Digital Communication System Block Diagram with Explanation

DIGITAL COMMUNICATION SYSTEM

Introduction

Digital technology is a branch of Electronics and communication. It makes use of digital signals, which are presented in discrete steps. A continuous signal is an analog signal. The output of a data source is transmitted from one point to another in a digital communication system. A data system can transmit rectangular pulses at speeds ranging from 100 to 500 Kbits per second.

A typical application of this system are:

• Computer to computer communication

• Computer interrogation (either for data storing or data manipulation and calculation)

• Programming

• Data collection

• Telemetry and alarm system

• Financial credit information

• Transfer travel and accommodation booking services

Digital communication may be classified into two categories. They are.

• A web-based system (Online System)

Data is transferred directly to or from a computer in an online system. The online system can either be real-time or non-real-time. A real-time system demands an immediate reaction. The data transmission speed in an on-real system is significantly slowed.

• Offline System

Data is sent to or from an intermediate storage location, such as a card, a paper tape punch, magnetic tape, or a disc, in an off-line system.

Based on transmission, digital communication can be classified into three categories:

1.  Simplex

Simplex refers to a one-way channel connection. here the communication is possible in only one direction.

Example: Radio, TV, etc.

2. Semi-Duplex

A semi-duplex connection is required for either direction transmission. Communication can be possible only one way at a specified time.

3. Full-Duplex 

A full-duplexer allows data to be transmitted in both ways at the same time.

BASIC ELEMENTS OF DIGITAL COMMUNICATION SYSTEM

A digital information source generates a limited number of messages. A nice example of a digital source is a typewriter. A source's ability to emit a finite number of characters (messages) is restricted.

A digital communication system is a system that sends data from a digital source to a digital sink. A digital waveform is a time function with just a discrete set of possible values. Only two values exist in the digital waveform since it is a binary waveform.

Voltages and currents with digital signals are common in electronic digital communication systems.

If an analog signal must be transmitted, it is first converted to a digital signal using an Analog to Digital Converter before being sent over the channels. Because an analog signal has a continuous range of values that is a function of time.

For example, a sine wave of 1000 Hz might be used to represent a binary 1 in a digital line transmission, whereas a sine wave of 500 Hz could be used to represent a binary 0. As a result, analog waveforms produced from an analog source are used to convey digital data.

Block diagram of Digital Communication System with Explanation

Digital signals are the coded representation of information that is transmitted in digital communication.

Figure: Block diagram of a Digital Communication System

The figure shows a block schematic of the essential parts of a digital communication system.

The block diagram explanation is:

INFORMATION SOURCE

The message signal to be sent is produced by the information source. In the case of a digital source, the information source generates a discrete and random message signal that does not change over time.

When using an analog source, the data is analog and changes over time. By sampling and quantizing the analog signal, an Analog to Digital Converter may transform it into a digital signal.

A message is created by a source of information, such as a human voice, a television image, teletype data, or the temperature and pressure of the atmosphere.

✓ Because the message in these cases is not electrical, a transducer is needed to transform it into an electrical waveform known as the message signal.

✓A baseband signal is another term for the waveform. The term "baseband" refers to the frequency range that the message signal generated at the source falls inside.

✓ An analog or digital message signal can be used.

✓ The amplitudes and times of analog signals fluctuate constantly across their intervals. A voice signal, a television signal, and the location of the signal are all examples of signal types.

✓ Both the amplitude and tome of a digital signal are discrete. Digital signals include computer data and telegraph transmissions.

✓ As illustrated in the block diagram, an analog signal may always be transformed into a digital signal by combining three fundamental operations: sampling, quantizing and encoding.

✓ Only sample values of the analog signal at regularly spaced discrete-time instants are maintained during the sampling procedure.

✓ Each sample value is approximated by the nearest discrete level in a limited collection of levels throughout the quantizing processes. The selected level is represented by a code word with a predetermined number of code components throughout the encoding procedure.

SOURCE ENCODER

The source encoder receives the symbols supplied by the information source. These symbols aren't capable of being directly communicated. The source encoder is responsible for converting them into digital format.

The code words for the symbols are assigned by the source encoder. A unique code word exists for each different symbol. They choose a unique value in the data set to give a series of codes to a specific piece of data. Pulse code modulators, delta modulators, vector quantizers, and other sources encoders are examples of common source encoders.

The encode function in source coding converts the digital signal created at the source output into other digital signals. It is a one-to-one mapping that is used to eliminate or minimize redundancy. As a result of this, we can deliver an effective service.

