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lec15 - ECE 333 Introduction to Communication Networks Fall...

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ECE 333: Introduction to Communication Networks Fall 2002 Lecture 15: Medium Access Control III ° Slotted Aloha ° Stability 2 In the previous lecture we introduced the ALOHA medium access control protocol. This protocol is a contention based or random access protocol in which there is some probability that multiple users transmit at the same time resulting in a collision. Using a simplified analysis for a network with many users we showed that the maximum throughput of ALOHA was 1/(2 e ) packets/ per transmission time. In this lecture we first look at an improvement on ALOHA, called "slotted ALOHA". We then consider the stability of these protocols.
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3 Slotted Aloha (Roberts '72) Slotted Aloha, as the name implies, changes the protocol from continuous time to slotted time operation. Specifically, we view the time axis a sequence of slots of length T , where one frame can be sent in each slot. The transmitters are assumed to all be synchronized so that all transmissions start at the beginning of a slot. When a frame arrives be transmitted during a slot, it is queued until the beginning of the next slot. Thus a frame only contends with frames generated during the same slot; this reduces the contention period from 2 frame times to 1 frame time. Repeating the argument from last lecture, we now find the probability of a successful transmission, 0 G P e ± ² , and the throughput vs. offered load is given by
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