4 Busy-- 口口题口口口口圈口@黑 Idle and busy--◆ ---Requests 口口口题圆 Multiaccess Communication 4.1 INTRODUCTION 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. There are many widely used communication media,such as satellite systems.radio broadcast,multidrop telephone lines.and multitap bus systems.for which the received signal at one node depends on the transmitted signal at two or more other nodes.Typ- ically,such a received signal is the sum of attenuated transmitted signals from a set of other nodes,corrupted by distortion.delay,and noise.Such media,called multiac- cess media,form the basis for local area networks (LANs),metropolitan area networks (MANs).satellite networks.and radio networks. The layering discussed in Chapters I and 2 is not quite appropriate for multiaccess media.One needs an additional sublayer.often called the meditm access control(MAC) sublayer,beween the data link control(DLC)layer and the modem or physical layer.The 2714 Busy ---- Multiaccess Communication 4.1 INTRODUCTION 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. There are many widely used communication media. such as satellite systems. radio broadcast. multidrop telephone lines. and multitap bus systems. for which the received signal at one node depends on the transmitted signal at two or more other nodes. Typ￾ically. such a received signal is the sum of attenuated transmitted signals from a set of other nodes. corrupted by distortion. delay. and noise. Such media. called multiac￾cess media. form the basis for local area networks (LANs). metropolitan area networks (MANs), satellite networks. and radio networks. The layering discussed in Chapters I and 2 is not quite appropriate for multiaccess media. One needs an additional sublayer, often called the medium access control (MAC) sublayer. beween the data link control (DLC) layer and the modem or physical layer. The 271