CHANNEL ENCODER

The channel encoder transforms a binary sequence into a transmittable message or information signal. Noise and interference may be introduced into the signal transmission process. Channel encoding is used to avoid these mistakes.

The input sequence is supplemented by the channel encoder by some superfluous binary bits. With some well-defined logic, these superfluous bits are inserted.

✓ The goal of channel coding is for the encoder to map the incoming digital signal into a channel input and for the decoder to map the channel output into an output digital signal, allowing for reliable communication across a noisy channel.

✓ This is accomplished by adding redundancy in a predetermined manner in the channel encoder and exploiting it in the decoder to precisely recreate the original encoder input.

✓ We reduce redundancy in source coding, but we incorporate controlled redundancy in channel coding to mitigate the channel noise impact.

✓ We can do source coding on its own, channel coding on its own, or both. Naturally, in the latter situation, the source encoding is done as indicated in fig.

✓ As illustrated in fig. 1, we create in reverse order in the receiver: channel decoding comes first, followed by channel encoding in the transmitter.

✓ In the receiver, we create in reverse order channel decoding first, followed by source decoding for any combination, resulting in a gain in system performance at the expense of higher circuit complexity.

CHANNEL

The communication channel is a physical medium that is used to transfer signals from a transmitter to a receiver located at a distance. Any digital communication system's information backbone is formed by it. Optical Fiber Cables can also provide a higher data rate.

This channel in a wireless system is made up of the atmosphere. To complete the link, a multi-hop system may include coaxial cables, fiber optic cables, and microwave links. Satellite channels make global communication relatively simple.

DIGITAL DEMODULATOR

The digital demodulator transforms the communication channel's input modulated signal to a binary bit sequence.

The digital modulator converts the input binary sequence into an analog signal waveform that can be sent via a communication channel without being distorted.

The following modulators are used to generate equivalent modulated analog signals appropriate for transmission across a band width-restricted analog transmission line of short-haul or long haul systems, using digital on-off signals as keys.

1 Amplitude Shift Keying (ASK)

2 Frequency Shift Keying (FSK)

3 Phase Shift Keying (PSK)

CHANNEL DECODER

The channel decoder reconstructs an error-free, precise bit sequence while reducing the impacts of noise and distortion in the channel.

SOURCE DECODER

The source decoder reverses the source encoder's action. It transforms the channel encoder's binary output to a symbol sequence. Decoders with variable and fixed lengths are also conceivable. Memory is used by certain decoders to store the code words.

Advantages of digital communication

• Digital circuits that are relatively affordable can be employed.

• Data encryption is used to protect privacy.

• The dynamic range (the difference between the highest and lowest value) can be increased.

• Data from Voice, Video, and Data sources may all be combined and sent via a single digital transmission system.

• Unlike analog systems, noise does not build from repeater to repeater in long-distance networks.

• Errors detected are minor, even when the received signal contains a lot of noise, i.e. the S/N Ratio in Digital Systems is high. • Errors are frequently rectified by using error correction coding.

Disadvantages of digital communication

• More bandwidth is necessary for digital systems than for analog systems and synchronization is required.

Satellite Multiple Access Techniques

MULTIPLE ACCESS TECHNIQUES IN SATELLITE COMMUNICATION NOTES

Mobile users share the wireless spectrum at the same time. The sharing might be based on frequency, time, or code. Wireless telephony employs duplexing technology, which permits talking and listening at the same time, similar to FDD frequency. Division Duplexing provides each user with two bands of frequencies. Because a simplex channel is made up of a forward and a reverse channel, each duplex channel is made up of one simplex channel.

The forward and reverse channels are separated by a frequency range across the system. TDD, or time division duplexing, uses time instead of frequency to have identical forward and reverse channels.

Both FDD and TDD have advantages and drawbacks. In wireless communication systems, these multiple access methods utilizing any of these approaches are utilized.

DIFFERENT MULTIPLE ACCESS TECHNIQUES:

THE THREE important multiple access techniques are:

1 TIME DIVISION MULTIPLE ACCESS

2 FREQUENCY DIVISION MULTIPLE ACCESS

3 CODE DIVISION MULTIPLE ACCESS

Time Division Multiple Access (TDMA)

The time slots of the whole available time are shared between several people. To achieve bidirectional communication, each duplex channel (TDD) includes distinct forward and reverse time slots. The figure represents a TDD time slot with the same frequency:

 

This TDD mode allows any transceiver to function as both a transmitter and a receiver. TDD is commonly seen in cordless phones. The TDMA concept is shown. In a TDMA system, the entire spectrum is split into time slots, with each time slot allowing just one user to utilize the radio channel.

