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[ pdf ] Multiaccess Communication

Multiaccess Communication Download
Topic under  Wireless Connectivity
Source: ee.sharif.edu 
File size: 3.63 MB
File type: pdf
Last download on: Sat Mar 25, 2017 02:51:19 PM
Short Desciption:
The subnetworks considered thus far have consisted of nodes joined by point-to-point communication links. Each such link might consist physically of a pair of twisted wires. a coaxial cable. an optical fiber. a microwave radio link. and so on. The implicit assumption about point-to-point links. however. is that the received signal on each link depends only on the transmitted signal and noise on that link.

The central problem of multiaccess communication is that of sharing a communication channel between a multiplicity of nodes where each node has sporadic service requirements. This problem arises in local area networks, metropolitan area networks, satellite networks, and various types of radio networks. Collision resolution is one approach to such sharing. Inherently, collision resolution algorithms can achieve small delay with a large number of lightly loaded nodes, but stability is a major concern. The joint issues of stability, throughput, and delay are studied most cleanly with the infinite node assumption. This assumption lets one study collision resolution without the added complication of individual queues at each node. Under this assumption, we found that throughputs up to 1/e packets per slot were possible with stabilized slotted Aloha, and throughputs up to 0.487 packets per slot were possible with splitting algorithms. Reservations provide the other major approach to multiaccess sharing. The channel can be reserved by a prearranged fixed allocation (e.g., TOM or FDM) or can be reserved dynamically. Dynamic reservations further divide into the use of collision resolution and the use of TOM (or round-robin ordering) to make the reservations for channel use. CSMA/CD (i.e., Ethernet) is a popular example of the use of collision resolution to make (implicit) reservations. Token rings, token buses, and their elaborations are examples of the use of round-robin ordering to make reservations. There are an amazing variety of ways to use the special characteristics of particular multiaccess media to make reservations in round-robin order. Some of these variations require a time proportional to (3 (the propagation delay) to make a reservation, and some require a time proportional to ,)/ m, where m is the number of nodes. The latter variations are particularly suitable for high-speed systems with (3 > I. Packet radio systems lie at an intermediate point between pure multiaccess systems, where all nodes share the same medium (and thus no explicit routing is required), and point-to-point networks, where routing but no multiaccess sharing is required. Radio networks are still in a formative and fragmentary stage of research.
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