Medium Access Control Modeling Why do we need

Medium Access Control Modeling

Why do we need Medium Access Control? Multiple Access Protocols Random Access © Tallal Elshabrawy Scheduling 2

ALOHA Protocols l Hawaii Islands 1970 s l University of Hawaii needed a means to interconnect terminals located at campuses on multiple islands to the host computer on main campus l Packet Radio l Messages are transmitted as soon as they become available © Tallal Elshabrawy Electrical Engineering Arts Management 3

ALOHA Operation Collision No Collision Terminal A Terminal B l l l Collision l Transmissions from two or more terminals overlap in time ALOHA Operation l Messages transmitted once available l From time-to-time messages collide (treated as erroneous frames) l The terminal knows that its messages was corrupted when it receives no acknowledgements within 2 tprop (i. e. , a timeout is used) l Recovery by the use of retransmissions Can we simply apply retransmission directly after timeouts? l NO! Retransmissions will collide again l SOLUTION: Retransmission after random intervals from timeouts (Backoff Algorithm) © Tallal Elshabrawy 4

ALOHA Operation First Transmission t 0 -X t 0+X Re-Transmission IF NECESSARY t 0+X+2 tprop+B o Backoff Period Vulnerable Period Timeout l Vulnerable Period: l Collision will occur if any other terminal commences transmission within this period l Backoff Period l Random time set for retransmission © Tallal Elshabrawy 5

Aloha Modeling l © Tallal Elshabrawy 6

Aloha Markov Model © Tallal Elshabrawy 7

Aloha Markov Model © Tallal Elshabrawy 8

Aloha Markov Model © Tallal Elshabrawy 9

Aloha Throughput © Tallal Elshabrawy 10