The communication channel provides the path for the message between the two communicating nodes in the model. Although the model in Figure 4-11 represents the channel as a direct point-to-point connection between the nodes, this is not generally the case.
In reality, the channel can take many different forms. In the simplest case, it might be a direct connection between nodes in a local area network. More typically, the communication channel is actually divided into segments, called links, with intermediate nodes between the links that forward packets from one link to the next.
Data originates at one end point and passes through each link to reach the destination end point.
As an example, consider Figure 4-12. In this example, data in the form of a http request originating from a home computer connects wirelessly through a router to a DSL modem. From there, the data passes through the DSL link to an Internet Service Provider, then through many additional connections to a computer somewhere on the Internet.
The computer communications revolution has produced several remarkable facts:
There is no fundamental difference between data processing (computers) and data communications (transmission and switching equipment).
There are no fundamental differences among data, voice, and video communications.
The distinction among single-processor computer, multiprocessor computer, local network, metropolitan network, and long-haul network has blurred.
In other words, the communication channel between your web browser and the web server on the Internet may be divided into many links, each with its own characteristics. This is true in general of most communication channel connections. Conversely, there may be many nodes sharing the use of a single channel or channel link. Thus, a channel or channel link may be required to carry several messages from different sources and bound for different destinations simultaneously. The requirements for data communication must include the ability to share the channel elements among many different sender-receiver pairs and to direct messages to their correct nodes, wherever those nodes might be located.
One way to view the channel is to consider the connection between the end point sender-receiver pair as the communication channel for that pair. If our prime objective is to consider the overall characteristics of the channel as a conduit for messages being sent between that pair, this view may be useful and sufficient. We noted in the previous paragraph, however, that the channel between two end points may actually consist of a number of links, with intermediate nodes connecting the links. Each link has its own characteristics of interest and in a limited sense, each link can also be described as a communication channel.
Since the channel may be made up of multiple links, the interfaces at each end of the connection may differ from each other and the characteristics of the end-to-end channel may differ from those of the individual links. For example,
the computer initiating a message might be connected to a network using a telephone modem, which transmits messages one byte at a time using audio tones as a signaling method. The receiving computer might be connected to the network using Ethernet, which expects messages formatted as digital packets consisting of many bytes of data, together with additional bytes that define the specific characteristics of the particular packet.
There are protocols and standards that define the makeup of the packets. The network must be capable of converting the message from one format to another at the intermediate nodes when required.