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Tuesday, October 31, 2006

What does Simplex (and half duplex) really mean?

In communications engineering there are three basic forms of communication:
- Simplex
- Half-Duplex
- Full-Duplex

The last of these terms (Full-Duplex) is probably the only clearly defined standard that everyone agrees on. Full-Duplex is where both stations on the communications link can simulatneously (or seemingly simultaneoulsy) send and receive data - like when you are using a mobile phone.

Simplex and Half-Duplex are where the problems come in. It doesn't help when the major communications standards organisations can't even agree on what they mean. For simplex the ITU definition is where signals flow in only one direction at a time. The ANSI definition is where signals flow in one direction - period!

For Half-Duplex the definitons are:
ITU: This seems to be the same as the simplex definition (although strangely according to wikipedia the ITU possibly define half duplex as the ANSI simplex definition - which makes no sense at all)
ANSI: Signals flow in only one directon at a time (i.e. the ITU's definition of simplex)

So which is correct? Well it really depends on what you are studying. I'm currently completing two final year communications subjects, with different lecturers choosing opposite definitions. I guess both are right in a silly sort of way - so much for having standards.

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