An Analysis of the Carrier-Sense Multiple-Access Protocol

The Carrier-Sense Multiple-Access (CSMA) protocol has been proposed for resolving conflicts when several sources attempt to use a single channel. In this paper we investigate the throughput and delay characteristics of CSMA. 1.1 Background When several sources attempt to use a single channel, a protocol (in this context, protocol is synonymous with queue discipline) is required to allocate the channel among the sources. When the channel is occupied, a signal is sent indicating that state of affairs. When a source wants to use the channel, it first listens for this signal. If the signal is not heard, the source starts transmitting. If the signal is heard, the 2023 source postpones transmitting and tries again at another time. The advantage of this system is that a device to control the sources is not required. The disadvantage is that occasionally messages will be destroyed because two sources will transmit at the same time. This is a consequence of the fact that signals travel at finite speeds, so there is a delay between the epoch when one source seizes an idle channel and the epochs when the other sources can first hear the busy-channel signal. Thus, soon after a source seizes the channel, another source may sense that the channel is free even though the channel is busy. When this occurs, both messages are destroyed because their bits have been merged. At the end (earlier if collision detection is used) of each transmission, the source determines whether the transmission was successful.