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Department of Electrical and Computer Engineering
Chapter 2: Bandwidth utilization techniques
Bandwidth
One characteristic that measures network performance is bandwidth. However, the term can be used
in two different contexts with two different measuring values: bandwidth in hertz and bandwidth in
bits per second.
Bandwidth in Hertz:
Bandwidth in hertz is the range of frequencies contained in a composite signal or the range of
frequencies a channel can pass. For example, we can say the bandwidth of a subscriber telephone
line is 4 kHz.
Bandwidth in Bits per Seconds:
The term bandwidth can also refer to the number of bits per second that a channel, a link, or even a
network can transmit. For example, one can say the bandwidth of a Fast Ethernet network (or the
links in this network) is a maximum of 100 Mbps. This means that this network can send 100 Mbps.
In real life, we have links with limited bandwidths. The wise use of these bandwidths has been, and
will be, one of the main challenges of electronic communications.
MULTIPLEXING:
Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the
devices, the link can be shared. Multiplexing is the set of techniques that allows the simultaneous
transmission of multiple signals across a single data link. As data and telecommunications use
increases, so does traffic. We can accommodate this increase by continuing to add individual links
each time a new channel is needed; or we can install higher-bandwidth links and use each to carry
multiple signals. In a multiplexed system, n lines share the bandwidth of one link. The figure below
shows the basic format of a multiplexed system. The lines on the left direct their transmission
streams to a multiplexer (MUX), which combines them into a single stream (many-to-one). At the
receiving end, that stream is fed into a demultiplexer (DEMUX), which separates the stream back
into its component transmissions (one-to-many) and directs them to their corresponding lines. In the
figure, the word link refers to the physical path. The word channel refers to the portion of a link that
carries a transmission between a given pair of lines. One link can have many (n) channels.
Figure 2.1 Dividing a link into channels
There are three basic multiplexing techniques: frequency-division multiplexing, wavelength-division
multiplexing, and time-division multiplexing. The first two are techniques designed for analog
signals, the third, for digital signals
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Department of Electrical and Computer Engineering
Figure 2.2 Categories of multiplexing
Frequency-Division Multiplexing: Frequency-division multiplexing (FDM) is an analog
technique that can be applied when the bandwidth of a link (in hertz) is greater than the combined
bandwidths of the signals to be transmitted. In FDM, signals generated by each sending device
modulate different carrier frequencies. These modulated signals are then combined into a single
composite signal that can be transported by the link. Carrier frequencies are separated by sufficient
bandwidth to accommodate the modulated signal. These bandwidth ranges are the channels through
which the various signals travel. Channels can be separated by strips of unused bandwidth-guard
bands-to prevent signals from overlapping. In addition, carrier frequencies must not interfere with
the original data frequencies.
Figure 2.3 Frequency-division multiplexing
The above figure 2.3 gives a conceptual view of FDM. In this illustration, the transmission path is
divided into three parts, each representing a channel that carries one transmission. We consider FDM
to be an analog multiplexing technique; however, this does not mean that FDM cannot be used to
combine sources sending digital signals. A digital signal can be converted to an analog signal before
FDM is used to multiplex them.
Multiplexing Process: Figure 2.4 is a conceptual illustration of the multiplexing process. Each
source generates a signal of a similar frequency range. Inside the multiplexer, these similar signals
modulate different carrier frequencies (f 1, f2 and f3). The resulting modulated signals are then
combined into a single composite signal that is sent out over a media link that has enough bandwidth
to accommodate it.
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Figure 2.4 FDM process
Demultiplexing Process: The demultiplexer uses a series of filters to decompose the multiplexed signal
into its constituent component signals. The individual signals are then passed to a demodulator that
separates them from their carriers and passes them to the output lines. Figure 2.5 is a conceptual
illustration of demultiplexing process.
Figure 2.5 FDM demultiplexing example
Example 2.1 Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three
voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration,
using the frequency domain. Assume there are no guard bands.
Solution
We shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure
2.6. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the
second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them as shown
in Figure 2.6. At the receiver, each channel receives the entire signal, using a filter to separate out its
own signal. The first channel uses a filter that passes frequencies between 20 and 24 kHz and filters
out (discards) any other frequencies. The second channel uses a filter that passes frequencies
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between 24 and 28 kHz, and the third channel uses a filter that passes frequencies between 28 and 32
kHz. Each channel then shifts the frequency to start from zero.