Digital data transmission and digital modulation are permitted in TDMA. Many customers can tune in to their favorite station within their designated time intervals. As demonstrated, several users' transmissions are interlaced into a single frame. 

The synchronization information and address information are included in the preamble field in this way. The guard bits are used to allow separate receivers to be synchronized across different frames and time periods.

It is believed that there are N time slots for N users, with each user accessing the channel during their allotted time period.

The following are some of the characteristics of TDMA: 1)Each user of the TDMA multiple access systems has the same carrier frequency, but no time periods overlap.

Frequency Division Multiple Access in Mobile Communication

All users share the satellite at the same time using FDMA, but each broadcasts in its frequency band. When using analog modulation, when signals are present all of the time, this is the most typical technique used. As shown in the diagram, the available transponder bandwidth is shared among the users, allowing them to all broadcast at the same time.

The most basic type of multiple access is FDMA. It assigns a carrier frequency (or many carrier frequencies for busy stations) and a bandwidth around that carrier frequency to an earth station permanently. All of the station's outgoing traffic is frequency modulated on that carrier, regardless of destination.

Each carrier is allocated a specific frequency band for the uplink in fixed-frequency operation, and other carriers share that band. Several carriers share frequency bands for demand multiple access (DMA), with a specific band assigned based on availability at the time of need.

In an FDMA system, each carrier spectra must be sufficiently separated from one another to avoid carrier crosstalk. The nearby low-power carrier will be affected by a high-power carrier. Excessive separation, on the other hand, causes satellite bandwidth to be squandered. For accessing, FDMA uses extremely easy frequency adjustment and provides virtually independent channel off operation.

A satellite's frequency plane is made up of the carrier frequencies and bandwidth allotted to all ground stations. Every FDMA station must be able to receive at least one carrier from all of the other stations in the network.

Bidirectional communication is feasible in conversational telephonic systems at the same time, and it is also necessary for cellular communication. A duplexer is a device that allows you to talk and listen at the same time. Frequency multiplexing/duplexing is referred to as frequency division duplexing (FDD) since it is done with frequency.

The duplex channel in FDD has two simplex channels, forward and reverse, as shown in the diagram. The forward frequency band transmits radio traffic from the base station BTS to the mobile unit, whereas the reverse frequency band transmits radio traffic from the mobile unit to the base station BTS. To enable simultaneous conversion, a duplex device is kept in both the mobile unit and the base station.

Some features of the FDMA scheme:

1) To eliminate adjacent channel interferences ACI, FDMA requires appropriate filtering on the receiver side.

2) In the FDMA scheme, if a channel is not in use, it is considered idle and is not used by other users. As a result, there is a risk of resource waste.

3) A single phone circuit can be handled by an FDMA channel at a time.

Types:

1) fixed assignment multiple access FAMA

2) Demand assignment multiple access DAMA

Non-linear effects in FDMA SCHEME OF MULTIPLEXING:

Multiple radio channels share the same antenna at the base station in this system. For optimum power and efficiency, power amplifiers are run at saturation, and they are non-linear. Intermodulation frequency production is caused by the dispersion of signals throughout the whole frequency domain. It will amplify interferences in the actual transmission, therefore IM should be kept to a minimum.

Code Division Multiple Access (CDMA)

Many earth stations broadcast orthogonally coded spread-spectrum signals in the same frequency range at the same time via CDMA. Decoding systems receive several stations' mixed broadcasts and retrieve one. Users can broadcast at the same time and share the frequency allotment under the CDMA system. The diagram represents the block diagram of the CDMA System.

 

Several users utilize the whole transponder bandwidth in this system at all times. Signals from various users are encoded so that information from a single transmitter can only be recognized and retrieved by a correctly synchronized receiving station that understands the code. That is, each receiving station has its code, referred to as its address, and anytime a transmitting station intends to send a message to that receiver, it simply modulates its broadcast with the desired receiver's address.

At an earth station, carrier separation is accomplished by identifying the carrier with the correct address. These addresses are often in the form of a periodic binary sequence that modulates the carrier or changes the carrier's frequency state. The carrier correlation procedure is used to identify addresses. For code generators, a digital address is obtained.

A station address generator repeats its address sequence, which is overlaid on the carrier alongside the data.

CDMA is better suited to military tactical communication environments where numerous small groups of mobile stations are only connected for a short period at irregular intervals.

