Chapter 4: Physical Layer Concepts 155
44. How different is multiplexing from switching, which you discussed in Chapter 2?
As we indicated earlier, multiplexing is a technique used to place multiple signals on a
single communications channel and is performed by a device called a multiplexer or mux
for short. Switching, on the other hand, is a process that involves establishing an appropri-
ate path, which a data message will follow as it travels throughout a network en route
between a sending source and a destination node. Switching is performed by switches,
which use speciﬁc predetermined criteria as the basis of path determination. Multiplexing
and switching are the two basic techniques used for transmitting data within a communi-
cations network. They make it possible for transmission facilities to be shared among
users in an efﬁcient and economical manner.
45. What types of switching strategies are there?
If you review Chapter 2, we provided you with some basic information between the
two primary strategies, namely, circuit-switching and packet-switching. For convenience, a
short summary follows.
Circuit-Switching In a circuit-switched network, a dedicated physical circuit must be
established between the source and destination nodes before any data transmission can
take place. Furthermore, this circuit must remain in place for the duration of a transmis-
sion. The public telephone system, known formally as the public switched telephone net-
work (PSTN), is a good example of a circuit-switched network. When we dial a telephone
number, a switch that resides at the telephone company’s central ofﬁce establishes a logi-
cal connection to a set of wires based on the number we dialed. This set of wires will
either connect to a wire center (also called a frame) that services a particular area, or it will
connect to another central ofﬁce that contains yet another switch. Ultimately, a circuit will
be established that connects the caller’s and receiver’s telephones. One feature of circuit-
switching is that it promotes link sharing. Circuit-switching enables different data trans-
FIGURE 4.10 In a typical ﬁber-optic installation (a), a ﬁber-optic transmitter converts an electron-
ic signal into light, which is sent through the ﬁber-optic cable to its destination. Repeaters, which are
optical ampliﬁers, regenerate the light signal at appropriate points along the way. With wavelength
division multiplexing (b), electronic signals originating from multiple data sources, each operating
at different wavelengths, are combined into a wavelength multiplexer (WDM) and output onto a sin-
gle ﬁber-optic cable. A demultiplexer (IWDM) is employed at the receiving end to split the signals.
Source: Adapted from Clark, 1997.