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Multiplexers
TDM and STDM data transport is accomplished through the use of a multiplexing
device. A multiplexer has three basic components: discrete channel inputs (DCI), a
framer, and a carrier interface.
• DCI—DCI is the input/output interface of the multiplexer that connects to
the data terminal device, which generates the data signal that is to be
multiplexed and transmitted over the carrier transport.
• Framer—The framer is a scanning and distributing device that samples the
various DCI signals and encodes them, assigns them a time-slot, frames
them, and transmits them over the carrier transport. Depending on the type
of multiplexing scheme used (TDM/STDM), buffering might be used to
accommodate the data flow over the transport.
• Carrier interface—Multiplexers can support one or more carrier transport
lines. The carrier interface is where the PSTN digital carrier line terminates.
Depending on the type of multiplexer, the carrier circuit might terminate
directly or through an external Channel Service Unit (CSU). We will look at
the functions performed by the CSU a little later in this section. Figure 5.1
illustrates the logical structure of a multiplexer.