Figure 2.6 Example 2.1
Wavelength-Division Multiplexing: Wavelength-division multiplexing (WDM) is designed to use
the high-data-rate capability of fiber-optic cable. The optical fiber data rate is higher than the data
rate of metallic transmission cable. Using a fiber-optic cable for one single line wastes the available
bandwidth. Multiplexing allows us to combine several lines into one. WDM is conceptually the same
as FDM, except that the multiplexing and demultiplexing involve optical signals transmitted through
fiber-optic channels. The idea is the same: We are combining different signals of different
frequencies. The difference is that the frequencies are very high.
Figure 2.7 gives a conceptual view of a WDM multiplexer and demultiplexer. Very narrow bands of
light from different sources are combined to make a wider band of light. At the receiver, the signals
are separated by the demultiplexer.
Figure 2.7 Wavelength-division multiplexing
Although WDM technology is very complex, the basic idea is very simple. We want to combine
multiple light sources into one single light at the multiplexer and do the reverse at the demultiplexer.
The combining and splitting of light sources are easily handled by a prism. Recall from basic physics
that a prism bends a beam of light based on the angle of incidence and the frequency. Using this
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technique, a multiplexer can be made to combine several input beams of light, each containing a
narrow band of frequencies, into one output beam of a wider band of frequencies. A demultiplexer
can also be made to reverse the process. Figure 2.8 shows the concept.
Figure 2.8 Prisms in wavelength-division multiplexing and demultiplexing
One application of WDM is the SONET network in which multiple optical fiber lines are
multiplexed and demultiplexed. A new method, called dense WDM (DWDM), can multiplex a very
large number of channels by spacing channels very close to one another. It achieves even greater
efficiency.
Time-Division Multiplexing: Time-division multiplexing (TDM) is a digital process that allows
several connections to share the high bandwidth of a line instead of sharing a portion of the
bandwidth as in FDM, time is shared. Each connection occupies a portion of time in the link. Figure
2.9 gives a conceptual view of TDM. Note that the same link is used as in FDM; here, however, the
link is shown sectioned by time rather than by frequency. In the figure, portions of signals 1, 2, 3,
and 4 occupy the link sequentially.
Figure 2.9 TDM
Note that in figure 2.9 we are concerned with only multiplexing, not switching. This means that all
the data in a message from source 1 always go to one specific destination, be it 1, 2, 3, or 4. The
delivery is fixed and unvarying, unlike switching. We also need to remember that TDM is, in
principle, a digital multiplexing technique. Digital data from different sources are combined into one
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timeshared link. However, this does not mean that the sources cannot produce analog data; analog
data can be sampled, changed to digital data, and then multiplexed by using TDM.
We can divide TDM into two different schemes: synchronous and statistical. We first discuss
synchronous TDM and then show how statistical TDM differs. In synchronous TDM, each input
connection has an allotment in the output even if it is not sending data. In synchronous TDM, the
data flow of each input connection is divided into units, where each input occupies one input time
slot. A unit can be 1 bit, one character, or one block of data. Each input unit becomes one output unit
and occupies one output time slot.
Figure 2.10 Synchronous time-division multiplexing
In synchronous TDM, a round of data units from each input connection is collected into a frame. If
we have n connections, a frame is divided into n time slots and one slot is allocated for each unit, one
for each input line.
Statistical Time-Division Multiplexing: In synchronous TDM, each input has a reserved slot in the
output frame. This can be inefficient if some input lines have no data to send. In statistical time-
division multiplexing, slots are dynamically allocated to improve bandwidth efficiency. Only when
an input line has a slot's worth of data to send is it given a slot in the output frame. In statistical
multiplexing, the number of slots in each frame is less than the number of input lines. The
multiplexer checks each input line in round robin fashion; it allocates a slot for an input line if the
line has data to send; otherwise, it skips the line and checks the next line.
Figure 2.11 shows a synchronous and a statistical TDM example. In the former, some slots are empty
because the corresponding line does not have data to send. In the latter, however, no slot is left
empty as long as there are data to be sent by any input line.
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Figure 2.11 TDM slot comparison
SPREAD SPECTRUM:
Multiplexing combines signals from several sources to achieve bandwidth efficiency; the available
bandwidth of a link is divided between the sources. In spread spectrum, we also combine signals
from different sources to fit into a larger bandwidth, but our goals are somewhat different. Spread
spectrum is designed to be used in wireless applications (LANs and WANs). In these types of
applications, we have some concerns that outweigh bandwidth efficiency. In wireless applications,
all stations use air (or a vacuum) as the medium for communication. Stations must be able to share
this medium without interception and without being subject to jamming from a malicious intruder (in
military operations, for example).
