The Network Layer

   Basic Data Transmission Methods

One of the most important services of the Network Layer is to relay data between disconnected nodes. The Network Layer provides other important services, but before we discuss them, we first need to explain a few basic concepts about data transmission.

Most data links fall into one of two categories: point-to-point, which are direct channels between two systems, and switched servers, which interconnect all nodes. Switched networks include two subcategories:

Circuit Switching

Telephone service is an example of a circuit-switched network. A telephone call sets up a temporary path, called a "virtual circuit," through the public telephone switching network. A portion of network capacity is set aside for each call, and all signals for that call are sent over the same physical path until the call is finished. If a caller hangs up and immediately calls the same destination, the network may establish a different pathway for the new call. However, once again, that path is reserved for that call until it is finished.

In either case, however, both ends of the circuit are unaware of the switching necessary to set up that circuit. As we can see on the Virtual Circuit Diagram, the two nodes only know that they are communicating across the switched network. The details of the physical path are invisible.

A circuit-switched network is dependent on a completed connection. When you place a telephone call, the connection is not complete until someone at the other end of the line answers. In this way, circuit switching is like being able to reserve a lane on the highway when you visit a friend. Before you can travel, you first make sure your friend is home and that the highway has a free lane. If both of these conditions are true, you will be guaranteed an exclusive lane to drive on, and assigned a particular route to travel.

On the negative side, when traffic is light on a virtual circuit, its unused capacity is wasted. For example, long-distance telephone charges add up until you end a call, even during periods when you are not talking.