| 单词 | CSMA (Carrier Sense Multiple Access) |
| 释义 | CSMA (Carrier Sense Multiple Access) A decentralized, contentious medium access control (MAC) protocol used in Ethernet and other bus-oriented local area networks (LANs). The carrier frequency is sensed by each of multiple stations, or nodes, to determine network availability before accessing the medium to transmit data. Each station must monitor the network to determine if a collision has occurred and the data require retransmission. CSMA variations include Nonpersistent CSMA, 1-Persistent CSMA and P-Persistent CSMA. • Nonpersistent CSMA: A machine may transmit data whenever it senses an idle channel. If the channel is busy, the machine backs off the network, calculates a random time interval, and again monitors the channel when that interval expires.This approach mathematically distributes the temporal monitoring of the network, thereby reducing the likelihood that multiple stations will sense its availability at approximately the same time and transmit simultaneously. • 1-Persistent CSMA: A machine may transmit data whenever it senses an idle channel. If the channel is in use, the machine will continuously sense it until the channel becomes free.The protocol is so named as the machine is persistent in its monitoring of the channel, and transmits with a probability of 1.0, i.e., 100 percent certainty of access success, whenever the channel is idle. If the network includes a large number of stations persistently monitoring the network, a great many of them might sense the availability of the network and begin to transmit simultaneously, virtually guaranteeing a collision. 74570c03.qxd 9/11/07 12:19 PM Page 118 |
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