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Kỹ thuật truyền số liệu 8

Tham khảo tài liệu 'kỹ thuật truyền số liệu 8', giáo dục - đào tạo, cao đẳng - đại học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 3APTER 8 Multiplexing Whenever the transmission capacity of a medium linking two devices is greater than the transmission needs of the devices the link can be shared much as a large water pipe can carry water to several separate houses at once. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data- and telecommunications usage increases so does traffic. We can accommodate this increase by continuing to add individual lines each time a new channel is needed or we can install higher capacity links and use each to carry multiple signals. As described in Chapter 7 today s technology includes high-bandwidth media such as coaxial cable optical fiber and terrestrial and satellite microwaves. Each of these has a carrying capacity far in excess of that needed for the average transmission signal. If the transmission capacity of a link is greater than the transmission needs of the devices connected to it the excess capacity is wasted. An efficient system maximizes the utilization of all facilities. In addition the expensive technology involved often becomes cost-effective only when links are shared. Figure 8.1 shows two possible ways of linking four pairs of devices. In Figure 8.1 a each pair has its own link. If the full capacity of each link is not being utilized a portion of that capacity is being wasted. In Figure 8.1b transmissions between the pairs are multiplexed the same four pairs share the capacity of a single link. 8.1 MANY TO ONE ONE TO MANY In a multiplexed system n devices share the capacity of one link. Figure 8.1 b shows the basic format of a multiplexed system. The four devices on the left direct their transmission streams to a multiplexer MUX which combines them into a single stream many to one . At the receiving end that stream is fed into a demultiplexer DEMUX which separates the stream back into its component transmissions one to many and directs them to their intended .