cate with the WAN via the gateway. To understand data network we have to know the con- Data communications involve the transfer of data cept of communications channels of different types of between computer programs. Just as humans must share networks. Broadcast channels are, as the name implies, a common language in order to communicate, the similar to short-wave radio conversations. A node programs must have a common protocol. The protocol broadcasts on the common channel and all other nodes simply defines the format and meaning of the data that connected to the channel listen to the message (see the programs interchange. Figure 3.1-3). An example of a very simple protocol might be the The channel might consist of electromagnetic emis- following: Suppose two programs connected by sions (that is, radio), or a wire or cable to which all nodes a communications channel need to exchange messages are attached. As with users of CB radio channels, more that vary in length. The protocol might specify that the than one node might begin to transmit at the same time. first three characters of each message be numeric When that happens, each garbles the other's message. characters giving the length of the message itself (in They all must stop and then start over again in such a way decimal, not counting the first three characters). For the that subsequent collisions are not likely to happen. message "HELLO WORLD" (containing 11 bytes in- Alternatively, transmissions can be scheduled SO that cluding the space), the sending program would transmit more than one transmission never takes place at the same the characters "O11 HELLO WORLD." The receiving time. program would accept the first three characters and then, Data networks can be classified according to the area knowing the length of the message, would expect 11 over which they extend. A LAN consists of a few up to more characters (see Figure 3.1-4). several hundred nodes, but will typically be confined to If the receiving program got a message that had a few buildings with