channels, 30kHz wide
Mobile Switching Center (MSC)
Handoff – the process of breaking up connection with an old cell                     The 832 channels are divided into four categories
and reestablish connection with a new cell                                                Control (base to mobile) to manage the system
At any time, mobile phone is logically in one cell and under control of                   21 channels
that cell’s base station
Paging (base to mobile) to alert users to calls for them
When mobile leaves a cell, its base station notices the phone signal
fading and asks surrounding base stations how much power they’re                      Access (bidirectional) for call setup and channel assignment
getting from the phone                                                                Data (bidirectional) for voice, fax, or data
Base station then transfers ownership to the cell getting the
strongest signal
Phone is then informed of its new base station and switches to a
new channel
EEC-682: Computer Networks I                                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                              41   Spring Semester 2005               - Wenbing Zhao                         42

Advanced Mobile Phone System                                                          Advanced Mobile Phone System

Call management                                                                       Making a call
Each mobile phone has 32-bit serial number, 10-digit phone                            Mobile user switches on phone, enters number to be called
number, in its PROM                                                                   on keypad, hits SEND button
When switched on, mobile scans list of 21 control channels                            Phone sends number to be called and its own id on access
to find most powerful signal                                                          channel
Mobile then broadcasts its serial number and phone number                             When base station gets request, it informs MTSO
When base station hears this, it tells MTSO, which records                            MTSO looks for idle channel for call and sends channel
the new customer and informs the home MTSO of new                                     number back on control channel
customer’s location                                                                   Mobile switches to selected voice channel and waits for
Mobile reregisters every 15 minutes                                                   called party to pick up phone

EEC-682: Computer Networks I                                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                              43   Spring Semester 2005               - Wenbing Zhao                         44
Digital Advanced Mobile Phone System (D-AMPS)
Advanced Mobile Phone System
2nd generation of AMPS system
Can co-exist with AMPS
Receiving a call
Void signal digitized and compression at the phone
All idle phones continuously listen to paging channel to
detect messages for them                                                             Multiple users per channel, using TDM
When a call is placed to a mobile phone, a packet is sent to
mobile’s MTSO, which sends a packet to mobile’s base
station
Base station broadcasts on paging channel
Mobile responds on control channel
Base station tells mobile phone which channel to switch to
Mobile switches to channel and starts ringing

(a) A D-AMPS channel with three users.       (b) A D-AMPS channel with six users.

EEC-682: Computer Networks I                                                          EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                            45   Spring Semester 2005                   - Wenbing Zhao                                   46

Global System for Mobile Communications (GSM)
Digital Advanced Mobile Phone System
D-AMPS widely used in US. Everywhere else uses GSM
To 1st approximation, GSM is similar to D-AMPS
MAHO – Mobile Assisted Handoff
Both use FDM, transmitting on one frequency, receiving on a higher
In D-AMPS, 1/3 of the time a mobile is neither sending nor                         frequency; a single frequency pair is split by TDM into time slots shared
receiving. It uses the idle slots to measure the line quality.                     by multiple mobiles
When it discovers that the signal is waning, it complains to MTSO              Diff: GSM channels are much wider than AMPS channels
MTSO then break the connection                                                     200kHz vs. 30kHz
In the meantime, the mobile can try to tune to a stronger signal               GSM uses 124 frequency channels, each of which uses an eight-slot
from another base station                                                      TDM system
Still takes ~300msec

EEC-682: Computer Networks I                                                          EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                            47   Spring Semester 2005                   - Wenbing Zhao                                   48
GSM                                                                                    CDMA – Code Division Multiple Access
A portion of the GSM framing structure.                                        Works completely differently from TDM and FDM
Each station transmits over entire frequency spectrum all
the time
Each station is assigned a unique m-bit code, called a chip
sequence
To transmit a 1, it sends its chip sequence
To transmit a 0, it sends the one’s complement of its chip
sequence
Assumes multiple signals add linearly – colliding frames are not
garbled!
Chip sequences for different stations are othogonal

EEC-682: Computer Networks I                                                                  EEC-682: Computer Networks I
Spring Semester 2005            - Wenbing Zhao                                    49   Spring Semester 2005                   - Wenbing Zhao                               50

Third-Generation Mobile Phones
CDMA – Code Division Multiple Access
IMT-2000 – ITU issued a blue print for 3rd Generation mobile
(a) Binary chip sequences for four                  telcomm in 1992, hoping to go service in 2000
stations                                                IMT - International Mobile Telecommunications
(b) Bipolar chip sequences (+1 for 1,
Basic services an IMT-2000 network should provide
and -1 for 0)
High-quality voice transmission
(c) Six examples of transmissions
Messaging (replace e-mail, fax, SMS, chat, etc.)
(d) Recovery of station C’s signal
Multimedia (music, videos, films, TV, etc.)
Internet access (web surfing, w/multimedia.)
Technology based on CDMA
W-CDMA from Ericsson and CDMA2000 from Qualcomm
Did not happen in year 2000
Let S be m-chip vector for station S
All chip sequences are orthogonal                                 2.5G services
Normalized inner product of any two                               EDGE (Enhanced Data rates for GSM Evolution) – GSM with more bits per
distinct chip seq, S and T, is 0, i.e., S*T=0                     baud
S*S = 1                                                               GPRS (General Packet Radio Service) – overlay packet network on top of D-
AMPS or GSM

EEC-682: Computer Networks I                                                                  EEC-682: Computer Networks I
Spring Semester 2005            - Wenbing Zhao                                    51   Spring Semester 2005                   - Wenbing Zhao                               52
Cable Television                                           Community Antenna Television

An early cable television system.
Community Antenna Television
Internet over Cable
Spectrum Allocation
Cable Modems

EEC-682: Computer Networks I                                        EEC-682: Computer Networks I
Spring Semester 2005         - Wenbing Zhao           53   Spring Semester 2005                  - Wenbing Zhao           54

Internet over Cable                                        Internet over Cable

The fixed telephone system.

EEC-682: Computer Networks I                                        EEC-682: Computer Networks I
Spring Semester 2005         - Wenbing Zhao           55   Spring Semester 2005                  - Wenbing Zhao           56
Spectrum Allocation                                                         Cable Modems

Frequency allocation in a typical cable                                     Typical details of the upstream and
TV system used for Internet access                                           downstream channels in North
America.

EEC-682: Computer Networks I                                                EEC-682: Computer Networks I
Spring Semester 2005              - Wenbing Zhao                       57   Spring Semester 2005              - Wenbing Zhao           58

Cable Modems

When cable modem is switched on, it scans the downstream
channels looking for a special packet periodically sent out by
the headend to provide system parameters
Cable modem then announces its presence and the headend
responds by assigning the modem to its upstream and
downstream channels
Modem then determines the distance from headend (ranging)
Upstream channels are divided in time in minislots. Modem must
request minislots before sending
Headend also assign each modem to a minislot for upstream
bandwidth request
All data traffic is encrypted

EEC-682: Computer Networks I
Spring Semester 2005              - Wenbing Zhao                       59

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