To achieve these goals, spread spectrum techniques add redundancy; they spread the original
spectrum needed for each station. If the required bandwidth for each station is B, spread spectrum
expands it to Bss' such that Bss » B. The expanded bandwidth allows the source to wrap its message
in a protective envelope for a more secure transmission. An analogy is the sending of a delicate,
expensive gift. We can insert the gift in a special box to prevent it from being damaged during
transportation, and we can use a superior delivery service to guarantee the safety of the package.
Figure 2.12 shows the idea of spread spectrum. Spread spectrum achieves its goals through two
principles:
1. The bandwidth allocated to each station needs to be, by far, larger than what is needed. This
allows redundancy.
2. The expanding of the original bandwidth B to the bandwidth Bss must be done by a process that is
Independent of the original signal. In other words, the spreading process occurs after the signal is
created by the source.
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Figure 2.12
After the signal is created by the source, the spreading process uses a spreading code and spreads the
bandwidth. The figure shows the original bandwidth B and the spreaded bandwidth Bss. The
spreading code is a series of numbers that look random, but are actually a pattern. There are two
techniques to spread the bandwidth: frequency hopping spread spectrum (FHSS) and direct sequence
spread spectrum (DSSS).
Frequency Hopping Spread Spectrum (FHSS):
The frequency hopping spread spectrum (FHSS) technique uses M different carrier frequencies that
are modulated by the source signal. At one moment, the signal modulates one carrier frequency; at
the next moment, the signal modulates another carrier frequency. Although the modulation is done
using one carrier frequency at a time, M frequencies are used in the long run. The band width
occupied by a source after spreading is BpHSS »B. Figure 6.28 shows the general layout for FHSS. A
pseudorandom code generator, called pseudorandom noise (PN), creates a k-bit pattern for every
hopping period Th• The frequency table uses the pattern to find the frequency to be used for this
hopping period and passes it to the frequency synthesizer. The frequency synthesizer creates a carrier
signal of that frequency, and the source signal modulates the carrier signal.
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Figure 2.13 Frequency hopping spread spectrum (FHSS)
Suppose we have decided to have eight hopping frequencies. This is extremely low for real
applications and is just for illustration. In this case, Mis 8 and k is 3. The pseudorandom code
generator will create eight different 3-bit patterns. These are mapped to eight different frequencies in
the frequency table (see Figure 2.14).
Figure 2.14 Frequency selection in FHSS
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The pattern for this station is 101, 111, 001, 000, 010, all, 100. Note that the pattern is pseudorandom
it is repeated after eight hoppings. This means that at hopping period 1, the pattern is 101. The
frequency selected is 700 kHz; the source signal modulates this carrier frequency. The second k-bit
pattern selected is 111, which selects the 900-kHz carrier; the eighth pattern is 100, the frequency is
600 kHz. After eight hoppings, the pattern repeats, starting from 101 again. Figure 6.30 shows how
the signal hops around from carrier to carrier. We assume the required bandwidth of the original
signal is 100 kHz.
Direct Sequence Spread Spectrum: The direct sequence spread spectrum (DSSS) technique also
expands the bandwidth of the original signal, but the process is different. In DSSS, we replace each
data bit with n bits using a spreading code. In other words, each bit is assigned a code of n bits, called
chips, where the chip rate is n times that of the data bit. Figure 6.32 shows the concept of DSSS.
Figure 2.15 DSSS
As an example, let us consider the sequence used in a wireless LAN, the famous Barker sequence
where 11 is 11. We assume that the original signal and the chips in the chip generator use polar NRZ
encoding. Figure 2.16 shows the chips and the result of multiplying the original data by the chips to
get the spread signal.
In Figure 2.16, the spreading code is 11 chips having the pattern 10110111000 (in this case). If the
original signal rate is N, the rate of the spread signal is lIN. This means that the required bandwidth
for the spread signal is 11 times larger than the bandwidth of the original signal. The spread signal
can provide privacy if the intruder does not know the code. It can also provide immunity against
interference if each station uses a different code.
Figure 2.16 DSSS Example
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Bandwidth Sharing
Can we share a bandwidth in DSSS as we did in FHSS? The answer is no and yes. If we use a spreading
code that spreads signals (from different stations) that cannot be combined and separated, we cannot
share a bandwidth. For example some wireless LANs use DSSS and the spread bandwidth cannot be
shared. However, if we use a special type of sequence code that allows the combining and separating of
spread signals, we can share the bandwidth. A special spreading code allows us to use DSSS in cellular
telephony and share a bandwidth between several users.
Chapter 2 ECEG-4503 Data Communications and Computer networks 11
