In telecommunication a communications system is a collection of individual communication networks, transmission systems, relay stations, tributary stations, and data terminal equipment(DTE) usually capable of interconnection and interoperation to form an integrated whole. The components of a communications system serve a common purpose, are technically compatible, use common procedures, respond to controls, and operate in unison. Telecommunications is a method of communication (e.g., for sports broadcasting, mass media, journalism, etc.).
II. COMMUNICATION SYSTEM:
A communications subsystem is a functional unit or operational assembly that is smaller than the larger assembly under consideration. Examples of communications subsystems in the Defence Communications System (DCS) are (a) a satellite link with one Earth terminal in CONUS and one in Europe, (b) the interconnect facilities at each Earth terminal of the satellite link, and (c) an optical fibre cable with its driver and receiver in either of the interconnect facilities. Communication subsystem (b) basically consists of a receiver, frequency translator and a transmitter. It also contains transponders and other transponders in it and communication satellite communication system receives signals from the antenna subsystem.
FIG NO: 1 BLOCK DIAGRAM OF COMMUNICATION SYSTEM
ELECTRONIC COMMUNICATION SYSTEM
Unidirectional / nature of transmission bidirectional information technique
Simplex Analog Base band transmission
Half duplex Digital Using modulation
ELECTRONIC COMMUNICATION SYSTEM
1) Continuous 2) Pulse modulation PCM
Modulation PAM/ PWM /PPM DM
A: Analog communication system
1).Definition: The modulation in which one of the characteristics of the carrier is changed in corresponded to the instantaneous value of the modulating signal.
2) If the carrier is sinusoidal then its amplitude, frequency or phase changes in accordance with the modulating signal to obtain AM, FM or PM
3) Analog modulation can be pulsed modulation as well. here the carrier is in the form of rectangular pulse. The amplitude, width duration or position of the carrier pulse varies in accordance with the modulating signal to obtain the PAM, PWM OR PPM
4) Examples: Amplitude Modulation(AM), Frequency Modulation (FM) Pulse Modulation(PM), Pulse Position Modulation (PPM) Pulse Amplitude Modulation(PAM), Pulse Width Modulation (PWM)
5) Advantages: a) transmission & receivers antenna is simple.
b) Low band width requirement.
c) FDM can be used.
6) Disadvantages: a) noise effect the signal quality.
b) Noise and signal cannot be separated
c) Repeaters can be used
d) Coding is not possible
e) Security is not available for data
7) Applications: a) Radio broadcasting(AM&FM)
b) TV broadcasting
B: Digital Communication system:
1) Definition: The modulation technique in which the transmitted signal is in the digital pulse of constant amplitude, constant frequency and phase
FIG NO: 02 DIGITAL COMMUNICATION SYSTEM
2) Examples: Pulse code modulation (PCM) and Delta
a) In the PCM and DM, a train of digital pulse is transmitted by the transmitter.
b) All the pulses are of constant amplitude, width and position. The information is contained in the combination of the transmitted pulse.
a) Reliable communication
b) Less sensitivity to changes in environmental conditions (temperature, etc.)
c) Easy multiplexing
d) Easy signaling
e) Hook status, address digits, call progress information
f) Voice and data integration
g) Easy processing like encryption and compression
h) Easy system performance monitoring
i) QOS monitoring
j) Integration of transmission and switching
k) Signal regeneration, operation at low SNR, superior performance
l) Integration of services leading to ISDN
a) Increased bandwidth
b) 64 KB for a 4 KHz channel, without compression (However, less with compression)
c) Need for precision timing
d) Bit, character, frame synchronization needed
e) Analogue to Digital and Digital to Analogue conversions
f) Very often non-linear ADC and DAC used, some performance degradation
g) Higher complexity
a) Long distance communication between earth and space
b) Satellite communication
c) Military communication which needs coding
d) Telephone system
e) Data and computer communication
III: ADVANTAGES OF DIGITAL COMMUNICATION OVER ANALOG COMMUNICATION SYSTEM
A) NOISE IMMUNITY: Due to the digital nature of the the transmitted signal, the interference of additive noise does not introduce many errors. So digital communication has better noise immunity
Repeaters can be used between transmitters and receivers to regenerate the signal which further improves the noise immunity.
INFORMATION PER BIT INCREASES
BANDWIDTH EFFICIENCY INCREASES
NOISE IMMUNITY INCREASES
FIG NO: 03 DIGITAL COMMUNICATION SYSTEM TREND
FIG N0: 04 INCREASED NOISE IMMUNITY
FIG NO 05: INCREASED BANDWIDTH EFFICIENCY
B) POSSIBILITY OF ERROR HANDLING: Due to the channel coding technique in digital communication, it is possible to detect and correct the errors introduced during the data transmission
The existence of Noise as well as the variations in propagation characteristics of some communication channels makes the correct handling of errors vital. Some of the ways in which errors are handled include, error detection, error correction, data acknowledgment and data resends.
Techniques for error correction, such as performing a Cyclic Redundancy Check (CRC) on the data can guarantee detection of errors with vastly greater certainty than a typical system BER. This detection process can then be used, if required, to send an acknowledgment (ACK)or a failed acknowledgment (NAK) back to the transmitting node to request re-transmission.
Typically a system based on a combination of error detection and acknowledgments can perform well in a stationary environment where data latency is not a problem. The principle advantages of this type of system are the simplicity and the high data through-put relative to the data rate for good SNR conditions. This type of system doesn’t, however, tend to work well in poor SNR conditions due to the large number of re-sends necessary. In order to improve the typical BER of a mobile communications systems error detection and correction is usually employed. This involves the transmission of some additional data in order to allow the detection, identification and correction of errors.
