Layers of Internet Protocols

For devices to communicate successfully over a packet-switched network, it is necessary for them to work to the same rules. A set of rules of this kind is called a protocol. Rather than a single protocol, the workings of packet-switched networks, including the Internet, were conceived as a hierarchy of layers. This has the advantage that different solutions can be substituted for one another at each layer. For example, the underlying transmission medium can be twisted-pair copper cable (which exists in vast quantities because that was the dominant form of wiring for voice services for a century), co-axial cable (which is used for cable-TV and for Ethernet), fibre-optic cable, or a wireless medium using some part of the electromagnetic spectrum. This layering provides enormous flexibility, which has underpinned the rapid changes that have occurred in Internet services.

The deepest layers enable sending devices to divide large messages into smaller packets, and generate signals on the transmission medium that represent the content of the packets; and enable receiving devices to interpret those signals in order to retrieve the contents, and to re-assemble the original message. The mid-layer protocols provide a means of getting the messages to the right place, and the upper-layer protocols use the contents of the messages in order to deliver services. Exhibit 3.3 provides an overview of the layers as they are currently perceived.
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