There may be an issue of near-far impact since the same channel is used by multiple users. When compared to other multiple access methods, the major advantage of CDMA is the lower degree of interference. At the receiving end, the receiver selectively adjusts to hearing the indented signal of the users since each user/subscriber is assigned a distinct pseudo-random codeword that is orthogonal to all other pseudo-random codewords of remaining users. To avoid near-far issues, CDMA uses proper power control methods.

Some features of CDMA multiple access schemes:

1) In CDMA, if the spreading sequences are not perfectly orthogonal from one user to another, there is a risk of self-jamming. As a result, this spreading sequence or pseudo-random noise code must be meticulously prepared before being multiplied by the message signal.

2) Compared to TDMA and FDMA, CDMA has a higher soft capacity limit.

3) The RSSI (radio signal strength indicator) is used in CDMA to improve power control.

Types

1) Fixed assignment multiple access

2) Demand assignment multiple access

Electrostatic Loudspeaker Working

Electrostatic Loudspeaker - Working and Construction

An electrostatic loudspeaker is a type of loudspeaker that generates sound by applying force to a membrane that is kept in an electrostatic field.

Figure: Electro Static Loudspeaker

Construction: The figure represents the construction and connections of an electrostatic loudspeaker. The electrostatic loudspeaker is made out of a thin flat diaphragm and two conducting grids (or stators). The diaphragm is placed between two electrically conducting grids, with a tiny air gap between the diaphragm and grids. The diaphragm is typically constructed of a polyester sheet (thickness 2 to 20 m) with outstanding mechanical characteristics. The grids are just perforated metal sheets.

Working: The diaphragm is kept at a DC potential of several kilovolts to the grids by the conductive coating and an external high voltage source. The audio signal controls the grids. The front and rear grids are driven in opposite directions. As a consequence, a homogeneous electrostatic field proportionate to the audio stream is created between both grids. This exerts a force on the charged diaphragm, and the ensuing movement pushes the air on each side of it. It is a high impedance gadget. As a result, impedance matching is required to utilize a standard amplifier. A transformer is most commonly utilized. It must maintain a consistent transformation ratio throughout the audio frequency spectrum to avoid distortion.

Advantages:

1. The diaphragm's extremely low weight and has a great frequency response

2. Improved stereo recording reproduction.

Disadvantages:

1. Poor bass response

2. Sensitivity to the amount of humidity in the surrounding environment.

HIGH FIDELITY (HI-FI) ELECTROSTATIC SPEAKER: 

The definition of hi-fi sound is a reproduced sound that has a similarity to the original or direct sound launched from the source and has undergone some conversion through the system or multiple systems. Hi-fidelity is considered to be attained when the published sound contains unnoticeable distortion from the original, when there is little external noise, and when the volume levels and acoustic effects of the room are readily audible. This replicated sound may be more pleasant to the listener at the system output than the original live sound at the source. 

(A) WOOFER, MIDRANGE, AND TWEETER: 

The capacity of a receiver to reproduce distinct frequency components is known as its fidelity. A single speaker cannot accurately replicate all frequency components.

Figure: Connection of Hi-Fi Speakers (or) Two Way Cross Over Network

Woofer:

i) Woofer is a cone with a huge diameter that is used to reproduce low-frequency signals.

Tweeter:

ii) Tweeter is a device used to recreate high-frequency signals with a tiny cone-diameter.

Driver or Squawker:

iii) The driver or squawker loudspeakers are located between the tweeter and woofer to reproduce mid frequencies ranging from 300Hz to 5000Hz.

The diagram represents the connecting of woofer and tweeter loudspeakers. It is a bidirectional cross-over network.

Frequency Response Characteristics: 

The figure represents the frequency response of the woofer, tweeter, and squawker.

Figure: Frequency Response Characteristics

(B) SPECIFICATION AND RANGE OF WOOFERS AND TWEETERS: 

Loudspeakers come in a variety of forms, including round, oval, and hexagonal. The speakers are identified by the letter The speakers are identified by:

a. Speaker frame's diameter

b. Type.

c. Power consumption.

d. The impedance of the voice coil.

CROSS OVER NETWORK: 

Several control circuits are employed to enhance the quality and performance of amplifiers and radio receivers. Cross over network refers to these circuits. In multi-way speaker systems, cross-over networks are utilized to divide the input sound into several frequency bands. The figure depicts a three-way cross-over network.

