An example of a technology that uses synchronous TDM is ISDN. ISDN basic rate (BRI) has three channels consisting of two 64 kb/s B-channels (B1 and B2), and a 16 kb/s D-channel. The TDM has nine timeslots, which are repeated in the sequence shown in the figure.
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.
The figure displays 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) x n. Thus traffic arriving at the SONET multiplexer from four places at 2.5 Gb/s goes out as a single stream at 4 x 2.5 Gb/s, or 10 Gb/s. This principle is illustrated in the figure, which shows an increase in the bit rate by a factor of four in time slot T.
The original unit used in multiplexing telephone calls is 64 kb/s, which represents one phone call. It 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 Mb/s for transmission over T1 lines. Outside North America, 32 DS0 units are multiplexed for E1 transmission at 2.048 Mb/s.
The signal level hierarchy for multiplexing telephone calls is shown in the table. As an aside, while it is common to refer to a 1.544 Mb/s transmission as a T1, it is more correct to refer to it as DS1.
T-carrier refers to the bundling of DS0s. For example, a T1 = 24 DSOs, a T1C = 48 DSOs (or 2 T1s), and so on. The figure shows a sample T-carrier infrastructure hierarchy. E-Carrier Hierarchy is similar.