More on T-1 and Time Division Multiplexing

All T carrier signals (e.g., T-1, T-3, E-3) are based on time division multiplexing. Each device that communicates over a T-1 line is assigned a time slot. If there are eight telephones contending for a T-1 circuit, a time slot is saved for each telephone for the duration of the particular telephone call. For example, telephone 1 might be assigned slot A; telephone 2, slot B; and so forth. During pauses in voice conversations, the slot is not assigned to another computer. The assigned time slot is transmitted without any bits. This is why time division multiplexing is not an efficient way to use a WAN. Pauses in data transmission result in idle time slots. In a network with millions of time slots, this can result in many idle time slots and wasted bandwidth.
More on time division multiplexing: http://en.wikipedia.org/wiki/Time_division_multiplexing