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Unformatted text preview: ced by one half. Throughput CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit. If channel is sensed busy, defer CSMA transmission
Note: collisions may still exist, since two stations may sense the channel Note: idle at the same time ( or better, within a “vulnerable” window = round trip delay) The amount of defer-time before retry is adjusted dynamically: the soThe called exponential back-off For the k-th retry, wait for Xk time-slots where For time-slots Xk ~ is randomly picked from [0..2k-1] by each node is Allow the protocol to adapt to different offered load, i.e. no. of active Allow stations sharing the channel Contrast this with the constant retry probability P in Aloha Contrast In case of collision, the entire packet transmission time is wasted In CSMA collisions CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing and deferral like in CSMA. But, CSMA/CD collisions are detected within a few bit times. Transmission is then aborted, reducing the channel wastage Transmission considerably. Collision detection is easy in wired LANs (eg, Ethernet): can Collision easy measure signal strength on the line, or code violations, or compare tx and receive signals Collision detection cannot be done in wireless LANs (the Collision cannot (the receiver is shut off while transmitting, to avoid damaging it with excess power) CSMA/CD can approach channel utilization =1 in LANs (low CSMA/CD ratio of propagation over packet transmission time) CSMA/CD collision detection “Taking Turns” MAC protocols
So far we have seen that channel partitioning MAC protocols So channel MAC (TDM, FDM and CDMA) can share the channel fairly; but a single station cannot use it all Random access MAC protocols allow a single user full channel Random MAC rate; but cannot share the channel fairly (in fact, capture is often capture is observed) Taking Turns MAC protocols achieve both fairness and full Taking MAC rate, at the expense of some extra control overhead (a) Polling: a Master station on a LAN in turn “invites” the (a) Master slave stations to transmit their packets (up to a Max). Problems: Request to Send/Clear to Send overhead, latency, single point of failure (Master) (b) Token passing: the control token is passed from one node (b) token is to the next sequentially. Can alleviate the latency and improve fault tolerance (...
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This note was uploaded on 12/08/2010 for the course IEG IEG3310 taught by Professor Wingc.lau during the Spring '10 term at CUHK.
- Spring '10