Lecture%20slides%202007%2003%2013

Lecture%20slides%202007%2003%2013 - Chapter 6 Medium Access...

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Chapter 6 Medium Access Control Protocols and Local Area Networks Random Access

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ALOHA z Wireless link to provide data transfer between main z Simplest solution: just do it z A station transmits whenever it has data to transmit z If more than one frames are transmitted, they interfere with each other (collide) and are lost z If ACK not received within timeout, then a station picks random backoff time (to avoid repeated collision) z Station retransmits frame after backoff time t t 0 t 0 -X t 0 +X t 0 +X+2t prop t 0 +X+2t prop + B Vulnerable period Time-out Backoff period B First transmission Retransmission
ALOHA Model z Definitions and assumptions z X frame transmission time (assume constant) z S : throughput (average # successful frame transmissions per X seconds) z G : load (average # transmission attempts per X sec.) z P success : probability a frame transmission is successful success GP S = X X frame transmission Prior interval z Any transmission that begins during vulnerable period leads to collision z Success if no arrivals during 2X seconds

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Abramson’s Assumption z What is probability of no arrivals in vulnerable period? z Abramson assumption: Effect of backoff algorithm is that frame arrivals are equally likely to occur at any time interval z G is avg. # arrivals per X seconds z Divide X into n intervals of duration Δ = X/n z p = probability of arrival in Δ interval, then G = n p since there are n intervals in X seconds = = = = = n as ) 1 ( p) - (1 intervals] 2n in arrivals 0 [ seconds] 2X in arrivals 0 [ 2 2 2n G n success e n G P P P
Throughput of ALOHA G success Ge GP S 2 = = 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 . 00 7 8 1 25 0.015 6 0.031 2 5 .0 0.125 .2 .5 4 G S z Collisions are means

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This note was uploaded on 10/09/2009 for the course ENSC 1156 taught by Professor Rhardy during the Spring '07 term at Simon Fraser.

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Lecture%20slides%202007%2003%2013 - Chapter 6 Medium Access...

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