INTRODUCTORY TOPICS IN TELECOMMUNICATIONS
Simplex, Half-Duplex, and Full Duplex
These are operational terms, and they will be used throughout this text. Simplex is one-
way operation; there is no reply channel provided. Radio and television broadcasting are
simplex. Certain types of data circuits might be based on simplex operation.
Half-duplex is a two-way service. It is defined as transmission over a circuit capable
of transmitting in either direction, but only in one direction at a time.
Full duplex or just duplex defines simultaneous two-way independent transmission on
a circuit in both directions. All PSTN-type circuits discussed in this text are considered
using full-duplex operation unless otherwise specified.
One-Way and Two-Way Circuits
Trunks can be configured for either one-way or two-way
operation. A third option is a
hybrid where one-way circuits predominate and a number of two-way circuits are provided
for overflow situations. Figure 1.8a shows two-way trunk operation. In this case, any trunk
can be selected for operation in either direction. The incisive reader will observe that there
is some fair probability that the same trunk can be selected from either side of the circuit.
This is called double seizure. It is highly undesirable. One way to reduce this probability
is to use normal trunk numbering (from top down) on one side of the circuit (at exchange
A in the figure) and to reverse trunk numbering, from the bottom up at the opposite side
of the circuit (exchange B).
Two-way and one-way circuits: two-way operation (a), one-way operation (b), and a hybrid
scheme, a combination of one-way and two-way operation (c).
Called both-way in the United States and in ITU-T documentation.