Business Data Communications Multiplexing and Inverse Multiplexing

Multiplexing Taking a single speed circuit and dividing it up into multiple, slower circuits. Two basic ways to do it Frequency division multiplexing (FDM) Time division multiplexing (TDM) DCE And makes it look like this. DCE Multiplexing takes a circuit like this…

Frequency Division Multiplexing You see an example every time you turn on your radio. Different stations are assigned different frequencies – a small part of the total available bandwidth. Each station broadcasts its own signal. You can tune your equipment to only pick up a certain signal at a certain frequency, excluding all others. Frequencies used cannot be right beside each other because of interference – guard bands must be left. So what’s the big deal?

Time Division Multiplexing The entire bandwidth is used by each signal but… Each party using the circuit has to take turns. Only one party can go at a time. Each party gets a crack at the circuit,its time slot, for a given amount of time before control passes to the next party. Eventually, control will come back around though. Obviously, each party is going to perceive the circuit as being slower than it really is.

Problems with Multiplexing FDM wastes bandwidth because of the necessity of having guard bands and because bandwidth remains reserved for a party whether it uses it or not. TDM wastes bandwidth because there will be times when the assigned party will not use its time slot.

Statistical Time Division Multiplexing STDM or Statmux allocates time slots based on demonstrated need. The more a party needs, the more it gets. Reduces the number of wasted slots, increasing the apparent throughput of the circuit. Parties that don’t need much still get all they need, so they’re happy. Parties that need more get more out of the slots that would have been unused.

Inverse Multiplexing DCE And makes it look like this. DCE Inverse multiplexing takes a group of circuits like this… Taking multiple slow(er) circuits and combining them to appear as one high(er) speed circuit. This is how T-carrier circuits are created.

Use of Muxxing and Imuxxing Both are very common. Muxxing is used where there is a need to share a circuit and collisions cannot be allowed. In the ether, for one place! There are other ways to multiplex (CDMA for one) but they are weird. Imuxxing is used to create T-carrier circuits, a common service provided by the phone companies for carrying lots of data.