One-way and two-way (both-way) circuits; two-wire and four-wire circuits
"One-way" and "two-way" circuits are terms familiar to the switching technologist. Most of us in the field of telecommunication just sort of assume that there is the capability of transmitting and receiving on a particular circuit and we would rather not get involved on how that is accomplished. Let us first look at it from point of view of the switching or network specialist. Consider switching node A and switching node B. There is a high community of interest between these two switching centers. We would expect some fair number of trunks inter-connecting these two nodes. Let's say 100. For some reason we always assume symmetry, thus 50 circuits in one direction and 50 in the other. Here a subscriber connecting through switch A requires one circuit to talk to another subscriber reached through switch B. But that subscriber through switch B wants to reply to the Switch A subscriber. This requires another circuit. So the 100 trunks will only support 50 duplex conversations. Now confusion arises.
This is a strategy we are using here to help switch design. One hundred trunk circuits are established between node A and Node B. The first fifty circuits will only carry traffic initiated at Node A bound for Node B. The second fifty trunk circuits will only carry traffic initiated at Node B bound for Node A. These circuits are one-way circuits
The IEEE dictionary defines duplex as "a simultaneous, two-way, independent transmission in both directions." This implies two separate paths. The incisive reader will ask why we can not do this with just one path that is "time-divided", allowing slots of time for one direction and slots of time for the other. We just assume that there are two wires for transmitting in one direction and another pair of wires to support transmission in the other direction. This is where we derive the terms "two-wire" transmission and "four-wire" transmission.
Under many circumstances we are forced to physically separate the signals being transmitted from those being received on a common conversation. The separation may be in space, time or frequency. Examples where separation is mandatory are: digital multiplex circuits, wide-area data networks and some types of local area networks. In other cases the operating entity can employ the more economical half-duplex operation.
Half-duplex operation is defined (IEEE Dictionary) as "pertaining to transmission over a. circuit capable of transmission in either direction but only (in) one direction at a time." Many military and public safety tactical radio circuits are half-duplex.
An example of two-wire, full-duplex transmission is the conventional telephone handset that is connected to its local serving switch. There is also the simplex mode of transmission that is exclusively one-way. Commercial television and radio broadcast are one-way or simplex systems.
Some switching entities find it efficient to have a group of circuits for one-way operation in one direction and an equal number of one-way circuits for operation in the other direction. They then will have a third, smaller group of circuits for both-way operation. This third group can accommodate service in either direction to compensate for asymmetrical operation. For example, in the evening there is a higher call density from urban to suburban areas when the working spouse calls home. In the morning on work days the opposite is true.
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