Multiplexing enables one physical medium to carry multiple signals. A transport network is a set of links between sites. To put more than one call on each link is to give each call a time slot (a place in a schedule) and transmit several calls simultaneously. This process is known as time division multiplexing (TDM). With multiplexing, the end users have the illusion of being on their own private links. Transmission systems that are designed according to North American rules work with groups of 24 telephone calls (see Figure 3).

Figure 3. 24 Telephone Lines Multiplexed into a 1.5 Mbps Signal

With SONET, multiplexing is TDM, not statistical. Hence, there is no concept of congestion or priority in SONET. The traffic flowing in and out of a node is exactly equal; there is no peak rate. A pipe of a certain size is given and is there whether data flows or not.