Multiple Access Schemes

1 Communication channels are major components of computer communication networks. They provide the physical mediums over which signals, representing data, are transmitted from one node of the network to another node. Communication channels can be classiied into two main categories: point-to-point channels and shared channels. Typically, the backbone of wide area networks (WANs) consists of point-to-point channels, while local area networks (LANs) use shared channels. Point-to-point channels are dedicated to connect a pair of nodes of the network. They are usually used in xed topology networks and their cost depends on many parameters such as distance, bandwidth etc. An important characteristic of these channels is that nodes do not interfere with each other, namely, transmissions between a pair of nodes has no eeect on the transmissions between another pair of nodes, even if a node is common to the two pairs. Shared channels are used when point-to-point channels are not economical, not available, or when dynamic topologies are preferable. In a shared channel, called also a broadcast channel, several nodes can potentially transmit and/or receive messages at the same time. Shared channels appear naturally in radio networks, satellite networks and some local area networks (e.g., Ethernet). Their deployment is usually easier than point-to-point channels. An important characteristic of shared channels is that transmissions of diierent nodes interfere each other, namely, one transmission coinciding in time with another may cause none of them to be received. This means that the success of a transmission between a pair of nodes is no longer independent of other transmissions. To have successful transmissions in shared channels, interference must be avoided or at least controlled. The channel allocation among the competing nodes is critical for proper operation of the network. This article focuses on access schemes to such channels known as Multiple Access Schemes. These schemes are nothing but channel allocation rules that determine who goes next on the channel aiming at some desirable network performance characteristics. Multiple access schemes belong to a sublayer of the data link layer called the Medium Access Control (MAC) which is especially important in LANs. Multiple access schemes are natural not only in communication systems but also in many other systems such as computer systems, storage facilities or servers of any kind, where resources are shared by a number of nodes. In this article we mainly address shared communication channels. 2 One way to classify multiple access schemes is …