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Compression and Multiplexing (pg. 2)
Multiplexing
Another method of manipulating data to achieve greater throughput is through multiplexing.
Multiplexing is a technique used in communications and input/output operations for
transmitting a number of separate signals simultaneously over a single channel or line.
To maintain the integrity of each signal on the channel, multiplexing can separate the
signals by time, space, or frequency. The device used to combine the signals is a
multiplexer, and the separate signals are recovered at the end by a demultiplexer.
Multiplexing combines traffic from multiple telephones and data devices into one single
stream so that many devices can share a particular telecommunication path. Multiplexing
makes more efficient use of telephone lines, as does compression. However, unlike
compression, multiplexing does not alter the actual data sent.
Multiplexing equipment is typically
located in long distance companies, local telephone companies and at end-user premises,
and is associated with both analog and digital services. Examples of multiplexing over
digital facilities include T-1, fractional T-1, T-3, ISDN and ATM technologies.
The oldest multiplexing techniques were devised by AT&T for use with analog voice
services. The goal was to make more efficient use of the most expensive portion of the
public telephone network, the outside wires used to connect homes and telephone offices to
each other. This analog technique is referred to as frequency-division multiplexing, which
allows multiple voice and later data calls to share paths between central offices.
Therefore, AT&T would not need not construct cable connections for each conversation,
since multiple conversations could share the same wire between a telephone company’s
central office.
Digital Multiplexing
Digital multiplexing schemes operate at greater speeds and carry more traffic than analog
multiplexing. For example, T-3 carries 672 conversations over one line at speeds of 45
megabits per second. With both digital analog multiplexing, a matching multiplexer is
required at both the sending and receiving ends of the communications channel.
T-3 is used for very large customers, telephone companies and Internet Service Provider
networks.
T-1 is the most common form of multiplexing for end-user organizations. T-1 is lower in
both cost and capacity than T-3. T-1 allows more than two voice and / or data
conversations to share a path. T-1 applications include linking organization sites
together for voice calls, email, database access, and links between end-users and
telephone companies for discounted rates on telephone calls. Like T-3 services, matching
multiplexers are required at both ends of a T-1 link.
Frequency-Division Multiplexing
Frequency-division multiplexing is a scheme in which numerous signals are combined for
transmission on a single communications line or channel. Each signal is assigned a
different frequency (sub-channel) within the main channel.
With frequency-division multiplexing, each channel has its own base frequency, and its own
carrier frequency. The carrier frequency can be modulated using several different methods
to derive either digital or analog channels.
The modulation method and the characteristics of the information on the channel (such as
the bit rate) determine the bandwidth needed per channel. The circuitry to handle a
channel in a frequency-division multiplexer is quite complicated and therefore costly. For
analog signals such as television signals, however, frequency-division multiplexing can
still be a good choice.
Frequency-division multiplexing can be used on optical fibers by using a different
frequency and thus a different wavelength of the light beam for each channel. With optical
systems, the term wavelength multiplexing is used. On radio links not only different
frequencies but also different polarization angles can be used. Suppose a long-distance
cable is available with a bandwidth allotment of 3 Mhz. This is 3,000 kHz, so
theoretically, it is possible to place 1,000 signals, each 3 kHz wide, into the
long-distance channel. The circuit that does this is known as a multiplexer. It accepts
the input from each individual end user, and generates a signal on a different frequency
for each of the inputs. This results in a high-bandwidth, complex signal containing data
from all the end users. At the other end of the long-distance cable, the individual
signals are separated out by means of a circuit called a demultiplexer, and routed to the
proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at
each end of the long-distance, high-bandwidth cable.
When frequency-division multiplexing is used in a communications network, each input
signal is sent and received at maximum speed at all times. This is its chief asset.
However, if many signals must be sent along a single long-distance line, the necessary
bandwidth is large, and careful engineering is required to ensure that the system will
perform properly.
In some systems, a different scheme, known as time-division multiplexing, is used instead.
Time-division Multiplexing
Time-division multiplexing has become a cost-effective method that is not only used on
trunk circuits between switching centers but is today even starting to be used on local
circuits to the customer. The basic interface of the ISDN is an example of this trend.
With time-division multiplexing, the whole bandwidth is assigned to each particular
channel for a fraction of the total transmission time. This fraction can vary from one bit
for bit-interleaved multiplexers, to a few thousand bits in the newest types of high
bit-rate multiplexers; the synchronous time-division multiplexing (STDM) designed for the
synchronous transfer mode.
Time-division can even be used to transfer samples of bits, derived by scanning the input
channels with a frequency at least 3 times higher than the highest bit rate on these
tributary channels. With this method digital signals from various sources with even
unknown or changing bit rates can be multiplexed and reproduced (with a tolerable
distortion) at the other end of the common channel (CCITT, 1988d).
All these time-division multiplexers are fixed slot time-division multiplexers, in that
they assign a fixed slot to each channel in a cyclic scan of all the tributary channels.
The fixed position of the slot in the cycle for each channel makes it possible to identify
the destination outlet for each portion of the information received over the common
channel.
This process requires synchronization in order to guarantee that the scanning of the
received information at that output side was at the same speed as the cyclic scan at the
input side.
All slots of one cyclic scan are arranged in a frame. In this frame, one generally finds
additional information to ensure correct synchronization and frame alignment, needed to
present the information from the input channels arriving at the wrong output channels as a
result of being out of phase.
The circuit that combines signals at the source (transmitting) end of a communications
link is known as a multiplexer. It accepts the input from each individual end user, breaks
each signal into segments, and assigns the segments to the composite signal in a rotating,
repeating sequence. The composite signal thus contains data from all the end users.
As with frequency-division multiplexing, at the other end of the long-distance cable, the
individual signals are separated out by means of a circuit called a demultiplexer, and
routed to the proper end users. Again, as in frequency-division multiplexing, a two-way
communications circuit requires a multiplexer/demultiplexer at each end of the
long-distance, high-bandwidth cable.
As always, if many signals must be sent along a single long-distance line, careful
engineering is required to ensure that the system will perform properly. An asset of
time-division multiplexing is its flexibility. The scheme allows for variation in the
number of signals being sent along the line, and constantly adjusts the time intervals to
make optimum use of the available bandwidth.
The Internet is a classic example of a communications network in which the volume of
traffic can change drastically from hour to hour.
Take the compression and
multiplexing quiz!
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