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Multiple access control in computer networks, comparing broadcast and point-to-point (ppp) channels. It also covers the channel allocation problem in lans and mans, introducing static and dynamic methods. Aloha, csma, and other multiple access protocols, as well as terminologies like contention systems and throughput.
Typology: Study notes
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propagation
transmission
nodal processing (processing delay)
queuing
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Fewer than N users A valuable chunk of time (TDM) or bandwidth (FDM) is wasted More than N users Some users are denied (even if another user is idle) Exactly N users Idle users waste bandwidth e.g. bursty traffic
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Station Model N independent stations generating frames Once a frame is generated, the station is blocked until the frame has been transmitted Single Channel Assumption A single channel is available for all communication All stations are equivalent Collision Assumption If two frames are transmitted simultaneously, they overlap in time and the resulting signal is garbled. This event is called acollision. All stations can detect collisions. Time Assumption Continuous time Slotted time Carrier assumption Carrier Sense (LAN) No Carrier Sense (Satellite)
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1-persistent When idle, transmit a frame When busy, continuously sensing When collision, wait for a random amount of time Non-persistent When idle, transmit a frame When busy, wait for a random amount of time When collision, wait for a random amount of time then re-sense p-persistent Slotted channels When idle, transmit with a probability p, defers until the next slot with probability 1-p CSMA with collision detection (CSMA/CD) As soon as a collision is detected, abort the transmission Basis of Ethernet LAN
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N stations with a unique address from 0 to N- Propagation delay is negligible
A reservation protocol: the desire to transmit is broadcast before the actual transmission
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Low load: improved efficiency High load: high delay
Low load: high delay High load: improved efficiency
Uses contention at low load Uses collision-free at high load Adaptive tree walk protocol
i q =log 2
1 1 Pr[successwithoptimalp]
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