Statistical Multiplexers Frequency Division Multiplexers function like modems, modulating digital data into analog form for transmission over phone lines. Time Division Multiplexers are all digital, dividing high speed digital channels into digital sub-channels. Bit interleaving maintains the order and number of bits from end to end. Synchronous protocols require maintaining the number and order of each block to ensure correct calculations are made at the receiving end to insure correct 8-bit bytes. Character Interleaving is used primarily for asynchronous data.
Asynchronous data is packaged within start and stop bits.
The extra space is used for more channels or to pass control signals. TDM's assign a slot to each channel whether the low speed channel is active or not. Statistical Multiplexers reduce high speed channel idle time by concentrating or compressing data from multiple channels together using microprocessors, buffers, and synchronous protocols.