3.2 CIRCUITS3.2.1 Circuit Configura

3.2 CIRCUITS
3.2.1 Circuit Configuration
Circuit configuration is the basic physical layout of the circuit. There are two fundamental circuit configurations: point-to-point and multipoint. In practice, most complex
computer networks have many circuits, some of which are point-to-point and some of
which are multipoint.
Figure 3.1 illustrates a point-to-point circuit, which is so named because it goes
from one point to another (e.g., one computer to another computer). These circuits sometimes are called dedicated circuits because they are dedicated to the use of these two
computers. This type of configuration is used when the computers generate enough data
to fill the capacity of the communication circuit. When an organization builds a network
using point-to-point circuits, each computer has its own circuit running from itself to the other computers. This can get very expensive, particularly if there is some distance
between the computers.
Figure 3.2 shows a multipoint circuit (also called a shared circuit). In this configuration, many computers are connected on the same circuit. This means that each must
share the circuit with the others. The disadvantage is that only one computer can use the
circuit at a time. When one computer is sending or receiving data, all others must wait.
The advantage of multipoint circuits is that they reduce the amount of cable required and
typically use the available communication circuit more efficiently. Imagine the number
of circuits that would be required if the network in Figure 3.2 was designed with separate point-to-point circuits. For this reason, multipoint configurations are cheaper than
point-to-point circuits. Thus, multipoint circuits typically are used when each computer
does not need to continuously use the entire capacity of the circuit or when building
point-to-point circuits is too expensive. Wireless circuits are almost always multipoint
circuits because multiple computers use the same radio frequencies and must take turns
transmitting.
3.2.2 Data Flow
Circuits can be designed to permit data to flow in one direction or in both directions. Actually, there are three ways to transmit: simplex, half-duplex, and full-duplex (Figure 3.3).
Simplex transmission is one-way transmission, such as that with radios and TVs.
Half-duplex transmission is two-way transmission, but you can transmit in only
one direction at a time. A half-duplex communication link is similar to a walkie-talkie
link; only one computer can transmit at a time. Computers use control signals to negotiate which will send and which will receive data. The amount of time half-duplex communication takes to switch between sending and receiving is called turnaround time (also
called retrain time or reclocking time). The turnaround time for a specific circuit can
be obtained from its technical specifications (often between 20 and 50 milliseconds).
Europeans sometimes use the term simplex circuit to mean a half-duplex circuit.
With full-duplex transmission, you can transmit in both directions simultaneously,
with no turnaround time.
How do you choose which data flow method to use? Obviously, one factor is the
application. If data always need to flow only in one direction (e.g., from a remote sensor
to a host computer), then simplex is probably the best choice. In most cases, however,
data must flow in both directions.
The initial temptation is to presume that a full-duplex channel is best; however,
each circuit has only so much capacity to carry data. Creating a full-duplex circuit means
that the available capacity in the circuit is divided—half in one direction and half in the
other. In some cases, it makes more sense to build a set of simplex circuits in the same
way a set of one-way streets can increase the speed of traffic. In other cases, a half-duplex
circuit may work best. For example, terminals connected to mainframes often transmit
data to the host, wait for a reply, transmit more data, and so on, in a turn-taking process;
usually, traffic does not need to flow in both directions simultaneously. Such a traffic
pattern is ideally suited to half-duplex circuits.
3.2.3 Multiplexing
Multiplexing means to break one high-speed physical communication circuit into several
lower-speed logical circuits so that many different devices can simultaneously use it but
still “think” that they have their own separate circuits (the multiplexer is “transparent”). It
is multiplexing (specifically, wavelength division multiplexing [WDM], discussed later
in this section) that has enabled the almost unbelievable growth in network capacity
discussed in Chapter 1; without WDM, the Internet would have collapsed in the 1990s.
Multiplexing often is done in multiples of 4 (e.g., 8, 16). Figure 3.4 shows a
four-level multiplexed circuit. Note that two multiplexers are needed for each circuit:one to combine the four original circuits into the one multiplexed circuit and one to
separate them back into the four separate circuits.