Figure: Three-Way Cross over Network

The mid-frequency range is a band that exists between the low and high pass frequencies. An inductor L1 serves as the low pass filter, while a capacitor C2 serves as the high pass filter. An inductor L2 and a capacitor C1 comprise the bandpass filter. The capacitor C2, which is linked in series with the tweeter, prevents low and mid-frequency sounds from reaching the tweeter. Similarly, putting inductance L1 in series with the woofer prevents high frequencies from reaching it. Inductance L2 and capacitor C1 connected in series with the squawker circuit keep low and high frequencies from reaching the squawker.

SURROUND SOUND SYSTEM TYPES

Surround sound, which utilizes Multichannel audio, refers to a set of techniques for improving (extending and deepening) the quality of sound reproduction from a recorded source. The additional recorded sound channels are replicated in this system by employing additional discrete speakers. With audio channels above and below the listener, the three-dimensional (3D) sphere of human hearing may be practically realized. Surround sound technology is utilized in both cinema and home theatre systems, as well as video game consoles and personal computers.

1. Creating Surround Sound:

Surround sound, which utilizes Multichannel audio, refers to a set of techniques for improving (expanding and deepening) the quality of sound reproduction from a recorded source. The additional recorded sound channels are replicated in this system by employing additional discrete speakers. With audio channels above and below the listener, the three-dimensional (3D) sphere of human hearing may be practically realized. Surround sound technology is utilized in both cinema and home theatre systems, as well as video game consoles and personal computers.

2. Mapping Channels to Speakers: 

Surround sound systems depend on each source channel being assigned to its own set of loudspeakers. The number and content of the source channels are recovered and applied to the appropriate loudspeakers using matrix systems. The transmission medium of a discrete surround system supports (at least) the same number of source and destination channels. One-to-one channel-to-speaker mapping is not the sole technique to provide surround sound signals.

3. Bass Management: 

Bass management may be used in surround sound systems. The basic concept is that bass content in the incoming signal, regardless of channel, should be sent exclusively to loudspeakers capable of handling it.

4. Surround Sound Specification: 

The number of channels, not the number of speakers, can be represented by each specification description. The many channel surround systems are described here.

5. Low-Frequency Effects (LEF) Channel: 

It was designed to transmit incredibly low sub – bass' cinematic sound effects on its channel. The LFE was originally a separate channel supplied to one or more subwoofers in the original movie theatre installation.

Types of Surround Sound System: 

(A) 3.0 Channel Surround (Analog multiplexed, Dolby Surround): 

This method extracts three audio channels from a two-channel source that has been specifically encoded.

• Two channels – left (L) and right (R) – for the front speakers (R)

• One channel for a surround speaker or a rear-facing speaker – surround (S)

Placement: Three identical speakers are arranged equidistantly around a central listening point. If two back speakers are utilized, they should be placed somewhat behind the listening position, above ear height. 

(B) 4.0 Channel Surround (Analog Multiplexed / Discrete, Quadraphonic): 

In this method, four audio channels are retrieved from either a specifically encoded two-channel source or a four-channel source.

• There are two channels for front-facing speakers – left (L) and right (R) (R)

• There are two channels for rear surround speakers – surround right (RS) and surround left (LS) (LS)

• Describes the early matrixes system and discrete quadraphonic surround systems.

Placement: This method is solely used for music. All speakers should be angled at 45 degrees. All speakers should be placed above the level of the ear. 

(C) 5.1 Channel Surround (3-2 Stereo) (Analog Matrixed, Dolby Pro Logic – II):

Five audio channels and one LFE channel are derived from a specially encoded two-channel or stereo source in this system. 5.1 Surround sound can also be referred to as 3-2 stereo. The phrase 3-2 refers to three front speakers and two back speakers.

• Front-facing speakers with two channels – left (L) and right (R) (R)

• One channel for a speaker in the center - (C)

• Two channels for rear surround speakers – surround left (LS) and surround right (SR) (RS)

• One low-frequency effects channel (LFE).

• Describes the Dolby Pro Logic – II matrixed surround system.

Placement: The left and right speakers are angled at ± 30°. The rear speakers should be angled around ± 110°. 

(D) 6.1 Channel Surround:

This method extracts six audio channels and one LFE channel from a specifically encoded two-channel or stereo source.

• Front speakers have two channels: left (L) and right (R) (R)

• One channel for the center speaker - center (C), side left (LS), and side right (SR) (RS)

• Two channels for side surround speakers – side left (LS) and side right (SR) (RS)

• One channel for rear surround speakers - back surround channel (BS)

• One low-frequency channel for driving a subwoofer (SW)

(E). 7.1 Channel Surround:

This method extracts seven audio channels from an eight-channel source. This system is employed in the home entertainment system.

