Teknik Akses Jaringan Carrier SensePertemuan 12

Matakuliah : H0484/Jaringan KomputerTahun : 2007

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Learning Outcomes

Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu :

• Menjelaskan teknik akses ke jaringan dengan carrier sense

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Outline Materi

• CSMA• Back-off strategy

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Carrier Sense Multiple Access

• ALOHA & Slotted ALOHA are inefficient because stations don’t take into account what other stations are doing before they transmit (Talk-before-listen)

• Sense for carriers (see if anyone else is transmitting) before begin transmitting

• Carrier Sense Multiple Access (CSMA) is Listen Before Talk

• In LAN’s it is possible for stations to detect what other stations are doing and reactively change.

• CSMA can improve performance• These protocols are called carrier sense protocols. • They are named 1-persistent CSMA, non-persistent

CSMA, p-persistent CSMA

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• When station has data to send, it listens to channel

• Channel idle: station transmits packet • Channel busy: station waits till channel is idle• When channel becomes free, a host transmits

its packet immediately (with probability 1)

1 - Persistent

Packet Atime

Station Blistens

Packet B

delay

Station Bsends

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• Propagation delay II– If propagation delay is small zero, collision may

still occur– Station A transmits, Stations B, X, Y

simultaneously realize that line is busy and wait– When line is free, stations B transmits then

station X, Y simultaneously transmit. Collision.

Collision in CSMA

Packet Atime

Station Blistens

Packet B Packet Y

delay Collision still possibleover long propagation delays

Packet X

Station Bsends

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• Before sending, station senses channel • If no transmission, station starts sending.• If channel is busy it does not continuously sense the

channel in order to start transmitting•Wait/sleep a random interval before sensing again

•As soon as channel is idle, then send a packet• Random interval reduces collisions

•Higher throughput than 1-persistent CSMA when many senders

Non Persistent

Packet Atime

Station Blistens

Packet B

Randomdelay

Station Bsends

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•Generalization of 1-persistent CSMA•Typically applied to slotted channels•Slot length is chosen as maximum propagation delay

•A station senses the channel, and•If slot is idle, transmit with probability p, or defer with probability q=1-p

•If next slot is idle, transmit with probability p, or defer with probability 1-p, repeat…

•If channel is initially busy, it waits until the next slot then sense channel continuously until it becomes free and applies the above algorithm

p-Persistent

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• IEEE 802.3 is a standard for a 1-persistent CSMA/CD LAN– If cable is busy, station waits until cable is

idle.– If 2 or more stations simultaneously

transmit on an idle cable, they will collide.– All colliding stations then terminate their

transmission and, wait random time and then start process again.

• Ethernet is a specific implementation.

CSMA with Collision Detection

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• Ethernet uses CSMA/CD• Listen-while-talk protocol

• A station listens even while it is transmitting, and if a collision is detected, stops transmitting

Ethernet - CSMA/CD

Packet Atime

Station Blistens

Packet B

delay

Packet B size

Not transmitted

Station Bsends

Packet B

Station B detects collision,Stops sending

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• Ethernet CSMA/CD requires a minimum size packet • If packet B arrives at A and A is no longer

transmitting, then Host A will• Fail to detect the collision• Thinks its packet got through• Thinks the incoming packet is a new packet

• Therefore, to detect a collision:• Minimum packet size >= 2*(prop. delay)*BW

A

Ethernet

Packet A t

d=propagation delay

B

Packet A

Packet B

Packet B

t+d

B transmit at t+d, just before packet A arrives. B sees collision and transmits a runt” packet Packet arrives at t+2d

Minimum size Ethernet packet

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Comparison of multiple access

G (transmission attempts per packet time)