C4 - Multiaccess Communication Sunghyun Choi Adopted from...

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ultiaccess Communication Multiaccess Communication Sunghyun Choi Adopted from Prof. Saewoong ahk’s material Bahk s material
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Channel Allocation Problem any broadcast network the key issue is • In any broadcast network, the key issue is how to determine who gets to use the channel AC: medium access control • MAC: medium access control – Between the data link layer and the physical layer llocates the multi cess medium among the – Allocates the multi-access medium among the competing nodes(hosts)
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hannel allocation • Channel allocation – Free for all or perfectly scheduled approach • Multi-access channels – Satellite channels – Multidrop telephone lines – Multitapped bus – Packet radio networks
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Idealized Slotted Multiaccess Model ansmitting nodes and one receiver m transmitting nodes and one receiver • Slotted system – Perfect synchronization and same packet length • Poisson arrivals with rate λ / m individually ollision or perfect reception • Collision or perfect reception – no channel error or capture effect • 0, 1, e(rror) and immediate feedback
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etransmission of collisions • Retransmission of collisions Backlogged node is with a packet to be retransmitted ither of the following two assumptions • Either of the following two assumptions – No buffering : packet arriving at active node is simply discarded finite set of nodes = packet always Infinite set of nodes ( m ): packet always arrives at a new node
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Slotted ALOHA very node tries to transmit packet of the • Every node tries to transmit packet of the same length only at the beginning of a slot. • Unbacklogged node simply transmit a new acket in the first slot after the arrival packet in the first slot after the arrival. – Achieves shorter delay in lightly loaded case – Cf. TDM w/ ave. delay = m /2 slots • Backlogged nodes wait for the random amount of time to transmit again. g
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ssuming infinite number of nodes and • Assuming infinite number of nodes and retransmissions are well randomized • New transmissions + retransmissions are proximated by Poisson approximated by Poisson G : combined arrival rate in a frame time unit (retransmission + new transmission λ ) rom the Poisson pmf the probability of a • From the Poisson pmf, the probability of a successful transmission in a slot = Ge -G
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t equilibrium arrival rate ould be the • At equilibrium, arrival rate λ should be the same as the departure rate Ge -G • Maximum at G =1, throughput = e -1 0.368 • Otherwise, two values of G result in the me departure rate! sa e depa u e a e! G < 1, too many idle slots G > 1, too many collisions
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recise model (Discrete Time Markov chain) • Precise model (Discrete Time Markov chain) : the prob. that a backlogged node retransmits r q n : number of backlogged nodes total n mber of nodes (ass ming no b ffering) m : total number of nodes (assuming no buffering) : the prob. that an unbacklogged node transmits a a q packet in a given slot .
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This note was uploaded on 04/29/2010 for the course CSE 4541.525A taught by Professor Choisunghyun during the Spring '09 term at Seoul National.

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C4 - Multiaccess Communication Sunghyun Choi Adopted from...

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