Suppose a company with a link between two cities wished to maximize the traffic between them. First, the data (or "cars") must be sent faster. Then, more lanes must be acquired. This is the basis of time division multiplexing (TDM) and wavelength division multiplexing (WDM). The capacity of this link is the sum of the speed of each of the lanes. Transmission technology deals with the concept of multiplexing, clarifying TDM and WDM.

The transport network has been defined as a set of links between telecommunication sites. Before multiplexing was discovered, each telephone call needed its own link to be transmitted. Many telephone calls needed many links, which was expensive.

A way to put more than one telephone call on each link must be found to save money. The best way to put more than one telephone call on each link is to multiplex the calls. This makes best use of the links.

The easiest way to understand multiplexing is to remember the transmission game one played as a child: two tin cans connected by a piece of string (see Figure 3). In essence, that was a private link. Multiplexing, however, enables several telephone calls to be sent on the same line. The end users have the illusion of being on their own private link. In effect, multiplexing creates a virtual telephone link for all of the users, which is an early telephone version of virtual reality. Synchronous multiplexing is TDM. Optical transmission uses a different type of multiplexing, WDM, which will be clarified later in the tutorial.

Figure 3. Multiplexing

As seen earlier, a way to put more than one call or streams of '1s' and '0s' on a line is to combine them together. Imagine a conveyer belt between two places getting fed by four other conveyer belts. If each of the feeding belts are dropping a widget to be transported every four seconds, then how fast does the middle one need to work to serve the four other belts? If it goes at the same speed, would it work?

The answer is yes, for if the system is to function effectively, the belts should be synchronized, and each widget should be sent one after the other. This means that the middle conveyor belt could carry four times more widgets than any of the other four feeding conveyor belts by placing the widgets closer together.

In TDM, all of the speeds add up, and, hence, neither SDH nor SONET have any concept of congestion or priority (see Figure 4). The size of the pipes going in a node equals the size of the traffic pipe out of a node. If the incoming traffic arrives from four places at 2.5 Gbps, the outgoing pipe will be 4 Gbps by 2.5 Gbps, which is 10 Gbps.

Figure 4. TDM Multiplexing

The resulting bit stream is converted into light using a laser and then becomes one of the input light streams of an optical multiplexer, if WDM is in the network.

In the first stage of multiplexing phone calls, the resulting bit rate is 1.5 Mbps and called T1/DS1 in North America, or it is 2 Mbps and called E1 outside of North America. It deals with the smallest stream handled by a transmission network.

Transmission systems that are designed according to European rules work with groups of 30 telephone calls. A group of 30 telephone calls is multiplexed into a 2–Mbps digital signal, and throughout most transmission documents and presentations, constant references to 2–Mbps channels may be found. These 2–Mbps streams are the basic building blocks for multiplexing.

Transmission systems that are designed according to North American rules work with groups of 24 telephone calls (see Figure 5). A group of 24 telephone calls is multiplexed into a 1.5–Mbps digital signal. These 1.5–Mbps channels are the basic building blocks for multiplexing in North America. A SONET multiplexing example shows how 84 T1 or DS1 combine into a 155–Mbps stream. An SDH picture would show how 63 E1 combine into a 155 Mbps stream.

Figure 5. Multiplexing Examples