Читать книгу Wireless Connectivity - Petar Popovski - Страница 13

We start by describing wireless communication through an analogy with a conversation within a group of people, named Zoya, Yoshi, and Xia. We will refer to these and some other characters throughout the book; the characters will stand for wireless devices, base stations, or similar. The data that they want to communicate to each other is the content of their speech, which is part of the conversation. Regardless of the speech content, the conversation can only take place if the participants follow some conversation protocol, such that at a given time only one person speaks while the others listen. How do they agree who gets to speak and who gets to listen? One way would be, before starting the actual conversation, to have them agree upon which conversation protocol should be followed. In that case the information exchanged in that preliminary conversation cannot be regarded as useful data, but rather as metadata, also called protocol information or control information. The metadata is necessary in order to enable the conversation to take place. But then, how do they agree on the protocol for exchanging the metadata?

These questions can go on to infinity, but in a normal situation the communication protocol is agreed upon by either sticking to certain rules of politeness or following visual cues and gestures that facilitate the conversation. In other words, the metadata is exchanged by using a visual communication channel that is different from the speech communication channel. However, in a commonly encountered wireless communication system there is only one communication channel through which both the data and the metadata should be sent. This is not to say that it is not possible to have one wireless communication channel for data and a separate one for metadata; even if such separation exists, then what is the protocol for exchanging the meta-metadata that is used to agree how to send the metadata?

This gets obviously complicated, but the bottom line is that we will always hit the problem of communicating over a single shared wireless channel. Now, taking the fact that there is a single channel for communicating both the data and the metadata, the key point of the analogy with the conversation is to put Zoya, Yoshi, and Xia in a dark room, such that they have only speech as a means of communicating (we exclude tactile communication) and no visual cues can be of help. In that setting, the audio channel should be used both to coordinate the conversation and to carry the actual content of the conversation.

This is the common situation in which wireless communication systems operate and will be the subject of this chapter. Here are some examples of the questions that will be discussed. If Zoya and Yoshi want to talk to each other, how do they agree who talks first and who listens first? If both Xia and Yoshi want to talk to Zoya, how should they agree who takes a turn to speak at a given time, so that they don't all talk simultaneously? Solutions to these problems are provided by various protocols for controlling the access to the medium; hence the name MAC (medium access control) protocols, and they are of central importance in wireless communication systems.