Thursday, 6 October 2011


Organisation chart showing the different types of multiplexing. Source: Created by GMcC 2007.
There are two basic forms of multiplexing used:

Frequency Division Multiplexing (FDM)

In FDM, multiple channels are combined onto a single aggregate signal for transmission. The channels are separated in the aggregate by their FREQUENCY.
There are always some unused frequency spaces between channels, known as "guard bands". These guard bands reduce the effects of "bleedover" between adjacent channels, a condition more commonly referred to as "crosstalk".
FDM was the first multiplexing scheme to enjoy widescale network deployment, and such systems are still in use today. However, Time Division Multiplexing is the preferred approach today, due to its ability to support native data I/O (Input/Output) channels.

FDM Data Channel Applications

Data channel FDM multiplexing is usually accomplished by "modem stacking". In this case, a data channel's modem is set to a specific operating frequency. Different modems with different frequencies could be combined over a single voice line. As the number of these "bridged" modems on a specific line changes, the individual modem outputs need adjustment ("tweaking") so that the proper composite level is maintained. This VF level is known as the "Composite Data Transmission Level" and is almost universally -13 dBm0.
Although such units supported up to 1200 BPS data modem rates, the most popular implementation was a low-speed FDM multiplexer known as the Voice Frequency Carrier Terminal (VFCT).

FDM Voice Channel Applications

Amplitude Modulation (AM), using Single Sideband-Suppressed Carrier (SSB-SC) techniques, is used for voice channel multiplexing. Basically, a 4 KHz signal is multiplexed ("heterodyned") using AM techniques. Filtering removes the upper sideband and the carrier signal. Other channels are multiplexed as well, but use different carrier frequencies.
Advances in radio technology, particulary the developments of the Reflex Klystron and integrated modulators, resulted in huge FDM networks. One of the most predominate FDM schemes was known as "L-Carrier", suitable for transmission over coaxial cable and wideband radio systems.

Time Division Multiplexing

Timeplex is probably the best in the business (IMHO) at Time Division Multiplexing, as it has 25+ years or experience. When Timeplex was started by a couple of ex-Western Union guys in 1969 it was among the first commercial TDM companies in the United States. In fact, "Timeplex" was derived from TIME division multiPLEXing!
In Time Division Multiplexing, channels "share" the common aggregate based upon time! There are a variety of TDM schemes, discussed in the following sections:
Conventional Time Division Multiplexing
Statistical Time Division Multiplexing
Cell-Relay/ATM Multiplexing

Conventional Time Division Multiplexing (TDM)

Conventional TDM systems usually employ either Bit-Interleaved or Byte-Interleaved multiplexing schemes as discussed in the subsections below.
Clocking (Bit timing) is critical in Conventional TDM. All sources of I/O and aggregate clock frequencies should be derived from a central, "traceable" source for the greatest efficiency.

Bit-Interleaved Multiplexing

In Bit-Interleaved TDM, a single data bit from an I/O port is output to the aggregate channel. This is followed by a data bit from another I/O port (channel), and so on, and so on, with the process repeating itself.
A "time slice" is reserved on the aggregate channel for each individual I/O port. Since these "time slices" for each I/O port are known to both the transmitter and receiver, the only requirement is for the transmitter and receiver to be in-step; that is to say, being at the right place (I/O port) at the right time. This is accomplished through the use of a synchronization channel between the two multiplexers. The synchronization channel transports a fixed pattern that the receiver uses to acquire synchronization.
Total I/O bandwidth (expressed in Bits Per Second - BPS) cannot exceed that of the aggregate (minus the bandwidth requirements for the synchronization channel).
Bit-Interleaved TDM is simple and efficient and requires little or no buffering of I/O data. A single data bit from each I/O channel is sampled, then interleaved and output in a high speed data stream.
Unfortunately, Bit-Interleaved TDM does not fit in well with today's microprocessor-driven, byte-based environment!

Byte-Interleaved Multiplexing

In Byte-Interleaved multiplexing, complete words (bytes) from the I/O channels are placed sequentially, one after another, onto the high speed aggregate channel. Again, a synchronization channel is used to synchronize the multiplexers at each end of the communications facility.
For an I/O payload that consists of synchronous channels only, the total I/O bandwidth cannot exceed that of the aggregate (minus the synchronization channel bandwidth). But for asynchronous I/O channels, the aggregate bandwidth CAN BE EXCEEDED if the aggregate byte size is LESS than the total asynchronous I/O character size (Start + Data + Stop bits). (This has to do with the actual CHARACTER transmission rate of the asynchronous data being LESS THAN the synchronous CHARACTER rate serviced by the TDM).
Byte-Interleaved TDMs were heavily deployed from the from the late 1970s to around 1985. These units could support up to 256 KBPS aggregates but were usually found in 4.8 KBPS to 56 KBPS DDS and VF-modem environments. In those days, 56 KBPS DDS pipes were very high speed circuits. Imagine!
In 1984, with the divestiture of AT&T and the launch of of T1 facilities and services, many companies jumped into the private networking market; pioneering a generation of intelligent TDM networks.
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