The document discusses digital modulation techniques used in modern telecommunication systems. It describes techniques like ASK, FSK, PSK and QAM that are used to modulate digital signals for transmission. It also discusses multiplexing techniques like FDM and TDM that allow multiple signals to be transmitted simultaneously over the same medium. The document outlines the evolution from analog to digital networks, including the introduction of techniques like PCM, TDM and optical fiber that enabled widespread digitization of telecom networks and improved quality, features and costs.

1. Telecommunication Systems
Lecture 05
Rana Muhammad Saleem
Emerson University Multan

2. Digital Modulation Techniques
In digital modulation techniques, the information-bearing signal is
digital, either binary or multilevel.
The techniques can be broadly classified as: ASK, frequency shift
keying (FSK), phase shift keying (PSK), and a mixture of ASK and
PSK called quadrature amplitude modulation (QAM).
The first three, illustrated in Figure are analogous to AM, FM, and
phase modulation (PM), with the difference of the modulation
signal being digital.

3. In binary ASK, the carrier amplitude has two discrete levels.
In binary FSK, the carrier has two discrete frequencies.
An early use of modulators/demodulators (modems) was for data
transmission over the telephone network.
The Bell System 103 modem used FSK to transmit data at 300 bps.
In practice, these modulation techniques use filtering to transmit digital
signals in the limited bandwidth channel without significant degradation.

4. Complex multilevel digital modulation, coupled with powerful data-
compression, error-correction, and channel-equalization techniques,
is used in modern high-speed telecommunications systems.

5. Multiplexing
In modern telecommunications systems, long-distance trunks are
implemented through microwave or optical fiber links simultaneously
used by tens of thousands of users.
This deliberate sharing of a communications medium by a number of
different signals is called multiplexing.
The frequency-shifting characteristics of modulation and the digital
encoding of signals using techniques such as PCM form the basis of
multiplexing.

6. The bundled signals are separated at the receiving end by a process
called demultiplexing.
There are two basic types of multiplexing, frequency division
multiplexing (FDM) and time division multiplexing (TDM).
In FDM, channel sharing is done in frequency, where each signal
occupies a separate frequency range in the channel.
FDM is the oldest approach to multiplexing in communications
networks.
In TDM, each signal occupies a separate time interval in the channel.

7. From Analog to Digital
• The term digital telephony refers to the use of digital technology in
the message path of voice communications networks. This implies
digitization of voice and its subsequent switching and transmission.
The first signals to be transmitted electrically were digital.
• A worldwide network of telegraph circuits existed at the time the
telephone was invented, and a considerable body of knowledge
existed on digital transmission. It was evident that with digital
transmission, the received signal’s quality need only be sufficient to
detect the presence or absence of a pulse. Codes, such as the
Morse code, were devised to ensure sufficient quality of the received
information.
• The introduction of voice telephony presented a completely new set
of problems, as the absolute quality of the analog signal was of
interest, rather than just a quality sufficient to detect the presence or
absence of a signal.

8. Thus, an entirely new body of knowledge grew related to speech
transmission over long distances.
By the 1930s, long-distance carrier systems using FDM were widely in
use, transporting groups of telephone circuits over long distances.
The bandwidth of these trunks based on cables became a concern as
telephony subscribers grew.
Though the concept of TDM was introduced, the lack of suitable
electronic components retarded its implementation.
Microwave radio was introduced in the 1930s as a new medium for
long-distance transmission.
This medium offered a large band-width but with severe problems of
noise and distortion.

9. As a solution to this problem, Alec Reeves patented PCM technique in
1938, showing that by converting analog signals into digital code and
transmitting them in place of analog signals, the problems of noise and
distortion could be overcome.
Shannon’s information theory expositions in 1948 made the potential for
PCM and digital transmission apparent.
However, practical electronic devices did not exist at that time to
implement and introduce PCM commercially.


10. Today, almost 100% of the world’s telecom networks use digital
transmission and switching. Figure 2.1 shows the evolution of the
analog telephone network to digital
in the following stages:
• Twenty-four-channel T1 systems were installed on
relatively short-haul inter-office trunks within the exchange
areas in the 1960s.
 In the 1970s, higher capacity T2 systems were introduced for long-
distance
digital transmission.
Digitization of the PSTN

11.  T1 coverage expanded greatly, and by the late 1970s digital loop
carrier (DLC) systems and digital private branch exchanges
(DPBXs) came into use.
 Integrated islands of digital switches (toll offices and end offices)
interconnected by digital transmission links grew.
 Realization of fully integrated and interconnected digital network
came about in the early 1980s with fiber-optic transmission for high-
density, long-haul routes.
 Digital connectivity to customer premises equipment occurred as
 T1/E1 became the preferred interface for large PABXs.
 End-to-end digital connectivity for voice or data became a reality in
the late 1980s with the introduction of ISDNs.

12. Digitization of Wireless Systems
Wireless systems, both fixed and mobile, have also evolved to
encompass digital techniques in their subscriber interface (air
interface), while adopting the same digital techniques as the PSTN
for transmission and switching.
The most important aspect of digital techniques in wireless systems
is the development of sophisticated voice coding techniques, which
have been developed to satisfy the need for utilizing RF bandwidth
efficiently and for operating in harsh environments.

