A transmission signal has two components. The first is the information to be sent, which is represented by '1s' and '0s'. The second is the optical signal that travels on the fiber. It is worth making the distinction here that all of the network elements' functions covered here exist in the synchronous as well as the optical layer.

The principles are the same; i.e., the elements in each layer just process different things. For example, the synchronous layer handles the '1s' and '0s' (the signal transmitted optically between the points is translated back to electrical for processing in the node), whereas the optical layer juggles light signals only and is all about prisms and mirrors.

How do light and '1s' and '0s' relate? The data to be sent is in the form of an electrical signal or a series of electrical voltages represented by '1s' and '0s'. The '1s' and '0s' are translated into pulses of light by a laser. As an appropriate analogy, imagine that you are in the dark with a torch, and your means of communication is to switch your torch on and off. After a code is established, communication may occur. The other end of the link needs someone with a good eye to detect the light pulses, and this is the equivalent of what the photo diode does to reconstitute the electrical signal from the pulses of light received.

The transmission path is like a road; it is capable of going in two directions. Most telecommunications circuits are bidirectional, meaning that they can transmit and receive—except, perhaps, when dealing with broadcasting or video distribution applications.

If each data switch, telephone exchange, or radio terminal is called a point, then a transport network is something that enables these points to be connected. The transport network provides the ability to carry traffic between the points. Transmission is like plumbing in that each pipe is set up according to a certain size, and it remains the same whether data flows through it or not. Other technologies such as asynchronous transfer mode (ATM) have some virtual circuits (VCs) that do not take bandwidth if no data flows. This does not occur in the transmission network; resources are allocated and reserved, and no facilities are overbooked.

Also, congestion does not take place in transmission networks. If two signals enter an element, the signal going out is designed to be big enough to accommodate the two coming in.