1 Kyung Hee University Multiple Access. 2 Kyung Hee University 13.1 Random Access MA – Multiple Access CSMA – Carrier Sense MA CSMA/CD – CSMA/Collision

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Kyung Hee University

Multiple AccessMultiple Access

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13.1 Random Access13.1 Random Access

MA – Multiple Access

CSMA – Carrier Sense MA

CSMA/CD – CSMA/Collision Detection

CSMA/CA – CSMA/Collision Avoidance

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Evolution of random-access methods

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• Base station is central controller

• Base station acts as a hop

• Potential collisions, all incoming data is @ 407 MHz

ALOHA network – Multiple Access

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Procedure for ALOHA protocol

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Collision in CSMA – Carrier Sense MA

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Persistence strategies

1- persistent

P-persistent

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CSMA/CD procedure – Collision Detection

- Used in Ethernet

CSMA/CD procedure – Collision Detection

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Interframe Gap

CSMA/CA procedure – Collision Avoidance

- Used in Wireless LAN

CSMA/CA procedure – Collision Avoidance

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13.2 Controlled Access13.2 Controlled Access

Reservation

Polling – Select and Poll

Token Passing

• Stations consult one another to find which station has the right to send

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Reservation access method

• A station need to make a reservation before sending data

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PollingPolling

If the primary want to receive data, it asks the seconda

ries if they have anything to send.

The secondaries are not allowed to transmit data unles

s asked (don’t call us - we’ll call you)

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Select

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poll

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Token-passing network A station is authorized to send data when it receives a

special frame called a token

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Token-passing procedure

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13.3 Channelization13.3 Channelization

FDMA – Frequency Division

TDMA – Time Division

CDMA – Code Division

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In FDMA, the bandwidth is divided into channels.

The available bandwidth is shared by all stations.

The FDMA is a data link layer protocol that uses FDM

at the physical layer

FDMA

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

TDMA

The entire bandwidth is just one channel.

Stations share the capacity of the channel in time

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In CDMA, one channel carries all transmissions simultaneously.

CDMA

Only one channel occupies the entire bandwidth of the

link

All Stations can send data simultaneously; there is no

time sharing.

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Chip sequences – Four Stations

CDMA is based on coding theory

Each station is assigned a code, which is a sequence

of numbers called chips.

All Stations can send data simultaneously; there is no

time sharing.

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Encoding Rules

When a station is idle, it sends no signal, which is

represented by a 0.

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Showing how four stations share the link during 1-bit

interval.

CDMA Multiplexer

Encoding Rules

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Encoding Rules

CDMA Demultiplexer

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Sequence Generation To generate sequences, we use a Walsh table, a two-

dimensional table with an equal number of rows and

columns.

Each row is a sequence of chips

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Sequence Generation

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Properties of Orthogonal SequencesProperties of Orthogonal Sequences

1. If we multiply a sequence by -1, every element in the

sequence is complemented

2. If we multiply two sequences, element by element and

add the result, we get a number called the inner

product. If two sequences are the same, we get N,

where N is the number of sequences; if different ,we

get

3. Inner product of a sequence by its complement is –N.

So A·B is 0.

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Example 1Example 1

Check to see if the second property about orthogonal codes holds for our CDMA example.

SolutionSolution

The inner product of each code by itself is N. This is shown for code C; you can prove for yourself that it holds true for the other codes.

C . C =

If two sequences are different, the inner product is 0.

B . C =

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Example 2Example 2

Check to see if the third property about orthogonal codes holds for our CDMA example.

SolutionSolution

The inner product of each code by its complement is N. This is shown for code C; you can prove for yourself that it holds true for the other codes. C . (C ) =

The inner product of a code with the complement of another code is 0.

B . (C ) =

Documents

1 Kyung Hee University Multiple Access. 2 Kyung Hee University 13.1 Random Access MA – Multiple Access CSMA – Carrier Sense MA CSMA/CD – CSMA/Collision