issues in communications

8/2/2019 Issues in Communications

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A: Issues in Communication System Design

OVERVIEW

To design a communication system according to user requirements is a challenging task because the

requirements differ from user to user and there are many design trade-offs to be considered. The design of a

communication system has to be carried out keeping in view the following factors:

What are the information sources? Data, voice, fax, video, or all of them? Depending on therequirements, the bandwidth requirements will be different.

What is the coverage area? The coverage area decides which transmission medium has to be chosen,and sometimes a combination of media may be required (for instance, a combination of twisted pair and

satellite).

Is a secure network required? For defense and corporate networks, a highly secure system isrequired to ensure that the information is kept confidential. Special security features have to be

incorporated in such cases.

What are the performance criteria? For data applications, particularly such as in banking and otherfinancial transactions, performance requirements are very stringent. For instance, even a one-bit error ina million bits transmitted is not acceptable. Such criteria call for efficient modulation techniques and

error detection/correction mechanisms. On the other hand, for voice and video applications, delay shouldbe minimum.

What are the signaling requirements? Is a separate signaling channel/network required? Which national/international standards have to be followed? The days of providing proprietary

solutions are gone; every communication system has to be designed according to the availableinternational standards. Before designing a communication system, the relevant standards need to be

studied, and the design has to be carried out.

Given an unlimited budget, we can design a world-class communication system for every user segment.

But the user is always constrained by budget. For a given budget, to design the optimal system thatmeets the requirements of the user is of course the biggest challenge, and as usual, we need to consider

the various trade-offs.

We will discuss the issues involved in designing communication systems. We also study the important aspects

in designing radio communication systems. The special attraction of the radio systems is that they providemobility to users, but every attraction comes with a premiumradio systems pose special design challenges.

When designing a communication system, the following requirements need to be considered: coverage area,information sources, security issues, performance issues, signaling requirements, international/national

standards to be followed, and the cost.


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A.1 DATA RATES

In a digital system, the information to be transmitted is converted into binary data (ones and zeros). In the case

of text, characters are converted into ASCII format and transmitted. In audio or video, the analog signal is

converted into a digital format and then transmitted.

To make best use of a communication channel, the data rate has to be reduced to the extent possible without

compromising quality. All information (text, graphics, voice, or video) contains redundancy, and thisredundancy can be removed using compression techniques. Use of low bit rate coding techniques (also called

source coding techniques) is very important to use the bandwidth efficiently. Particularly in radio systems,

where radio bandwidth has a price, low-bit rate coding is used extensively.

When designing a communication system, the designer has to consider the following issues related to

information data rates:

What are the information sources to the communication system: data, voice, fax, video, or acombination of these?

How many information sources are there, and is there a need for multiplexing them beforetransmitting on the channel?

How many information sources need to use the communication channel simultaneously?This determines whether the channel bandwidth is enough, whether multiple access needs to be used,

etc.

If the channel bandwidth is not sufficient to cater to the user requirements, the designer has to considerusing data compression techniques. A trade-off is possible between quality and bandwidth requirement.

For instance, voice signals can be coded at 4.8kbps, which allows many more voice channels to be

accommodated on a given communication channel, but quality would not be as good as 64kbps PCM-coded voice.

The services to be supported by the communication system

data, voice, fax, and video services

decide thedata rate requirement. Based on the available communication bandwidth, the designer has to consider low bit

rate coding of the various information sources.

NoteCompression techniques can be divided into two categories: (a) lossless compression techniques; and(b) lossy compression techniques. The file compression techniques such as Winzip are lossless because

the original data is obtained by unzipping the file. Compression techniques for voice, image, and videoare lossy techniques because compression causes degradation of the quality.


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A.2 MODULATION TECHNIQUES

For designing an analog communication system, analog modulation techniques such as AM and FM are used. If

you are designing a digital communication system, the choices are digital modulation techniques such as ASK,

FSK, and PSK. Many variations of these basic schemes are available that give slightly different performancecharacteristics. Choice of a modulation scheme needs to take into consideration the performance requirements

as well as the availability of hardware for implementing the modulator and demodulator.

In a communication system, an important design parameter is Bit Error Rate (BER). To achieve a good BER (to

reduce the bit errors as much as possible), the Signal-to-Noise Ratio (SNR) should be high. SNR and E b/No are

related by the formula

Where: No is the noise power density in watts/Hertz. The total noise in a signal with bandwidth of B is given by

N = No B. Hence,

The BER can be reduced by increasing the Eb/No value, which can be achieved either by increasing the

bandwidth or by decreasing the data rate.