• Front speakers have two channels: left (L) and right (R) (R)

• One Channel for the Center Speaker – Center (C)

• Two channels for side surround speakers – left surround (LS) and right surround (RS) (RS)

• There is just one low-frequency effect (LFE) channel.

(F). 10.2 Channel Surround:

10.2 Channel Surround has 14 distinct channels.

• Five front speakers are available: left wide, left, center, right, and right wide.

• There are five surround channels: left surround diffuse, direct, rear surround, right surround diffuse, and direct.

• Two LFE channels (LFE left and LFE right) and two height channels (LFE left and LFE right).

Facsimile Communication System

Facsimile Communication Systems

In a communications system, it is necessary to send visual signals in addition to fundamental signals such as speech, music, or telegraph codes. The receiving end of a facsimile transmission receives an identical replica of a document, image, or still photograph. A television system varies from a facsimile in that the scene can be ‘live' (i.e., include movement). Television transmission needs a greater bandwidth, whereas facsimile transmission is achievable over telephone lines.

Uses

i) Photographic transmission ( For example, in the press)

ii) Transmission of papers, weather maps, and so forth.

iii) Transmission of linguistic texts for which the teleprinter is insufficient (example: Chinese)

Facsimile sender

The scannable message can take one of three forms:

i)  A single sheet, which is often wrapped around a cylindrical drum in the sender to allow scanning.

ii) Tape that is too narrow to be continuous.

iii) Continuous sheet paper, often known as wide tape.

Scanning is accomplished in two ways.

1. Optical scanning, in which a laser spot moves through the message.

2. Resistance scanning, in which the message's letters have variable resistance and are brought into the circuit by touching and sliding a stylus over them.

Cylindrical scanning

The message is initially clipped around the drum in this technique. The drum is then simultaneously rotated around its axis and traversed along with it under a fixed scanning point. The light reflected from the scanning region is focused on a photocell. The photocell's electrical output represents the signal.

Cylindrical Scanning

This system's configuration is represented in Figure. The signal is converted into a modulated wave by the chopper disc, and the carrier frequency is determined by the speed of the disc. The modulated signal is simpler to amplify than the straight photocell signal.

The message lighting area is rather big, and a mask with a tiny aperture produces the spot that illuminates the photocell. In most scanning configurations, the spot takes a spiraling route around the drum. Scanning in a succession of closed rings is an alternative configuration. As the fixing clips pass beneath it, the spot moves from one ring to the next. However, this approach is not widely utilized.

Facsimile receiver

The mechanical characteristics of scanning in the receiver are similar to those in the transmitter, and identical equipment is usually used on both ends. Scanning in the receiver produces an optical output from the electrical input. This is the inverse of what happens in the transmitter. For the received signal to have the right link to the broadcast signal, the signals must be synchronized, passed appropriately, and have the same height/breath ratio.

Synchronization

If the information is documented, it is sufficient to utilize synchronous motors for both transmitter and receiver and to work on frequency control supply mains. A synchronization signal with a frequency of 1020 Hz must be delivered when the image is sent (international standards). The transmitter speed has a known connection to this, and the receiver speed is changed via a stroboscope to match the relationship with an accuracy of 1 in about 10⁵.

When the signal is modulated onto a carrier, the carrier must be sent together with the sideband. Since the carrier is present, the precise 1020Hz synchronizing signals may be retrieved. For signal recovery, a local oscillator at the receiver is enough. The consequence of improper phasing is seen in the figure.

Figure:  A – Input, B – Effect of incorrect Synchronization, C – Effect of Incorrect Phasing  Phasing

To ensure that the image of the clips holding the paper to the drum does not overlap the transmitted picture, proper phasing is required. The following procedure is used to modify the pulley phasing for each transmitted image.

The receiver operator first adjusts the speed to the proper value using the synchronizing signal and then places the drum in the correct starting position. A switch secures this position. A pulsed signal is sent from the sender to mark the commencement of transmission, and the pulse releases the switch that holds the receiver drum. The figure depicts the result of improper phasing.

Index of co-operation

The height/breadth ratio must be the same for both transmitted and received images, which is determined by the scanning pitch and the diameters of the drums used in the transmitter and receiver. The product of total line length and the number of lines per unit length divided by π is the Index of Cooperation.

Figure: Sender and Receiver Correlation in FAX.

