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DIGITAL NETWORKS WEEK 1 DN/02/01/01

NUWAN BALASURIYA PAGE 1 OF 5 BATCH 02

INTRODUCTION TO DIGITAL NETWORKS

Introduction

Until 1980’s the communication was totally analog. All wired communication systems like

PSTN and ISDN and also the wireless systems like AMPS based on FDMA were all using

analog technologies. However, in today’s communication industry digital communication has

overtaken the analog communication. This dominance is a result of a blend of many advantages

that digital communication processes. Before moving in to further details it is worthwhile to

clarify what a digital signal and digital communication is.

Digital signals / Digital communication

Digital signals are a set of signals (electrical, optical or other), which are discrete in time and

also discrete in amplitude.

Similarly whenever a communication takes place between two or more parties using digital

signals, we call it a digital communication.

Some historical milestones

The first electrical communication system, the Morse code was invented by Samuel Morse in

1840 which started the electrical communication. In fact it was a digital communication system

which used a sequence of pulses with different duration to transfer different characters. The

Morse code was extensively used and is a method of communication in naval travel, even today.

After the Morse code many other similar schemes appeared which were based on character

encoding, however are not considered as very important milestones.

Gradually analog communication which could carry voice signals over distances were invented

and developed.



However in 1937, the invention of the voice digitizing technique by Alex Reeve showed a new

way for voice communication. This technique is widely used even today and known as Pulse

Code Modulation or simply PCM. With the invention of PCM analog telephony started moving

towards “all digital”.

Later in 1960’s a huge step towards the “all digital” goal was taken with the introduction of

improved digital transmission facilities (digital hierarchies) and also with the introduction of

X.25 technology in 1970’s. Frame relay, ATM, ISDN they all helped in many ways for the

advancement of digital communication.

Today the world is almost 100% digital in communication and storing of data in all forms.

Moreover the preference has been centering on ip based communications.

Advantages of using digital signals / communication

Due to some advantages digital signals are preferred over analog signals. These advantages can

be listed as follows.

1. High reliability

2. Ease of multiplexing

3. Ease of signaling

4. Capability for voice and data traffic integration

5. Ease in processing

6. Ease of performance monitoring

7. Ease in transmission and switching

High reliability

Digital signals have discrete levels. Therefore even with some amount of superimposed noise,

the resultant signal9 after reception) can be re approximated to one of the discrete levels. Up to

some amount of noise, the probability of miss approximating is very low. In other words, the

probability of an error is very low in digital domain, compared to analog domain.



Ease of multiplexing

With the digital signal streams, multiple communication streams can be multiplexed together and

then at the receivers they can be effectively de-multiplexed.

Ease of signaling

Signaling is the process of exchange of control information between the transmitters. Signaling

information can be embedded inside the message bit stream. Further multiple signaling

information can be multiplexed together.

Capability for voice and data traffic integration

Data and voice can be easily multiplexed and can be transmitted using the same physical link.

(eg: ISDN)

Ease in processing

Error control and compression coding are defined in digital domain. Therefore the information

can be easily compressed and coded in digital domain.

Ease of performance monitoring

In communication, original signal get distorted in the transmissions due to the phenomenon like

fading, noise, etc.. Therefore even with sophisticated error control schemes, there is an

associated error. Knowing this error quantitatively is very important. In digital domain this error

can be expressed in terms of bit error rate, where as in analog domain such quantitative direct

measure is not available.



Disadvantages of digital signals / communication

1. Need for tight synchronization

2. Increased bandwidth requirement

3. Complexity and cost in digital systems

Need for tight synchronization

Upon reception at the receiver and demodulation the digital level should be identified. In order to

achieve this we need to sample our pulse stream within each and every bit duration. Without a

properly synchronized sampling, the sample stream will not correctly represent the original bit

stream. Therefore properly synchronized clock is needed at the receiver.

Increased band width requirement

Consider a voice signal with filtered bandwidth of 4 kHz. With PCM coding we get a bit rate of

64kbps. Without any vocoding or compression, this is clearly an increment in bandwidth

requirement.

Complexity and cost in digital systems

Digital systems involve ADC and also DAC. Moreover they involve advanced processing

circuits. All these make the digital system more complex than its analog counterpart.



Different types of communication systems

Table 1: Different types of communication systems

Signal type Transmission Example

Analog Analog Telephony

Analog Digital PCM

Digital Analog Modems

Digital Digital ISDN

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