CS440 Computer Networks 1

EthernetEthernet

Neil TangNeil Tang9/17/20089/17/2008

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OutlineOutline

Major Concern for MAC Design

Basic Architecture

Physical Properties

MAC Protocol

Strengths and Weaknesses

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Major Concerns for MAC DesignMajor Concerns for MAC Design

Throughput

Fairness

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Ethernet (802.3)Ethernet (802.3)

Basic Architecture: multiple hosts share a physical medium.

MAC Protocol: CSMA/CD

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Basic ArchitectureBasic Architecture

Hub Hub

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Physical Properties Physical Properties

Broadcast: Any signal placed on Ethernet is broadcast. Encoding: Manchester Error Detection: CRC Physical Distance: 2500m Number of Hosts: 1024 Data Rate: 10Mbps/100Mbps, 1Gbps Cables: 10Base2(200m), 10Base5(500m), 10BaseT(100m)

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Frame Format Frame Format

Min/Max Body Size: 46/1500 bytes

Destaddr

64 48 32

CRCPreamble Srcaddr Type Body

1648

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AddressingAddressing

48-bit Address: e.g., 08:00:2b:e4:b1:02

The address is burned to the ROM of each adaptor. Every adaptor has a world-wide unique address.

Each manufacturer is allocated a different prefix.

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MAC AlgorithmMAC Algorithm

In the receiver

The receiver in an adaptor receives all frames but only accepts: frames addressed to its own address frames addressed to the broadcast address (FF:FF:FF:FF:FF:FF) Frames addressed to a multicast address (e.g., 01:00:5E:80:00:00 -

01:00:5E:FF:FF:FF (reserved by IANA)), if it is in that multicast group All frames, if it is placed in promiscuous mode.

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MAC AlgorithmMAC Algorithm

In the transmitter

If it has data to send and the line is busy, it waits for the line to go idle and transmit immediately.

At the moment it detects collision, it transmits a 32-bit jamming sequence and stops transmission.

It waits a “certain” amount of time and tries again. It uses binary exponential backoff, delay k 51.2μs , where k=0…2n-1(random selection).

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MAC AlgorithmMAC Algorithm

1-persistent Vs. p-persistent

1-persistent protocol (Ethernet): It transmits once the medium is idle

p-persistent protocol (Aloha): It transmits with probability p and defers until

next empty time with probability q=1-p. Make the same decision next time.

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MAC AlgorithmMAC Algorithm

Why min frame length 512 bits

Worst case propagation round trip delay (2500m, 4 repeaters) = 51.2 μs

Transmission time = 512/10M = 51.2 μs

(a)

(b)

(c)

A B

A B

A B

A B

(d)

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Strength and WeaknessStrength and Weakness

Strength:

Simple: No routing, no switching and very simple MAC protocol.

Low Cost: Cheap cables and cheap equipments

Weakness:

Inefficient in Throughput: Under heavy traffic condition, collisions happen frequently.