Internet Core Protocols: The Definitive Guide

Data-Link Services

When two devices on a network communicate with each other, they don’t use IP to do so. Rather, they use protocols that are specific to the wire itself. For example, devices on an Ethernet segment use a predefined series of electrical impulses to communicate with each other. Whenever an Ethernet device wants to send data to another device on the same network, it raises and lowers the voltage of the shared medium so that a series of “on” and “off " voltage patterns are generated. These changes in voltage are interpreted as bits by the other devices on the network.

The changes in voltage are dictated by protocols that are specific to the different types of physical networks. Ethernet networks have data-link protocols that will not work with technologies like Token Ring. Similarly, modems use protocols specific to different types of modem technology.

Much of IP’s functionality is determined by the physical media that the IP device is connected to. When an IP device has information that it needs to send to another device on the same wire, it has to understand the characteristics of the wire in order to prepare the information so that is usable for that particular medium.

One of the issues that IP has to deal with is the mechanisms used for the network-specific addressing. Just as physical networks have to provide mechanisms for encapsulating and disseminating data on the wire, they also have to provide a way for devices to locate each other, using addressing methods ...