Simplex and duplex channels
If two people want to talk to each other, it seems to make sense for them both to use the same frequency. There is, however, one slight snag with this idea…
When you are using one frequency, you can either receive, or transmit. You can’t do both at once. Even if you had two separate radios and aerials, you still couldn’t receive the other boat’s transmissions at the same time as you were transmitting yourself, because your own transmissions would drown out those from the other vessel.
So using a single-frequency channel demands a degree of discipline and standard procedures to decide who is to talk and who is to listen, and how they are to switch roles when necessary to achieve two-way communication. The principle, however, is simple, so this method of operating on a single frequency is known as simplex, and the single-frequency channels are known as simplex channels.
Simplex operation is all very well between two operators who know what they are doing, and understand the limitations of a simplex channel. It is far less satisfactory if, for instance, you are using VHF to make a link call to your office, or to wish your mother in law a happy birthday.
If you are talking on the telephone to someone who remains totally silent, you very quickly begin to wonder whether there is something wrong with the phone! That’s because normal conversation is a constant, two way process, even if one party does little more than grunt.
Normal, two-way conversations can be achieved by radio, by using two frequencies. One station transmits on one frequency, while the other transmits on the other. Of course, they must both have their receivers set up to receive the frequency that the other is transmitting.
This two-way, two-frequency operation is known as duplex, so the two-frequency channels are known as duplex channels. They are used, in the main, for public correspondence (link calls), for which two-way traffic is essential, and for some port operations channels.
You can compare the flow of radio traffic in simplex and duplex channels to the flow of traffic along a road:-
A simplex channel is like a single track road: traffic can only flow in one direction at a time, and two way traffic can only be achieved by taking it in turns
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