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lec16 - ECE 333 Introduction to Communication Networks Fall...

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1 ECE 333: Introduction to Communication Networks Fall 2002 Lecture 16: Medium Access Control IV CSMA CSMA/CD 2 Carrier Sense Multiple Access (CSMA) In studying Aloha, we assumed that a node simply transmitted a packet and was informed afterwards if a collision occurred. In many multi-access channels, it is possible for a node to detect when other nodes are transmitting after a small propagation delay, this is referred to carrier sensing. In such cases it is silly for a node to begin transmission if it detects another transmission on the link. A family of "Aloha"-like protocols for this situation are called Carrier Sense Multiple Access (CSMA) protocols. There are several different variations of CSMA - including persistent CSMA , nonpersistent CSMA , and p- persistent CSMA. In each of these protocols, users only transmit if they detect that the channel is idle. Collisions can still occur, if two users begin transmitting within close enough in time so that they cannot detect each other. If a collision occurs, we assume (as in Aloha) that the users find out about this after they finish transmitting their packets, in which case they wait a random time before retransmitting. The variations of CSMA differ in their behavior when the channel is detected to be busy.
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3 Persistent CSMA: 1) If channel is idle, transmit. 2) If channel is busy, wait until the channel is idle then transmit. 3) If a collision occurs, wait a random amount of time then go to 1. Other variations of CSMA replace the first 2 steps as follows: Non-persistent: 1) If channel idle, transmit. 2) If channel busy, wait a random amount of time and repeat step 1. p – persistent 1 : (for slotted system) 1) If channel idle, transmit with probability p . Wait one slot with probability 1 - p , and repeat step 1. 2) If channel busy, wait for idle and go to step 1. 1 “Persistent” is also called “1-persistent” (i.e. p = 1). 4 With persistent CSMA, if multiple stations are waiting for the channel to become idle, then a collision will occur. Using non-persistent or p-persistent CSMA will reduce this probability, but at the expense of longer delays. The maximum time from when one node begins transmitting until another node can sense this is a fundamental performance parameter for all of these CSMA protocols. This time depends on the propagation delay as well a detection delay required for the receiver to determine that a transmission is occurring. Intuitively, one would expect the performance to degrade as the maximum propagation and detection delay increases.
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