The curves shown inFigure A.1give the performance of the modulation schemes. These curves are known as

the waterfall curves. In designing a communication system, based on the required BER, the Eb/No value is

obtained for a given modulation scheme. PSK and QPSK perform better compared to ASK and FSK because fora given BER, the value of Eb/No is less, and hence with less energy of the signal, we can achieve good

performance. Another criterion to be considered is the ease of implementing the modulator/ demodulator. It is

much easier to implement ASK and FSK modulators and demodulators as compared to PSK and its variations.

Figure 10.1: Performance curves for Digital Modulation Systems.

For digital communication systems, the BER is an important design parameter. BER can be reduced by

increasing the value of Eb/No (where Eb is the energy per bit and No is the noise power density).

Note: Choosing a specific modulation technique depends on two important factorsBER performance andcomplexity of the modulator/demodulator circuitry.


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A.3 PERFORMANCE CRITERIA

The performance of a digital communication system is measured in terms of the Bit Error Rate (BER). BER is

measured in terms of the ratio of the number of bits in error as a percentage of the total number of bits received.

Depending on the application, the BER requirements vary. For applications such as banking, a very high BER is

required, of the order of 1012

i.e., out of 1012

bits only one bit can go wrong (even that has to be corrected or aretransmission requested). For applications such as voice, a BER of 104 is acceptable.

As we discussed in the previous section, BER is dependent on the modulation technique used. The performancein terms of the BER also is dependent on the transmission medium. A satellite channel, for example, is

characterized by a high bit error rate (generally, around 106

; in such a case, a higher-layer protocol (data link

layer) has to implement error detection techniques and automatic repeat request (ARQ) protocols such as stop-

and-wait and sliding window.

Because it is not possible to achieve a completely error-free transmission, errors should be detected or correctedusing error-detection or error-correcting codes. After detection of errors, using automatic repeat request (ARQ)protocols, the receiver has to request retransmission of data.

A.4 ERROR DETECTION AND CORRECTION

The source coding techniques are used to reduce the redundancy in the signal. Because the transmissionmedium introduces errors, we need to devise methods so that the receiver can either detect the errors or correct

the errors. To achieve this, error detection techniques and error correction techniques are used. These techniquesincrease the bandwidth requirement, but they provide reliable communication. Error detection is done throughCRC and error correction through FEC.

Because the communication channel introduces errors in the bit stream, error detection techniques need to beincorporated. If errors are detected, the receiver can ask for retransmission. If retransmissions have to be

reduced, error correction techniques need to be incorporated.

Note

In voice and video communications, a higher bit error rate can be tolerated. Even if there is one error in

10,000 bits, there will not be perceptible difference in voice/video quality. However, for dataapplications, a very reliable data transfer is a must.


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A.5 SECURITY ISSUES

Security is of paramount importance because systems are prone to attacks. The various security threats can be

the following:

Interruption: The intended recipient is not allowed to receive the data this is an attack on the availability ofthe system.

Interception: The intended recipient receives the data, but unauthorized persons also receive the datathis isan attack on the confidentiality of the data.

Modification: An unauthorized person receives the data, modifies it, and then sends it to the intendedrecipientthis is an attack on the integrity of the data.

Fabrication: An unauthorized person generates the data and sends it to a personthis is an attack on theauthenticity of the system.

To overcome these security threats, the data has to be encrypted. Encryption is a mechanism wherein the user

data is transformed using an encryption key. Only those who have the encryption key can decrypt the data.

There are two possibilities: link encryption and end-to-end encryption. In link encryption, at the transmittingend, the data is encrypted and sent over the communication link. At the receiving end of the link, the data is

decrypted. In end-to-end encryption, the user encrypts the data and sends it over the communication link, and

the recipient decrypts the data. To provide high security, both types of encryption can be employed. Note that

encryption does not increase the data rate (or bandwidth). Length of the encryption key decides how safe theencryption mechanism is. Though 56- and 64-bit keys were used in earlier days, now 512- and 1024-bit keys are

being used for highly secure communication systems.

The major security threats are: interruption, interception, modification, and fabrication. The data is encrypted atthe transmitting end to overcome these security threats. At the receiving end, the data is decrypted.

NoteFor encryption, there will be an encryption algorithm and an encryption key. The encryption algorithmspecifies the procedure for modifying the data using an encryption key. The algorithm can be made

public (known to everyone), but the encryption key is kept secret.


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A.6 RADIO SYSTEM DESIGN ISSUES

Radio system design poses special problems because of the special nature of the radio signal propagation. The

important design issues are:

Frequency of operation: Radio systems cannot be operated in any frequency of our choice. Frequencyallocation needs to be obtained from the centralized authority of the government. Only certain bands such as

ham radio band and Industrial, Scientific, and Medical (ISM) band are unlicensed, and anyone can use thesebands without getting a license from the government authorities.

Radio survey: Radio frequency propagation characteristics depend on many factors, such as natural terrain(presence of hills and valleys, lakes) and artificial terrain (presence of high-rise buildings). A radio survey mustbe carried out to decide where to keep the antennas to achieve the maximum possible coverage. Multipath

fading causes signal degradation. Measures have to be taken to reduce the effect of multipath fading.

