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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)