Sec.4.1 Introduction 273 Finally.Section 4.6 explores packet radio.Here the interesting issue is that each node interferes only with a limited subset of other nodes:thus.multiple nodes can transmit simultaneously without interference. Before going into any of the topics above in detail,we briefly discuss some of the most widely used multiaccess media. 4.1.1 Satellite Channels In a typical geosynchronous communication satellite system,many ground stations can transmit to a common satellite receiver,with the received messages being relayed to the ground stations (see Fig.4.1).Such satellites often have separate antenna beams for different geographical areas.allowing independent reception and relaying between areas. Also.FDM (or TDM)can be used,permitting different earth stations within the region covered by a single antenna beam to be independently received. It is thus possible to use a satellite channel as a collection of virtual point-to-point links.with two virtual links being separated either by antenna beam or multiplexing.The potential difficulty with this approach is the same as the difficulty with using FDM or Satellite Ground stations (a)Satellite multiaccess channel Primary Secondaries (b)Multidrop telephone line Figure 4.1 Common multiaccess (c)Multitap bus channels.Sec. 4.1 Introduction 273 Finally, Section 4.6 explores packet radio. Here the interesting issue is that each node interferes only with a limited subset of other nodes: thus, multiple nodes can transmit simultaneously without interference. Before going into any of the topics above in detail, we briefly discuss some of the most widely used multiaccess media. 4.1.1 Satellite Channels In a typical geosynchronous communication satellite system, many ground stations can transmit to a common satellite receiver, with the received messages being relayed to the ground stations (see Fig. 4.1). Such satellites often have separate antenna beams for different geographical areas, allowing independent reception and relaying between areas. Also, FDM (or TDM) can be used, permitting different earth stations within the region covered by a single antenna beam to be independently received. It is thus possible to use a satellite channel as a collection of virtual point-to-point links, with two virtual links being separated either by antenna beam or multiplexing. The potential difficulty with this approach is the same as the difficulty with using FDM or Satellite o o Ground stations o o Primary (a) Satellite multiaccess channel Secondaries (b) Multidrop telephone line (c) Multitap bus Figure 4.1 Common multiaccess channels