NoteThe propagation characteristics differ for different frequency bands. A number of mathematical modelsare available to analyze radio propagation. The natural terrain (presence of hills, lakes, greenery) and

artificial terrain (presence of tall buildings) also affect radio propagation.

Line of sight communication: Some radio systems are line of sight systems, that is, there should not be anyobstructions such as tall buildings/hills between the transmitting station and the receiving station. In the case of

broadcasting applications, the transmitting antennas have to be located at the right places to obtain the

maximum coverage. Systems such as AM broadcast systems do not have this limitation because the radio waves

are reflected by the ionosphere, and hence the range is very high.

Path loss calculations: When a signal is transmitted with particular signal strength, the signal traverses alarge distance and becomes attenuated. The loss of signal strength due to the propagation in the atmosphere and

attenuation in the communication subsystems (such as filters and the cable connecting the radio equipment to

the antenna) is calledpath loss. The path loss calculations have to be done to ensure that the minimum required

signal strength is available to the receiver to decode the information content. The receiver should be sensitiveenough to decode the signals. The required BER, SNR, gain of the antenna, modulation technique used, rain

attenuation, and gain of the amplifiers used are some of the parameters considered during the path loss

calculations.

NoteFor all radio systems, path loss calculations are very important. Based on the path loss calculations, thereceiver sensitivity, antenna gain, amplifier gains, etc. are calculated when designing radio systems.

Rain attenuation: The attenuation of the radio signals due to rain varies, depending on the frequency band.For instance, in satellite communication, at 17/12GHz the rain attenuation is very high as compared to 6/4GHz.

This aspect has to be taken into consideration in the path loss calculations.

Radio bandwidth: Radio spectrum being a limited natural resource, the bandwidth of a radio channel has tobe fully utilized. To achieve this, efficient source coding techniques have to be used. For example, to transmitvoice over a radio channel, it is not advisable to use 64kbps PCM (though many systems still use it). A better

approach would be to use low bit rate coding techniques (such as ADPCM, LPC, or its variations) so that in a

given radio bandwidth, more voice channels can be pumped in.

NoteAs radio spectrum is a limited natural resource, the radio channel has to be fully utilized. Using low bitrate coding of voice/video signals and choosing an efficient modulation technique are very important inradio system design.


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Radio channels: A radio channel consists of a pair of frequenciesone frequency from base station to the endstation and one frequency from end station to the base station. A minimum separation is required between the

uplink and downlink frequencies.

Multiple access : Radio systems use multiple access techniques to make efficient use of bandwidth. FDMA,TDMA, and CDMA systems, as discussed earlier, have different spectrum requirements and different

complexities.

All these issues need to be kept in mind when designing radio systems.

Design of radio systems involves special issues to be addressed. These include frequency of operation, radiopropagation characteristics, path loss calculations, rain attenuation, efficient usage of radio spectrum through

low-bit rate coding of voice and video signals, and usage of multiple access techniques.

A.7 TELECOMMUNICATION STANDARDS

In telecommunication system design, the standards play a very important role. The days when organizations

used to develop proprietary interfaces and protocols are gone. Before embarking on a system design, the

designer has to look at international/national standards for the interfaces and protocols. The various standardsbodies for telecommunication/data communications are:

American National Standards Institute (ANSI) Electronics Industries Association (EIA) European Telecommunications Standards Institute (ETSI) Internet Engineering Task Force (IETF) International Organization for Standardization (ISO)

International Telecommunications Union Telecommunications Services Sector (ITU-T), earlier knownas CCITT

Institute of Electrical and Electronics Engineers (IEEE)Throughout this book, we will mention a number of relevant standards related to telecommunication systems

and interfaces as well as communication protocols. Referring to the standards documents is very important toget an in-depth knowledge of the specifications, particularly during implementation.

Nowadays, communication system design is driven by international standards. The standards formulated bystandardization bodies such as ANSI, EIA, ETSI, IETF, ISO, ITU-T and IEEE need to be followed while

designing communication systems and protocols.


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A.8 COST

The cost is the most important of the design parameters. To design a communication system that meets all the

performance criteria at minimum cost is the major challenge to communication engineers. The choice of the

transmission medium (twisted pair, coaxial cable, radio or fiber, etc.) needs to keep in view the cost. Incommunication system design, engineers do not develop each and every subsystem. The various subsystems are

procured from different vendors and integrated. In such a case, the engineer has to choose the subsystem that

meets all the performance requirements and is cost effective. Experience is the best teacher for choosing theright subsystems.

Cost is the most important design parameter while designing communication systems. To design a system thatmeets all the performance requirements at the lowest cost is the challenge for all communication engineers.

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