# Amateur Extra: Digital Time Division Multiplexing

The Extra-Extra-Extra Read All About it question of the week is a real beauty. [E8B12]

## What is digital time division multiplexing?

A. Two or more data streams are assigned to discrete sub-carriers on an FM transmitter
B. Two or more signals are arranged to share discrete time slots of a data transmission
C. Two or more data streams share the same channel by transmitting time of transmission as the sub-carrier
D. Two or more signals are quadrature modulated to increase bandwidth efficiency

Much like last time, whoa, come on!  Trust me, though, this one is pretty easy to remember.

Take a deep breath, close your eyes, and contemplate your navel, I mean the question.  We can automatically throw answers A and D out, because they make no mention of time.  Time is the key here.

Now take another breath, and think of the next word, division.  Discrete?  Bingo.  You just solved it.  The correct answer is B. Two or more signals are arranged to share discrete time slots of a data transmission.  C. Two or more data streams share the same channel by transmitting time of transmission as the sub-carrier is close, at least it sounds close, but is actually meaningless.  Transmitting the time of the transmission is rather pointless.

But what is “Time Division Multiplexing” anyway.  Short version:  If your mobile phone uses TDMA, then you’re using it on a daily basis.  It’s what allows all those conversations to co-exist on the same radio frequencies simultaneously.

Think of it like this:  Pretend there are 10 people sitting in a circle.  Each of them has one second to talk before the next one starts talking.  The cycle goes round-and-round, and if you were to then remove every other voice except for the one you wanted to listen to, you could understand what was being said.  Now imagine that happening at a much higher speed.  All you have to do is grab the “chunk” of the signal that happens every .002 seconds (as opposed to .001 or .005) and ignore the rest for that cycle.