Page 86 - DCAP406_DCAP_207_Computer Networks

Unit 6: Multiplexing

which signals is carried. The signals may be analog, binary or digital. This has been shown in the Notes
Figures 6.1 and 6.2. In Figure 6.1 and Figure 6.2 the link between PC and modem, modem, link
between modem and CTO and so on constitute a circuit. The circuit may be a physical path
consisting of wires or it may be wireless. A network, which is wired or wireless involves a
number of circuits consisting of a number of intermediate switches. A circuit is classified based
upon its uses like dial up connection, leased line, etc. The connections between two or more
points are also established virtually unlike to the connection made by dial up and leased line as
physical in nature. Such circuits are defined as Virtual Circuit (VC) based upon the type and
nature of the connection. A virtual circuit is a logical path selected out of many possible physical
paths available between two or more points. However, the connection in a virtual circuit is not
guaranteed. The virtual circuits that ensure a permanent connection between two points is
referred as Permanent Virtual Circuit (PVC). The PVC provides guaranteed connection between
two or more points when needed without having to reserve a specific physical path in advance.
A Switched Virtual Circuit (SVC) is similar to a permanent virtual circuit and allows users to
dial in to the network of virtual circuits. A circuit may contain many channels together. The
Integrated Services Digital Network (ISDN) supports 2 Basic Rate Interface (BRI) service channel
and 1 signaling channel. A Digital Signal 1 (DS1) circuit supports 24 64-Kb/s channels, while a
DS3 circuit supports 612 64-Kb/s channels. These numbers of channels on a single circuit are
possible because of multiplexing techniques.

6.2 Multiplexing

Multiplexing is the process in which multiple channels are combined for transmission over a
common transmission path. There are following different techniques for multiplexing:

Frequency Division Multiplexing (FDM)
Time Division Multiplexing (TDM)

Code Division Multiplexing (CDM)
Wavelength Division Multiplexing (WDM)
6.2.1 Frequency Division Multiplexing (FDM)

Multiple channels are combined together for transmission over a single channel. The channels
are separated by their frequency. It is explained in the Figure 6.3 where a frequency dimension
is divided into several non-overlapping frequency bands. Each channel ci is allotted its own
frequency band as depicted in Figure 6.3.

Figure 6.3: Frequency Division Multiplexing

t

C 1 C 2 C 3 C 4 C 5
f

Guard spaces

There are always some unused frequency spaces between channels. They are known as guard
bands and also shown in Figure 6.3. They are used to reduce the effects of overlapping between
adjacent channels. Overlapping of adjacent channels tends to produce crosstalk.

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