13. Telephony over Packet-Switched Networks
 The most popular application of packet-based techniques for
telephony services is IP telephony or Internet telephony, the
transmission of voice over the Internet.
 The underlying principle is that the digitized voice is broken into
packets and transmitted and switched individually, without an end-
to-end circuit as in the traditional PSTN.
 These services not only bring telephony at a lower cost, but also
enable a variety of services, the integration of many services, and
the creation of new services.

14. Advantages of Digital Techniques
 The introduction of digital technology was motivated by the desire to
improve the quality, add new features, and reduce costs of conventional
voice services.
 Digitization did not arise from the needs of the data communications
industry for better data transmission services.
 As more and more of the network became digitized, more and more
support for direct use of the facilities became available for data
 applications.
 The following section summarizes the advantages.

15.  Integrated Networks
 Techniques used for digital telephony can be used uniformly for
any kind of digital traffic.
 A digitally encoded message presents a common format for
transmission, irrespective of the source.
 Thus, it need not provide any special attention to individual
services and can be indifferent to the nature and origin of the
traffic.
 Signals from different sources can be multiplexed, switched, and
transmitted in a uniform manner.

16.  Modern Semiconductor Technology and
Software Support
 As the cost of microelectronics dropped, digital techniques
gradually became more attractive.
 The circuitry used to multiplex and switch digital signals is much
cheaper than the complex amplifiers and filters used to handle
analog signals.
 Great advantages have been achieved by using very large-scale
integrated circuit (VLSI) devices such as analog to digital (A/D)
and digital to analog (D/A) converters, voice codecs, multiplexers,
DSPs, and switching matrices.

17. Regeneration
 In digital transmission, the requirement is only to detect the presence or absence of a
 pulse to be detected in the presence of channel impairments. To transmit signals
over
 long distances, intermediate devices that remove some of these impairments are
 therefore needed.
 In analog transmission, such devices are amplifiers, which simply amplify the signals.
 In digital transmission, regenerators are used. These are devices that catch the
 bit stream before it is submerged in the noise and then remove noise and distortion
 from it by creating it afresh.
 An amplifier cannot eliminate noise and distortion from a signal, while a repeater can.

18. Ease of Signaling
 In digital transmission, control information is indistinguishable from
message traffic. One means of incorporating control information
involves time division multiplexing these as separate, but easily
identifiable,control channels. This is called common channel
signaling (CCS).
 Special control commands can also be inserted into the control
channel, with digital logic in the receiver decoding them.
 The result is that the transmission and signaling equipment can be
designed, modified, or upgraded independently of each other.

19. Integration of Transmission and Switching
 The transmission equipment is independent of switching
equipment, with standardized interfaces.
 The incoming signals are demultiplexed, switched to the
appropriate outgoing trunk, and then remultiplexed with other
circuits routed to the same trunk.
 This particular switching technique is known as space division
switching, as the operation involves the physical connection of a
 signal from one route to another.

20.  Multiplexing can easily be integrated into the switching
equipment in a digital system.
 Digital encoding/decoding is needed only at the end
switches, while intermediate switches carry out the
switching function simply by rearranging time slots taken
from incoming trunks on the appropriate outgoing trunk.
 This not only reduces the equipment used, but also
improves end-to-end voice quality by eliminating multiple
A/D/A conversions and by using low
 error rate transmission links.

21. Use of Optical Fiber Technology
 The technological development that has had the single
most significant impact on telecommunications networks is
fiber-optic transmission.
 Digital transmission dominates fiber applications due to
the ease of interfacing digital (on/off) signals to a fiber.
 Very large numbers of channels can be time division
multiplexed to fill the capacity of optical fiber transmission
facilities.

22. DSP
 Once a signal is encoded, DSP enables many enhancements that result in
 the improvement of quality of services and efficient use of network and
transmission facilities.
 They also enable high-quality communication in harsh environments.
 Some examples of DSP applications in transmission systems are error
control,
 echo cancellation, and equalization. With respect to communication services
or
 applications, data compression, encryption, noise, and interference
 cancellation are useful DSP functions.

23. Disadvantages of Digital Techniques
 Digital techniques are, however, not without disadvantages.
 A digitized voice signal at a standard rate of 64 kbit/s requires at least
eight times the bandwidth as the analog signal to be transmitted in
binary form. Although moresophisticated voice encoding
(compression) and modulation schemes exist, none of them can
provide the same quality voice without some bandwidth or power
penalty.
 Especially in long-distance trunks, this bandwidth penalty is a
significant disadvantage and is directly dependent on the encoding
and modulation schemes used.

24.  With multilevel encoding and modulation, greater efficiency in
terms of the bit rate in a given bandwidth is achievable.
 However, with limited transmit power, such a multilevel signal is no
longer as immune to noise and distortion as before.
 When digital information is transmitted, a timing reference is
needed for the receiver to sample the incoming signal. The
generation of such a timing signal is known as synchronization.
When a number of digital transmission links and switches are
interconnected, networkwide timing and synchronization must
 be established.
 In analog systems, synchronization requirements are much less
stringent.