Let

D→ diameter of sending drum

d→ diameter of receiving drum

P→ scanning pitch of sender

p→ scanning pitch of receiver

n→ number of lines scanned

The transmitted picture has a breadth of nP, while the received picture has a breadth of np. The broadcast image's height is proportional to D, whereas the received image's height is proportional to 'd.' As a result, the proper height/breadth ratio must be maintained.

Figure:  Effect of Index of Cooperation

IOC has a fixed value of 352 as per CCITT. The figure represents the consequences of receiving facsimile images with varying IOC levels.

Direct recording reception

This method uses a highly absorbent chemically treated paper. When a voltage is given to the paper using a metal stylus, the electrolyte dissociates, and one of the dissociation products reacts with the stylus to produce a metallic salt.

This, in turn, interacts with a color chemical in the paper, resulting in a mark on the paper. The intensity of the mark is determined by the quantity of dissociation, i.e. the signal voltage. Since it generates black coloration, a steel stylus is frequently utilized. The paper must be maintained in sealed containers. It has a shelf life of around one month after opening.

A resistant paper marketed commercially as Teledeltos paper is used in another form of direct recording reception. This comprises a metalized backing with a carbon black-like material placed on top and a very thin layer of insulation on top of it. The paper is pressed down with the help of a stylus. The burning happens when the signal voltage is applied, resulting in the blackening of the paper.

GSM Architecture Block Diagram

Digital Cellular System: Digital cellular systems are those which incorporate digital modulation techniques. Digital systems provide significant improvements in capacity and system performance. The United States Digital Cellular System (USDC) was created in the late 1980s to handle more users in a given spectrum allotment. USDC is a TDMA (time division multiple access) technologies. USDC can provide up to six times the capacity of AMPS.

The USDC standard has the same 45MHZ FDD scheme as AMPS. The dual-mode USDC/AMPS system, which had been implemented in Canada and Mexico, was standardised as interim standard 54 ( IS-54) by the electronic industry north American Digital Cellular (NADC). The USDC system is intended to share the same frequency reuse scheme and base stations as the AMPS system, enabling base stations and consumer units to be supplied with both AMPS and USDC channels from within the same type of hardware. USDC forward and reverse control channels utilise the same signalling method as AMPS phones to ensure compatibility.

Global System for Mobile Communications (GSM)

The secondary standard is the Global System for Mobile (GSM). It was created to address the fragmentation issues that plagued Europe's first cellular infrastructure. GSM was the world's first cellular system to define digital modulation as well as network-level architecture and services. It is the most widely used second-generation (second generation) technology in the world.

GSM was originally designed to be a pan-European cellular service that offered several communication services via ISDN, but it is now the world's most advanced standard for new cellular radio and personal communication devices globally. As of 2001, GSM has over 350 million consumers worldwide.

In 1991, GSM was initially launched on to European market. For marketing purposes, GSM changed its name to the global system for mobile communications in 1992. By the end of 1993, numerous non-European nations in South America, Asia, and Australia had embraced GSM, which supports personal communication service (PCS) in the newly established 1.8GHZ to 2.0GHZ radio bands throughout the world.

GSM Services

GSM services adhere to ISDN standards and are classed as teleservices or data services. Teleservices include both conventional mobile phone traffic and mobile generated traffic. Data service includes both computer-to-computer communication and packet-switched traffic. User services are classified into three kinds.

Telephone Services — This service includes emergency dialling and facsimiles. Videotex and Teletex are also supported by GSM, although they are not part of the GSM standard.

Bearer or data services - These services include packet-switched methods and transfer rates ranging from 300 bps to 9.6 kbps. Data can be transmitted in either transparent or non-transparent mode (where GSM provides standard channel coding for user data) (where GSM offers special coding efficiency based on specific data services).

Supplementary ISDN services — These are digital services that include all diversions, closed user groups, and caller identifications. These services are not available in analogue mobile networks. Short message services (SMS) are another supplementary service that allows GSM customers and base stations to send alphanumeric pages of a certain length.

SMS can be used for safety and advisory purposes, such as broadcasting highway or weather information to all GSM users within the reception range.

GSM's most notable feature is the Subscribers Identity Module ( SIM ). SIM cards are memory devices that contain information such as the subscriber's identification number, the networks and countries where the subscriber is eligible for services, private keys, and other user-specific information. A SIM card with a four-digit personal ID number is used by a subscriber to activate services from any GSM phone.

