ABSTRACT
The most familiar telecommunications system is the public telephone network. The basic building block of this system is the voice channel. Amplifiers in the transmit and receive pairs prohibit two-way communications in the channel. Dial-up telephone networks consist of two simplex transmission paths between central offices and a full-duplex transmission path in each subscriber loop. Half-duplex modems must delay the transmission of data long enough to ensure that the echo suppressors are out of the circuit. Modems, when classified according to their required transmission bandwidth, can be divided into three categories: subvoice, voice, and wideband. Frequency-division multiplexing (FDM) or time-division multiplexing (TDM) techniques are used to fill the voice channel. FDM and TDM are equally suitable for voice-grade channels. However, TDM, because it is more efficient in its bandwidth utilization, and because more channels can be multiplexed on a single channel, is seldom used to multiplex low-speed signals on one single-voice channel.
