Chi-Cheng Lin, Winona State University

CS 313 Introduction to Computer Networking &

Telecommunication

Mobile Telephone System and Wireless

LANs

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Topics

Mobile Telephone System

CDMA

Wireless LANs

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Mobile Telephone System

First-Generation Mobile PhonesAnalog Voice

Second-Generation Mobile PhonesDigital Voice

Third-Generation Mobile PhonesDigital Voice and Data

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Advanced Mobile Phone System

Area is divided into cells with an antenna control by a cell office in each cell

Cell offices communicate with MTSO Transmission frequencies cannot be

the same in adjacent cells Cell size is not fixed

Smaller cells used in higher populated area

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Cellular System

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Advanced Mobile Phone System

(a) Frequencies are not reused in adjacent cells.(b) To add more users, smaller cells can be used.

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Frequency reuse patterns

©The McGraw-Hill Companies, Inc., 2004

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Cellular Transmission

Traditionally analogFM used to minimized noise

Digital transmissionCDPD (Cellular Digital Packet Data)

Low-speed digital service over existing cellular network

Based on OSI ModelModem needed

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Cellular System

HandoffWhen a mobile telephone leaves a cell1. Its base station notices the signal

fading out2. The base station asks all the

surrounding base stations how much power they are getting from it

3. Ownership is transferred to the neighbor base station that receives strongest power

4. The telephone is informed of its new boss

5. If a call is in progress, it will be asked to switch to a new channel

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Channels

832 full-duplex channelsEach channel consists of 2 simplex

channelsTransmission channels

(849-824)MHz/30KHz 832

Receiving channels(894-869)MHz/30KHz 832

Typically, actual number of voice channel per cell 45

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Channel Categories

The 832 channels are divided into four categoriesControl (base to mobile) to manage

the systemPaging (base to mobile) to alert users

to calls for themAccess (bidirectional) for call setup

and channel assignmentData (bidirectional) for voice, fax, or

data

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Second-Generation Mobile Phones

D-AMP

GSM

CDMA

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D-AMPS Digital Advanced Mobile Phone System

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

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GSMGlobal System for Mobile

Communications GSM uses 124 frequency channels,

each of which uses an eight-slot TDM system

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GSM

A portion of the GSM framing structure.

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Third-Generation Mobile Phones:Digital Voice and Data

Basic services an IMT-2000 network should provideHigh-quality voice transmissionMessaging

Replace e-mail, fax, SMS, chat, etc.

Multimedia Music, videos, films, TV, etc.

Internet accessWeb surfing, w/multimedia

2.5G, 4G, …

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Code Division Multiple Access (CDMA)

In FDMA, the bandwidth is divided into channels.

In TDMA, the bandwidth is just one channel that is timeshared.

In CDMA, one channel carries all transmissions simultaneously.

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CDMA

Every station has a chip sequence Bipolar encoding

1 is represented by +10 is represented by -1Silence is represented by 0

The set of chip sequences are orthogonalA chip sequence is represented by a vector of +1’s

and -1’s (for example, 4 sequences A, B, C, & D)A ● B = A ● C = A ● D = B ● C = B ● D = C ● D = 0A ● A = B ● B = C ● C = D ● D = length of chip sequence

Transmission: s = sum of (bit chip_sequence) Receiving: bit = (s ● chip_sequence) /

chip_length● : inner product of two vectors

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CDMA(a) Binary chip

sequences for four stations

(b) Bipolar chip sequences

(c) Six examples of transmissions

(d) Recovery of station C’s signal

Q: Why does it work?A: _ _ _ _

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Wireless LANs

(a) Wireless networking with a base station.(b) Ad hoc networking.

The range of a single radio may not cover the entire system

Multipath fading Noisy

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Wireless LANs Problems

A multicell 802.11 network

ISM band

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Wireless LANs

802.11 BroadbandCellular

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The 802.11 Protocol Stack (part of)

54Mbps5GHz ISM band

11Mbps2.4GHz ISM band

54Mbps2.4GHz ISM band

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The 802.11 MAC Sublayer Protocol

The hidden station problem.

CSMA/CD does not work (why?) Hidden station and exposed station

problems

The exposed station problem.

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The 802.11 MAC Sublayer Protocol

Two modes of operations Contention mode

DCF: distributed coordination function CSMA/CA

Collision-free mode PCF: Point coordination function Polling w/ beacon frame

NAV: Network Allocation Vector

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The 802.11 MAC Sublayer Protocol

Noisy, unreliable Smaller frames reduce probability of frame damages

Fragmentation Each fragment, with its own checksum and sequence

number, is acknowledged individually Stop-and-wait Fragment burst: sequence of fragments

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The 802.11 MAC Sublayer Protocol

ProblemsNAV terminates at end of the first fragmentHow do DCF and PCF coexist

Solution: interframe spacing

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The 802.11 Frame Structure

Three classesData, control, management

4 addresses in the frame