SMS services are available as smart cards (credit card-sized cards that can be put into any GSM phone) or plug-in modules. All GSM phones are similar and inoperable unless a SIM card is inserted. Subscribers may insert their SIM card into any appropriate terminal, such as a hotel phone, public phone, or any portable or mobile phone, and have all incoming GSM calls routed to that terminal and all outbound calls billed to their home phones, regardless of where they are in the globe.

The system's provision of air privacy is the second prominent characteristic of GSM. In contrast to analogue FM cellular phone systems, which may be easily observed, eavesdropping on a GSM radio transmission is nearly difficult. Encrypting the digital bitstream sent by a GSM transmitter with a secret cryptographic key known only to the cellular carrier allows for privacy. For each user, this key evolves. Before designing GSM equipment or developing a GSM system, each carrier and GSM equipment manufacturer must sign a memorandum of understanding (MOU). The Memorandum of Understanding (MOU) is an international agreement that permits nations and carriers to share encryption algorithms and other private data.

GSM System Architecture

The GSM framework is made up of three primary linked subsystems. They are as follows: 1) Base station subsystem (BSS), 2) Network and switching subsystems, and 3) Operation support subsystem (OSS). Through specific network interfaces, the subsystems communicated with one another and with the users.

The mobile station (MS) is also a subsystem, although it is typically regarded to be part of the BSS for architectural purposes. The BSS, also known as the radio subsystem, offers and controls radio transmission routes between the mobile switching centre and the base station (MSC). The BSS also controls the radio interface between mobile stations and all GSM subsystems. Each BSS is composed of numerous base station controllers (BSCs), which connect the MS to the NSS through MSCs.

NSS manages system switching and connects MSCs to other networks such as PSTN and ISDN.

Figure: GSM Architecture

The OSS permits engineers to monitor, diagnose, and troubleshoot all elements of the GSM system and hence supports its operation and maintenance. This subsystem communicates with the others in the GSM network.

The picture is a block diagram of the GSM architecture. The radio air interface connects the mobile stations (MSs) to the base station subsystem (BSS). The BSS is made up of several BSCs that are linked together by a signal MSC. Each BSC may manage hundreds of Base Transceiver Stations (BTSs). Some BTSs may be co-located at the BSC, while others may be dispersed widely and physically linked to the BSC through a microwave connection or special leased lines.

The picture represents the various interfaces used in GSM. The interface that links a BTS to a BSC is known as the Abis interface. GSM has standardised the Abis interface for all manufacturers, which transfers traffic and maintenance data. BTS and BSC equipment from the same vendor might be used to minimise small differences.

Physically, the BSCs are interconnected to the MSC through dedicated/leased lines or a microwave link. The A interface, which is specified within GSM, is the interface between a BSC and an MSC. The A-interface employs the signalling correction control portion (SCCP) of the SS7 protocol. It allows for communication between the MSC and the BSS, as well as network communications between individual subscribers and the MSC. An interface enables a service provider to employ multiple manufacturers' base stations and switching equipment.

The NSS is in charge of routing GSM calls between external networks and the radio subsystem's BSCs. The MSC is the NSS's core unit, and it manages traffic between all of the BSCs.

NSS has three databases. They are as follows: 1) Home location registers ( HLR ), 2) Visitor location registers ( VLR ), and 3) Authentication centres ( AUC ). The HLR is a database containing subscriber and location information for each user who lives in the same city as the MSC. Each GSM user in a certain GSM market is given a unique international mobile subscriber identifier (IMSI). Each home user is identified by this number.

The VLR is a database that temporarily saves the IMSI and customer information for each roaming subscriber who visits a certain MSC's coverage area. The VLR is linked to multiple neighbouring MSCs in a certain region and stores subscription information for every visiting user in the area.

When a travelling mobile is registered in the VLR, the MSC provides the required information to the visiting subscriber's HLR so that calls to the roaming mobile can be routed correctly over the PSTN by the roaming user's  HLR.

The Authentication Center is a highly secure data centre that maintains each HLR and VLR subscriber's authentication and encryption credentials. The Authentication Center maintains a database known as the Equipment Identify Register (EIR), which identifies stolen or fraudulently altered phones that communicate identify data that does not match the information in either the HLR or VLR.

The OSS provides support for one or more operation maintenance centres (OMC), which monitor and maintain the operation of each MS, BS, BSC, and MSC in a GSM system.

The OSS performs three functions: 1) maintain all telecommunications hardware and network operations with a specific market, 2) manage all pricing and invoicing procedures, and 3) manage all mobile equipment in the system.

Various interfaces used in GSM