Multiplexing Techniques

Multiplexing is the process where multiple channels are combined for transmission over a common transmission path.

There are two predominant ways to multiplex:

Frequency Division Multiplexing
Time Division Multiplexing

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).

VFCT Go here for more information about the Voice Frequency Carrier Terminal.


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.

L_CARRIER Go here for more information about L-Carrier FDM systems.


Time Division Multiplexing

Timeplex was probably the best in the business (IMHO) at Time Division Multiplexing, as it had 30+ years of 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.

T1 FRAMING Go here for more information about North American T1 framing.


E1 FRAMING Go here for more information about CCITT (ITU-T) E1 framing.


Statistical Time Division Multiplexing (STDM)

Statistical TDMs are such that they only utilize aggregate bandwidth when there is actual data to be transported from I/O ports. Data STDMs can be divided into two categories:

Conventional STDM
Frame Relay/X.25 Networking

An additional ANALOG TDM system, known as Time Assignment Speech Interpolation is also discussed.

Conventional STDM

The Statistical Multiplexer (or “statmux”) utilizes a different form of Time Division Multiplexing. These multiplexers typically use a HDLC-like frame for aggregate communications between units. As I/O traffic arrives at the mux it is buffered, then inserted into the I-Field of the HDLC frame. The receiving units removes the I/O traffic from the aggregate HDLC frame.

Statistical Multiplexers are ideally suited for the transport of asynchronous I/O data; as it can take advantage of the inherent latency in asynchronous communications. However, they can also multiplex synchronous protocols by “spoofing” and prioritization; again taking advantage of the latency between blocks/frames.

Statistical Multiplexers are typically faster at transporting I/O data End-To-End than X.25 systems, but some of these multiplexers can also perform network switching functions between I/O ports. The total I/O bandwidth can (and usually does) exceed the aggregate port bandwidth.

Later, many of these multiplexers incorporated “switching” mechanisms that allowed I/O ports to “intelligently” connect themselves to other destination ports upon user command. While somewhat functioning as an X.25 switch, these Statistical Multiplexers were usually faster, and provided more transparent I/O data-carrying capacity.

Statistical TDM’s biggest disadvantage is that it is I/O protocol sensitive. Therefore, they have difficulty supporting “transparent” I/O data and unusual protocols. To support these I/O data types, many statmux systems have provisions to support Conventional TDM I/O traffic through the use of adjunct/integrated modules.

Conventional STDM was very popular in the late 1970s to mid 1980s and is still used today, although the market for these units is dwindling.

Frame Relay and X.25 Networking

Frame Relay and X.25 systems are also categorized as Statistical TDMs. Both of these systems utilize aggregate HDLC frame structures, and both of these systems can interoperate with both Private and Public systems.

The advantage of Frame Relay over X.25 is that it can support the same traffic as X.25, while, while facilitating “bandwidth on demand” requirements for “bursty” traffic (e.g. LANs). Public Frame Relay services are available, offering customers additional methods to interconnect LANs, rather than having dedicated Wide Area Network (WAN) links.

Frame Relay, however, cannot adequately support voice or video traffic because of variable End-To-End delivery times (e.g. variable delay). Voice and video transmissions are of a “Constant Bit Rate” (CBR) nature, and do not fare well sitting in a queue waiting for a big LAN packet to finish transmitting.

Care must be taken when deploying Frame Relay technologies. In the Internet world, it is so easy to overload trunk capacities at the end-points of an IP connection. In fact, this overloading is a “cheap” way to add increased I/O traffic (and users) without increasing aggregate bandwidth. Unfortunately, when Frame Relay starts chucking out I/O data frames, the impact on Internet applications is very noticeable, since IP retransmissions are so long! This same detrimental effect is also experienced in wireless LANs.

Time Assignment Speech Interpolation (TASI)

TASI systems represent an example of an ANALOG Statistical Time Division Multiplexing scheme. These systems enjoyed limited use in the 1980s, and were particularly adept at sharing voice circuits; specifically PBX trunks.

