Carrier-Sence Multiprle Access (CSMA) Protocols

Communication of information between two or more parties takes place over a variety of physical media called channels. Such channels can be simple twisted pair cables, coaxial and optical cables, or the free space. Sometimes the channel is dedicated to a specific transmitter-receiver pair. This may be the case when two parties establish a telephone conversation over a dedicated cable. Channels of this type are called point-to-point. There are situations, however, where multiple users need to have access to the same channel. The most familiar one is human speech communication. When a number of humans are located in the same room they all use the same channel, the atmosphere, for their conversation exchange. Computer Local Area Networks (LANs) is another example: a common approach in this case is to attach a number of computers to the same cable as in Figure 1. Hence, each computer can listen to the transmission of every other computer attached to the same cable. For a third example, consider Satellite communication. As shown in Figure 2 a number of terminals need to communicate between each other but due to physical obstacles they cannot all listen to each other directly. Instead, each terminal first sends the information to the satellite through the upstream channel. The satellite listens to the upstream channel, receives the information and then retransmits to the downstream channel, to which all terminal can listen. Hence the upstream channel needs to be accessed by all terminals. Channels of this type are called multiple-access. If the terminals in a multiple-access channel are left unchecked so that they can transmit information whenever the need to do so, then the possibility arises that more than one terminals attempt to use the channel at the same time. In such a situation, the concurrently transmitted messages interfere with each other and generally cannot be received correctly by the intended receivers. Hence, a fundamental issue in multiple-access channel communication is how to coordinate the transmitting terminals in order to avoid