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MultipleAccess

Protocols

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Multiple Access Multiple hosts sharing the same medium

What are the new problems?

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Shared Media Ethernet bus

Radio channel

Token ring network

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Multiple Access protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes:interference Collisionif node receives two or more signals at the same time

Multiple Access Protocol

Distributed algorithm that determines how nodes share

channel, i.e., determine when node can transmit Communication about channel sharing must use channel

itself! No out-of-band channel for coordination

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Channel Partitioning Frequency Division Multiplexing

Each node has a frequency band

Time Division Multiplexing Each node has a series of fixed time slots

What networks are these good for?

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Computer Network Characteristics Transmission needs vary

Between different nodes

Over time Network is not fully utilized

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Ideal Multiple Access ProtocolBroadcast channel of rate R bps

1. When one node wants to transmit, it can send atrate R.

2. When M nodes want to transmit, each can send ataverage rate R/M

3. Fully decentralized:

no special node to coordinate transmissions no synchronization of clocks, slots

4. Simple

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Random Access Protocols When node has packet to send

transmit at full channel data rate R.

no a prioricoordination among nodes

two or more transmitting nodescollision, random access MAC protocolspecifies:

how to detect collisions

how to recover from collisions (e.g., via delayed retransmissions)

Examples of random access MAC protocols: slotted ALOHA

ALOHA

CSMA, CSMA/CD, CSMA/CA

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Data link layer divided into two functionality-oriented sublayers

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Taxonomy of multiple-access protocols discussed in this chapter

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RANDOM ACCESS

In random accessor contentionmethods, no station is superior to another stationand none is assigned the control over another. No station permits, or does notpermit, another station to send. At each instance, a station that has data to senduses a procedure defined by the protocol to make a decision on whether or not to

send.

ALOHACarrier Sense Multiple AccessCarrier Sense Multiple Access with Collision DetectionCarrier Sense Multiple Access with Collision Avoidance

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Frames in a pure ALOHA network

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Procedure for pure ALOHA protocol

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Vulnerable time for pure ALOHA protocol

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Frames in a slotted ALOHA network

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Vulnerable time for slotted ALOHA protocol

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Space/time model of the collision in CSMA

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Vulnerable time in CSMA

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Behavior of three persistence methods

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Flow diagram for three persistence methods

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Figure 12.12 Collision of the first bit in CSMA/CD

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Collision and abortion in CSMA/CD