The primary benefit of multiplexing is to save money by reducing the amount of
cable or the number of network circuits that must be installed. For example, if we did not
use multiplexers in Figure 3.4, we would need to run four separate circuits from the clients
to the server. If the clients were located close to the server, this would be inexpensive.
However, if they were located several miles away, the extra costs could be substantial.
There are four types of multiplexing: frequency division multiplexing (FDM), time
division multiplexing (TDM), statistical time division multiplexing (STDM), and wavelength division multiplexing WDM.
Frequency Division Multiplexing Frequency division multiplexing (FDM) can be
described as dividing the circuit “horizontally” so that many signals can travel a single
communication circuit simultaneously. The circuit is divided into a series of separate
channels, each transmitting on a different frequency, much like series of different radio
or TV stations. All signals exist in the media at the same time, but because they are on
different frequencies, they do not interfere with each other.
Figure 3.5 illustrates the use of FDM to divide one circuit into four channels. Each
channel is a separate logical circuit, and the devices connected to them are unaware that
their circuit is multiplexed. In the same way that radio stations must be assigned separate
frequencies to prevent interference, so must the signals in a FDM circuit. The guardbands
in Figure 3.5 are the unused portions of the circuit that separate these frequencies from
each other.
With FDM, the total capacity of the physical circuit is simply divided among the
multiplexed circuits. For example, suppose we had a physical circuit with a data rate of
64 Kbps that we wanted to divide into four circuits. We would simply divide the 64 Kbps
among the four circuits and assign each circuit 16 Kbps. However, because FDM needs
guardbands, we also have to allocate some of the capacity to the guardbands, so we might
actually end up with four circuits, each providing 15 Kbps, with the remaining 4 Kbps
allocated to the guardbands. There is no requirement that all circuits be the same size,
as you will see in a later section. FDM was commonly used in older telephone systems,
which is why the bandwidth on older phone systems was only 3,000 Hz, not the 4,000 Hz
actually available—1,000 Hz were used as guardbands, with the voice signals traveling
between two guardbands on the outside of the channel.
Time Division Multiplexing Time division multiplexing (TDM) shares a communication circuit among two or more computers by having them take turns, dividing the
circuit vertically, so to speak. Figure 3.6 shows the same four computers connected using
TDM. In this case, one character is taken from each computer in turn, transmitted down
the circuit, and delivered to the appropriate device at the far end (e.g., one character from
computer A, then one from B, one from C, one from D, another from A, another from
B, and so on). Time on the circuit is allocated even when data are not being transmitted,
so that some capacity is wasted when terminals are idle. TDM generally is more efficient
than FDM because it does not need guardbands. Guardbands use “space” on the circuit
that otherwise could be used to transmit data. Therefore, if one divides a 64-Kbps circuit
into four circuits, the result would be four 16-Kbps circuits.
Statistical Time Division Multiplexing Statistical time division multiplexing
(STDM) is the exception to the rule that the capacity of the multiplexed circuit must
equal the sum of the circuits it combines. STDM allows more terminals or computers to be connected to a circuit than does FDM or TDM. If you have four computers
connected to a multiplexer and each can transmit at 64 Kbps, then you should have a
circuit capable of transmitting 256 Kbps (4 × 64 Kbps). However, not all computers
will be transmitting continuously at their maximum transmission speed. Users typically
pause to read their screens or spend time typing at lower speeds. Therefore, you do not
need to provide a speed of 256 Kbps on this multiplexed circuit. If you assume that
only two computers will ever transmit at the same time, 128 Kbps would be enough.
STDM is called statistical because selection of transmission speed for the multiplexed
circuit is based on a statistical analysis of the usage requirements of the circuits to be
multiplexed.
The key benefit of STDM is that it provides more efficient use of the circuit
and saves money. You can buy a lower-speed, less-expensive circuit than you could
using FDM or TDM. STDM introduces two additional complexities. First, STDM can
cause time delays. If all devices start transmitting or receiving at the same time (or
just more than at the statistical assumptions), the multiplexed circuit cannot transmit
all the data it receives because it does not have sufficient capacity. Therefore, STDM
must have internal memor