David Sudjiman

Mul-ti-plex [m´ulte pl`eks] n (plural mul-ti-plex-es)
electronic engineering multiple transmission: the simultaneous transmission
of two or more signals along one communications channel

(Microsoft Encarta Reference Library, 2004) [3].

‘Multiplexers (MUXs) act as both concentrators and contention devices to allow multiple, relatively low-speed terminal devices to share a single, highcapacity circuit (physical path) between two points in a network. ‘ (Horak 1996) [2]. Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. (Whatis.com 2004) [4].

MUX exists in both end-point. At one point, it combines several circuits, usually a set of four or multiples of four into one high-capacity circuit and the other end-point is to split them back into several circuits. By putting the wire all together in a single high-capacity line, multiple communications can now be achieved. Multiplexing can be implemented in current transmission media such as twisted pair, coaxial, fiber optic cables, microwave, satellite and radio systems.

The main advantage to use multiplexing is for economic reason. (Horak 1996) [2]. Imagine that without using multiplexer shown in Figure 1, each computer will need its own dedicated physical connection which can cost more rather than combining several circuit into one.

Multiplexing works in a such way that the hosts will not experience what is happening in between. It is MUX job to craft the electric signal and put it into one high-capacity circuit. On another end, other MUX will examine the signal and forward it to a designated host.

As the technology of transmission medium is constanly advancing time to achieve faster delivery and bigger amount of data that can be carried, Multiplexing evolves as well to take advantages of newer technology.

Figure 1: Multiplexed Circuit

Nowadays, there are several methods to carry data in a multiplexed environment. This article will describe the varieties of multiplexing technology based on a chronological order. These methods are Frequency Division Multiplexing, Time Division Multiplexing, and Statistical Division Multiplexing.

Frequency Division Multiplexing

Horak (1996) [2] explain that Frequency Division Multiplexing (FDM) can be described
by dividing the single high-capacity channel into several smaller-capacity
channels (sub-channel). Each sub channel transmits data simultaneously using
different frequency so that each sub-channel has its own frequency to use and
is not affecting other subchannels.

A radio is A good example to explain how FDM works. Note, that we
are only using one broad range of radio frequency and there are several radio
stations broadcasting its service using different frequency. All we need to do is
to adjust the radio to catch only certain radio broadcast on certain frequency.
(Fitzgerald 1002, p.96) [1].

According to Horak (1996) [2] FDM has drawback by dedicating such frequency
to several smaller circuits even though the designated channel is not
using it.

Figure 2 illustrate how FDM works by dividing one channel into several frequencies including the Guard-band act as delimiter for each logical sub-channel
so that the interference from other sub-channel using the same physical circuit
can be minimized. For example, in Figure 2, the multiplexed circuit is divided
into 4 frequencies. Channel #1 using 0-800 Hz for its data transfer and delimited
by 200 Hz Guard-band. Channel #2 using 1000-1800 Hz and delimited by
200 Hz too; and so on.

Figure 2: Frequency Division Multiplexing

In regards to speed, we simply need to divide the main circuit amongst
the availabel subchannels. For example, if we have a 64 Kbps physical circuit
and wanted to use 4 sub-channel, each sub-channel will have 16 Kbps. However,
Guard-band is also using this 64 Kbps physical circuit and therefore each channel
will be using only 15 Kbps with 4 Guard-bands (1 Kbps per Guard-band).
This calculation is subject to change while there are many ways to define the
bandwidth for subchannels and Guard-bands. (Fitzgerald 1002, pp.96-97) [1].

Horak (1996) [2] give the example of FDM usage in digital communication
area. It is currently use on broadband Local Area Network, cellular radio, and
some digitized voice applications.

Time Division Multiplexing

Time-division multiplexing (TDM) is a method of putting multiple data
streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled
at the receiving end based on the timing.

(Whatis.com 2004) [7].

According to Horak (1996) [2] data for each device are sent in a serial fashion
from one end multiplexer to the other end so that device #1 transmit on time
slot #1, device #2 transmit on the next time slot and so on. On the receiver,
multiplexer will try to recognize that the first data time slot is for device #1
and the next data time slot is for the next device which is #2. Therefore, the
multiplexer must have the proper time synchronization with other end multiplexer.

On the other hand, Fitzgerald and Dennis (2001, p.97) [1] believes that
TDM is more efficient than FDM as TDM does not use Guard-bands anymore.
Therefore, a 64-Kbps circuit can be fully occupied for 4 circuits with each of
the circuit will have 6-Kbps speed.

Horak (1996) [2] also mentioned that the main drawback of TDM is that
one channel is dedicated to its own use even though the channel is broken or
inactive. This means that if the Device #1 uses Channel #1 and Device #1 is
broken, the time slot for data transmission will still be dedicated to Device #1.

Figure 3: Time Division Multiplexing

Statistical Time Division Multiplexing

In comparison to TDM, the STDM method analyses statistics related to
the typical workload of each input device (printer, fax, computer) and
determines on-the-fly how much time each device should be allocated for
data transmission on the cable or line.

(Whatis.com 2004) [6].

STDM provides a better way of using idle devices on multiplexed TDM
environment. While in TDM, certain amount of time is dedicated to certain
devices whether they are using it or not, STDM is able to define how much time
should be allocated for frequent devices.

Horak (1996) [2] states that ‘…they (STDM) can invoke flow control options
that cause a transmitting terminal to cease transmission temporarily in the
event that the MUX’s internal buffer, or temporary memory device, is full‘.

Figure 4: Statistical Time Division multiplexing

According to Horak (1996) [2] example usage of STDM can be found in
T/E Carrier. T-1, which is known is US, is able to provide 24 time slots with
maximum 64 Kbps for each slot. E-1, which is known in Europe provide 30 time
slots.

Horak (1996) [2] also mentioned that each individual channels can be grouped
to provide faster transmission rate (super-rate). This usually necessary for Voice
data or Video Conferencing. On the other hand, each individual channel also can be subdivided into slower speed (sub-rate).

Conclusion

The multiplexing techniques offer different ways to transmit data from several
slower-speed circuit through one single high-speed circuit.

FDM, which comes to the first part of this discussion, offers the simplest
multiplexing technology by dividing the high-speed circuit using different frequency.
While TDM and STDM provides different ways to split one high-speed
circuit using dedicated time allocation for each data for each channel, STDM
provides better ways than its predecessor (TDM) by relying on statistic to define
which channel needs more dedicated time to get more bandwidth.