2. Communications Systems
In a well-defined communications system, there are three essential components for
effective information transfer: a transmitting device, a transport mechanism, and a
receiving device.
The transport mechanism may range from a simple single channel to a complex
communications network consisting of many circuits, switching devices, and other
cponents.
3. Modes of Transmission
A communications system may carry information only in one
direction (simplex), in both directions sequentially (duplex or
full duplex), or alternately in each direction (half duplex).
It may be point to point as in a telephone conversation, point to
multipoint as in a radio or TV broadcast, or between many
points simultaneously as in the case of a computer network.
4.
5.
6. Types of Communiction Networks
A communications network facilitates communication between users when
the need arises.
It may be a structured framework without geographical boundaries such as
the PSTN(Public Switched Telephone Network).
it may be a network spanning over a small area serving a single
organization, such as a private automatic branch exchange (PABX) or a
local area network (LAN).
PSTN
A switched network such as the PSTN can set up a channel over the
switched network on demand.
A switching device at each subscriber enables appropriate line selection on
demand.
Now the transport mechanism is a complex switch and circuit arrangement.
7. This concept is extended to larger telecommunications networks with more
subscribers.
In the PSTN, the number of switches (exchanges) and their locations are
governed by the overall number and the geographical distribution ofstations
requiring service.
Interconnections among exchanges are multiple circuits, called trunks, with
sufficient capacity to cater for the demand.
These carry multiple conversations simultaneously using special
multiplexing and transmission techniques.
8. Multiplexing refers to the bundling of several conversations together,
and transmission refers to the transport of these bundles.
Signaling is another important concept in the telecommunications
network.
It conceptually uses specialized messages to set up, maintain,
terminate, and bill calls.
9. PABX
The term “private automatic branch exchange (PABX)” is often used as a
synonym for “telephone system”.
Private automatic branch exchanges are switchboards which connect
several devices, e.g. telephones or faxes and answering machines with
each other and with the public telephone network.
This allows a number of phones to be operated using a relatively low
number of public telephone lines.
Internal calls can be placed free of charge without using the public
telephone network.
10. Depending on the type of telephone system and the configuration, internal
lines can be called directly from outside by using a direct dial in number, or
indirectly through an operator.
Key components include toggle, conference calls and picking up calls.
Toggle is used to switch back and forth between multiple calls.
11. Advances in switching, signaling, and transmission have enabled modern
networks to handle a complex combination of subscribers, systems, and
a variety of advanced services.
Applications that are implemented on telecommunications systems are
referred to as communications services. Older examples are telegraphy
and voice telephony.
Modern examples are cellular telephony and multimedia.
We are now in the Information Age, where information has become an
important commodity for both the business community and the greater
public.
12. Signals in Communications
Types of Signals
Signals can be classified on the basis of the way in which their values vary over time:
• Analog: This is a continuous function that assumes smooth changes in value with finite
rates of change. The microphone output is an example of an analog signal.
13. Discrete: This is a noncontinuous function whose values form a discrete set and occur
at isolated points in time.
Digital: A function that assumes a limited set of discrete values for constant durations
of time is called a digital signal. Changes of values are instantane- ous.
A binary waveform is one that has only two allowable values and is commonly used for
the communication of digital data.
14. Characteristics of Signals
A signal can be characterized in the time or frequency domain.
In the time domain, the signal’s characteristics are identified as a function
of time.
In the frequency domain, the signal’s characteris-tics are represented as its
constituent frequency components, as viewed on a spectrum analyzer.
As most signals in communications are random, in addition to time and
frequency domain characterization, statistical characterization is also
possible.
15. 1.Frequency Domain Characteristics of Signals
Any complex waveform can be decomposed into either a discrete set
of sinusoids or a continuum of sinusoids, each having a different
amplitude, frequency, and phase.
Fourier analysis
The tool for this characterization is Fourier analysis.
Fourier analysis shows that any periodic signal is composed of a dc
component, a fundamental component, and a series of harmonics.
16. Fourier analysis
The fundamental is a sinusoid having the same frequency as the periodic
signal, and harmonics are sinusoids at integer multiples of the fundamental
frequency. Figure shows two examples of periodic signals and their
frequency content, known as the Fourier series.
17. Fourier Transform
Nonperiodic signals are composed of a continuous range of frequency
components, called the Fourier transform.
For example, Figure 1.6 shows the time and frequency domain
representations of a rectangular pulse and a pulse train. The Fourier
series (fundamental and harmonics) or the Fourier transform (continuous
range of frequency components) is called the spectrum of the signal.
18. The range of frequencies that encompasses all of the energy present in a
signal is known as the bandwidth of that signal.
2. Statistical Characteristics of Signals
A deterministic signal can be expressed in the form of an equation, where all
terms are completely known.
However most signals in practical communications systems are not
deterministic.
To predict the behavior of random, or nondeterministic signals, statistical
techniques such as probability density function, autocorrelation, and cross-
correlation functions are used.
Auto correlation represents a signal’s similarity to a time-delayed replica of
itself.
Hence, it is a measure of how fast the signal changes and, correspondingly,
its frequency characteristics.
19. 3. Practical Examples
Examples of analog signals are signals from a microphone (a low-audio-
bandwidth case), musical performance (a high-bandwidth audio), signals
from a video camera (video bandwidth), or signals from a sensor circuit to
measure such variables as temperature, humidity, and acceleration. Analog
signals in communications have wide-ranging characteristics. As an
example, Table 1.1 compares voice and music.
Table 1.1 Comparison of Electrical Parameters of Speech and Music.
Signal Frequency Frequency of Dynamic
Range Peak Energy Range*
Speech 100–7,000 Hz 250–500 Hz 35–40 dB
Music 20–20,000 Hz 200–600 Hz 75–80 dB
*
The dynamic range is the ratio of the power produced by the loudest and the softest portion of signal.
20. Digital signals may originate from a computer or other processor-controlled
device, or through digitization or digital encoding of an analog signal.
One common way in which computers represent and output data is in the
form of binary-encoded characters.
Binary encoding of information is well known—the earliest example is the
Morse code.
The Extended Binary Coded Decimal Inter- change Code (EBCID) and the
American Standard Code for Information Inter- change (ASCII) are other
known methods of binary encoding of information.
21. The ASCII code is shown in Table 1.2. In ASCII, the character A is
encoded as the seven- bit binary sequence 1000001.
Analog signals are digitized by sampling, approximating the samples to a
discrete set of values (quantization) and then expressing the sample
values as binary codes (encoding or digitization.
