An example of a technology that uses synchronous TDM is ISDN. ISDN basic rate interface
(BRI) has three channels: two 64-kbps B channels (B1 and B2) and a 16-kbps D channel.
The TDM has ten time slots, which are repeated in the sequence shown in Figure 2-8.

Figure 2-8 TDM Example: ISDN


On a larger scale, the telecommunications industry uses the SONET or SDH standard for
optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere,
are two closely related standards that specify interface parameters, rates, framing formats,
multiplexing methods, and management for synchronous TDM over fiber.

Figure 2-9 shows an example of statistical TDM. SONET/SDH takes n bit streams, multiplexes
them, and optically modulates the signal, sending it out using a light-emitting device
over fiber with a bit rate equal to (incoming bit rate) * n. Thus, traffic arriving at the
SONET multiplexer from four places at 2.5 Gbps goes out as a single stream at 4 * 2.5
Gbps, or 10 Gbps. This principle is illustrated in the figure, which shows an increase in the
bit rate by a factor of 4 in time slot T.

Figure 2-9 STDM Example: SONET 


The original unit used in multiplexing telephone calls is 64 kbps, which represents one
phone call. This is referred to as a DS0 (digital signal level zero). In North America, 24 DS0
units are multiplexed using TDM into a higher bit-rate signal with an aggregate speed of 
1.544 Mbps for transmission over T1 lines. Outside North America, 32 DS0 units are multiplexed
for E1 transmission at 2.048 Mbps.

Table 2-1 shows the signal level hierarchy for multiplexing telephone calls. As an aside,
although it is common to refer to a 1.544-Mbps transmission as a T1, it is more correct to
call it DS1.

Table 2-1 DS0 Units
 
T-carrier refers to the bundling of DS0s. For example, a T1 equals 24 DSOs, a T1C equals
48 DSOs (or two T1s), and so on. Figure 2-10 shows a sample T-carrier infrastructure hierarchy.
The E-carrier hierarchy is similar.

Figure 2-10 T-Carrier Hierarchy