Читать книгу Wireless Connectivity - Petar Popovski - Страница 14

The main feature of the wireless communication medium is the fact that the medium is shared, in the same way in which the air through which the sound propagates is shared among the people having a conversation. MAC protocols enable multiple wireless communication users and devices to share the medium and send/receive data.

First, we must agree on how the system operates and what it takes to have a signal from one communication node received correctly at another node. In other words, we need to settle on a suitable system model: a set of assumptions that will allow us to talk about communication protocols and principles in a setting that is simple, but sufficient to contain the necessary properties of a shared wireless medium. We build the initial model by relying on a common sense analogy with the spoken conversation, as it captures three fundamental properties of wireless communication: broadcast, interference, and half-duplex operation. We illustrate these features by observing a conversation between Zoya, Yoshi, and Xia:

 Half–duplex: A given person, e.g. Zoya, cannot speak and listen at the same time.

 Broadcast: If Zoya has information to convey to Yoshi and Xia, then, provided that both Yoshi and Xia are listening, Zoya needs to say her message only once, and not repeat it individually to Yoshi and to Xia.

 Interference: If Yoshi and Xia speak simultaneously, Zoya will not understand either of them.

The descriptions above are arguably not always correct, but they do represent what is common sense for a conversation. Furthermore, the analogy of the communication problems with the conversation between Zoya, Yoshi, and Xia is useful, but it has its limitations, which will be pointed out when necessary.