Multiplexing

Multiplexing is the transmission of multiple data communication sessions over a common wire or medium. Multiplexing reduces the number of wires or cable required to connect multiple sessions. A session is considered to be data communication between two devices: computer to computer, terminal to computer, etc..

Individual lines running from 3 terminals to one mainframe is not a problem but when the number of terminals increases to 10 and up, it becomes a problem. Imagine a mainframe computer with 1200 terminals connected and each terminal running its own wire to the mainframe. If each wire was 1/4" in diameter (typical Cat 5 cable), you would have a wiring bundle going into the computer, roughly 2 feet in diameter.

A multiplexer allows sharing of a common line to transmit the many terminal communications as in the above example. The connection between the multiplexer and the mainframe is normally a high speed data link and is not usually divided into separate lines.

The operation of multiplexers (abbreviated MUXs) is transparent to the sending and receiving computers or terminals. Transparent means that as far as everyone is concerned, they appear to be directly connected to the mainframe with individual wires. The multiplexer does not interfere with the normal flow of data and it can allow a significant reduction in the overall cost of connecting to remote sites, through the reduced cost of cable and telephone line charges.

Multiplexers are used to connect terminals located throughout a building to a central mainframe. They are also used to connect terminals located at remote locations to a central mainframe through the phone lines.

There are 3 basic techniques used for multiplexing:

  • Frequency Division Multiplexing (FDM)
  • Time Division Multiplexing (TDM)
  • Statistical Time Division Multiplexing (STDM)

FDM - Frequency Division Multiplexing

Frequency Division Multiplexing (FDM) is an analog technique where each communications channel is assigned a carrier frequency. To separate the channels, a guard-band would be used. This is to ensure that the channels do not interfere with each other.

For example, if we had our 3 terminals each requiring a bandwidth of 3 kHz and a 300 Hz guard-band, Terminal 1 would be assigned the lowest frequency channel 0 - 3 kHz, Terminal 2 would be assigned the next frequency channel 3.3 kHz - 6.3 kHz and Terminal 3 would be assigned the final frequency channel 6.6 kHz - 9.6 kHz.

The frequencies are stacked on top of each other and many frequencies can be sent at once. The downside is that the overall line bandwidth increases. Individual terminal requirement were 3 kHz bandwidth each, in the above example: the bandwidth to transmit all 3 terminals is now 9.6 kHz.

FDM does not require all channels to terminate at a single location. Channels can be extracted using a multi-drop technique, terminals can be stationed at different locations within a building or a city.

FDM is an analog and slightly historical multiplexing technique. It is prone to noise problems and has been overtaken by Time Division Multiplexing which is better suited for digital data.

TDM - Time Division Multiplexing

Time Division Multiplexing is a technique where a short time sample of each channel is inserted into the multiplexed data stream. Each channel is sampled in turn and then the sequence is repeated. The sample period has to be fast enough to sample each channel according to the Nyquist Theory (2x highest frequency) and to be able to sample all the other channels within that same time period. It can be thought of as a very fast mechanical switch, selecting each channel for a very short time then going on to the next channel.

Each channel has a time slice assigned to it whether the terminal is being used or not. Again, to the send and receiving stations, it appears as if there is a single line connecting them. All lines originate in one location and end in one location. TDM is more efficient, easier to operate, less complex and less expensive than FDM.

STDM - Statistical Time Division Multiplexing

Statistical Time Division Multiplexing uses intelligent devices capable of identifying when a terminal is idle. They allocate time only to lines when required. This means that more lines can be connected to a transmission medium as this device statistically compensates for normal idle time in data communication lines. Newer STDM units provide additional capabilities such as data compression, line priority, mixed speed lines, host port sharing, network port control, automatic speed detection and much more.

Telecommunication Multiplexing

Traditional telecommunication multiplexing is used between switching offices on Interoffice trunks and Intertoll trunks. The Telcos (telecommunication companies such as Bell Canada, AGT, BC-Tel, etc..) share communication facilities which can be either analog FDM or the digital TDM. A communication path can change in mid-stream from FDM to TDM and back again depending on where or whose communication facility is being used.

FDM is analog and has been updated to TDM throughout the world. There are only a very few locations left where FDM is being used.

FDM - Channel Groups

Telecommunications FDM is based on channel groups. The basic channel is called the Voice Channel and it has a bandwidth of 0-4 kHz. The channel groups are based on multiples of the voice channel:

Multiplex LevelVoice Circuits Freq Band
(kHz)
BW
(kHz)
Voice Channel10 - 44
Group1260 - 10848
Supergroup 60312 - 552240
Mastergroup600564 - 3,0842520
Jumbogroup3600564 - 17,54816984

  • The Mastergroup and Jumbogroup have guard-bands added to the bandwidth.
  • A Group is made of 12 Voice Channels.
  • A Supergroup (60 Voice channels) is made of 5 Groups (12 Voice Channels).
  • A Mastergroup (600 Voice Channels) is made of 10 Supergroups (60 Voice Channels).
  • A Jumbogroup (3600 Voice Channels) is made of 6 Mastergroups (600 Voice Channels).

TDM - T1 Carrier System

Telecommunications TDM is based on the T1 Carrier System. It is a digital system that digitizes the analog Voice Channel into 8 bit data. This means that there are 2^8 or 256 levels that the 8 bit data can represent.

It samples the analog signal 8000 times a second (2x 4 kHz - makes Nyquist happy!). It is a serial data stream so we transmit the 8 bit data 1 bit at a time. This means that for a digitized Voice Channel, the data rate is:

8 bits x 8000 samples = 64 Kbps

The basic Carrier used in the T1 Carrier System is called the T1 (sometimes called DS-1) and it carries 24 Voice Channels. The Bit Rate for the T1 Carrier is 1.544 Mbps. If we multiply:

24 Voice Channels x 64 Kbps per Voice Channel = 1.536 Mbps

The missing 8 K is used to "frame" the data. It is information used for the Start Frame bytes, End Frame, Error Checking, Routing information, etc..

Digital CircuitVoice ChannelsBit Rate# of T1 Circuits
T1 (DS-1)
24
1.544 Mbps
1
T2 (DS-2)
96
6.312 Mbps
4
T3 (DS-3)
672
44.736 Mbps
28
T4 (DS-4)
4032
274.176 Mbps
168

Typically:

  • T1 - Twisted Pair or Coax Cable
  • T2 - Coax Cable
  • T3 - Coax, Fibre Optics or Light Route Radio
  • T4 - Coax or Fibre Optics

You can rent any quantity of a T1 line, you don't have to rent the complete circuit. You basically rent a time-slot on the line based on 64 kbps channels. This is called Fractional T-1.

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Copyright July 2013 Eugene Blanchard