A TASI multiplexer is interconnected between the PBX and the trunk facilities. Usually, one analog trunk circuit is used for signaling purposes between TASI units at each end of the link. The remaining voice trunks support analog TASI TDM voice conversations.

In normal telephone conversations, a majority of time is spent in a latent (idle) state. TASI trunks will allocate “snippets” of voice from another channel during this idle time. If an individual were to monitor these TASI trunks, they would hear bits and pieces of various conversations. The signaling channel is used for the signaling conversion between End-Point PBX (Private Branch Exchange) units and also for the allocation of bandwidth once incoming speech energy has been detected.

As digital speech processing became more common, TASI systems were created that had analog inputs, and digital outputs. This type of multiplexing technique is more commonly known as “Digital Speech Interpolation” (DSI).

Unfortunately, TASI and DSI systems suffer from a few drawbacks. First, there can be a lot of voice “clipping” noticed by users. This occurs when a little bit of speech is lost while waiting for the TASI mux to detect valid speech and allocate bandwidth. Clipping also occurs when there just isn’t bandwidth present at the moment. Also, TASI and DSI units are very susceptible to audio input levels and may have problems with the transport of voiceband data (e.g. VF modem) signals.

Cell-Relay Transmission

In Cell-Relay systems, data is broken up into basic units (called “cells”) and transported through the network. A standard cell-size is defined as consisting of 53 8-bit bytes. These 53 bytes consist of 48 bytes of Payload (data) and 5 bytes of Header (routing) information.

Cell-Relay operation is somewhat analogous to a processor bus. Instead of a 32-bit data buss, there is a 53-byte data buss. Instead of a 64-bit address buss, there is a 5-byte address buss. While the bus operates synchronously, under control of a buss clock, the buss function itself is asynchronous (similar in operation to an ordinary microprocessor bus). But instead of the transfer being parallel, a a high-speed serial ATM facility is used. That’s Cell Relay!

As mentioned in the paragraph above, the buss function is asynchronous. That means that the I/O data (CPU modules in the above example) will immediately arbitrate for the facility (processor buss) when there is data destined for it. If there is a conflict, somebody loses, and data is lost. It is up to the application to recover (or not!) from the error condition.

Cell-Relay operation can be summarized as being similar to Conventional TDM, except that is has properties of asynchronous transfer. When operating with very high speed facilities, Cell-Relay has the ability to integrate Local Area Networks (LANs) and Wide Area Networks (WANs).

Some Cell-Relay transmission services are now under development, or available on a limited basis:

Asynchronous Transfer Mode (ATM)
Switched MultiMegabit Data Service (SMDS)

Asynchronous Transfer Mode (ATM)

ATM is a cell-based transport mechanism that evolved from the development of the Broadband ISDN (B-ISDN) standards. ATM does not stand for Automatic Teller Machine, or Ascom Timeplex Multiplexers (although that might apply!); rather, it defines the asynchronous transport of cells (Cell-Relay). Perhaps even more important, ATM is associated with a process known as ATM Adaptation Layer. AAL describes how various I/O traffic types are converted into cells!

The Adaptation process and the serial transport of cells is commonly referred to as “Fast Packet Multiplexing” (FPM). While similar in concept, FPMs do not necessarily conform to ATM standards or switching conventions.

Switched MultiMegabit Data Service (SMDS)

Similar to ATM, but designed for operation at lower rates (64 KBPS – 155.520 MBPS). SMDS services ARE AVAILABLE NOW from many of Local Exchange Carriers (LECs). International and domestic Carrier services are available as well.

SMDS offers customers alternatives to Frame Relay transport. SMDS addressing utilizes the CCITT (now ITU) E.164 addressing scheme, making addressing much more manageable for customers. Also, SMDS is available at higher rates than Frame Relay (which typically tops out at T1 rate – 1.544 MBPS).