C) EASY MULTIPLEXING, MULTIPLE ACCESS AND FREQUENCY SPREADING: TDM (Time Division Multiplexing) technique can be used to transmit many voice channel over a common single transmission channel.
Ttime-division multiplexing (TDM) is a type of digital multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel.
A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel plus a synchronization channel and sometimes error correction channel before the synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.
D) SECURITY: Digital communication is useful in military applications where only few permitted receivers can receive the transmitted signals
Communications are secure when the receiving end reliably detects whether the received information differs from the transmitted information. The standard IEC 834-1  contains recommendations for blocking, permissive, and direct tripping pilot schemes, in terms of their susceptibility to noise bursts. The short table below gives the expected minimum number of noise bursts required to produce an undesirable output
. To help detect noise bursts, we can add some redundant information to the transmitted message. Shannon gives a formal definition of redundancy:
Redundancy = Total Bits Transmitted – Information
For example, if we transmit a total of 10 bits, and there are eight bits of information, then the redundancy is 10 – 8 = 2 bits.
Redundancy is necessary, but not sufficient for security. One of the objectives of encoding is to make each of the distinct messages (e.g., for eight bits there are 256 different messages) as different as possible from the rest. The quantitative measure of this difference is the Hamming distance. It is defined as the minimum number of bits that could be corrupted in one distinct
message, which would result in a different distinct valid message. The simplest form of redundancy that increases Hamming distance is repetition.
Consider a single bit of information (permissive trip for instance) that must be received by the remote relay from the local relay. If the local relay transmits only that bit, the remote relay cannot detect whether the bit has been corrupted. The remote relay receives a one or a zero, and has no indication if the received value is the same as the transmitted value.
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Now assume that the local relay transmits the bit of information twice. The receiving relay compares the two bits. If they are the same, the receiving relay assumes they are correct and accepts the bit of information. But, if they differ, the receiving relay discards the information. When we inject noise bursts onto the channel, we generate messages almost at random, because the bit error rate is so high. There are two valid messages (00 and 11) and two invalid messages(10 and 01). Therefore, we expect that a randomly generated message will be accepted by the receiving relay half the time, or once per two noise bursts. Since that is a long way from 107, add more redundancy.
If the transmitting relay adds another bit of redundancy, then there are still only two valid messages (000 and 111), but there are now six invalid messages (001, 010, 011, 100 101, and110). The receiving relay will accept a randomly generated message two out of eight times, or one out of four, on average.
Every time we add a bit of redundancy, we cut the probability of accepting a randomly generated message by half. Thus, the expected number of randomly generated messages is 2n detected error, where n is the number of redundant bits.
We need log2 (107), or about 23.3, bits of redundancy to get 107 security. Our 1 bit of information, plus 24 redundant bits, yields a 25-bit message. This method does not make very good use of our channel however, because we must transmit 25bits to securely communicate a single bit of information. Now that we have the required security, we can add more information bits with no loss of redundancy.
Suppose we decide to repeat eight bits of information three times, and add some channel framing bits for 36 bits total. Four of these channel framing bits do not count as redundant bits, so there are 36 – 4 – 8 = 24 redundant bits. We have increased our information transmission capability by a factor of eight over the original single bit, decreased the rate of transmission by 1 – 36/25 =44%, and maintained 224 = 1.7 x 107 security to randomly generated messages.
E) SIGNAL PROCESSING: It is possible to use advanced data processing techniques such as digital signal processing. , image processing, data compression etc.
Digital signal processing (DSP) is concerned with the representation of signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing is subfield of signal processing. DSP includes subfields like: audio and speech signal processing, sonar and radar signal processing, sensor array processing, spectral estimation, statistical signal processing, digital image processing, signal processing for communications, control of systems, biomedical signal processing, seismic data processing, etc.
The goal of DSP is usually to measure, filter and/or compress continuous real-world analog signals. The first step is usually to convert the signal from an analog to a digital form, by sampling it using an analog-to-digital converter (ADC), which turns the analog signal into a stream of numbers. However, often, the required output signal is another analog output signal, which requires a digital-to-analog converter (DAC). Even if this process is more complex than analog processing and has a discrete value range the application of computational power to digital signal processing allows for many advantages over analog processing in many applications, such as error detection and correction in transmission as well as data compression,
Data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an encoded representation would use, through use of specific encoding schemes. Compression is useful because it helps reduce the consumption of expensive resources, such as hard disk space or transmission bandwidth. On the downside, compressed data must be decompressed to be used, and this extra processing may be detrimental to some applications. For instance, a compression scheme for video may require expensive hardware for the video to be decompressed fast enough to be viewed as it is being decompressed (the option of decompressing the video in full before watching it may be inconvenient, and requires storage space for the decompressed video). The design of data compression schemes therefore involves trade-offs among various factors, including the degree of compression, the amount of distortion introduced (if using a loss compression scheme) and the computational resources required to compress and uncompress the data.
F) ACCURACY: To design analog system : analog components like resistors capacitors and inductors are used .these components reduces accuracy of analog system but this is not so in digital systems.
G) EASY PROCESSING LIKE ENCRYPTION AND COMPRESSION: Encryption is the process of transforming information using an algorithm (called cipher) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information (in cryptography, referred to as cipher text). In many contexts, the word encryption also implicitly refers to the reverse process, decryption (e.g. “software for encryption” can typically also perform decryption), to make the encrypted information readable again (i.e. to make it unencrypted). Encryption has long been used by militaries and governments to facilitate secret communication. Encryption is now commonly used in protecting information within many kinds of civilian systems. Encryption is also used to protect data in transit, for example data being transferred via network (e.g. the Internet, e-commerce mobile telephones, wireless microphones wireless intercom systems, Bluetooth devices and bank automatic teller machines.. There have been numerous reports of data in transit being intercepted in recent years .Encrypting data in transit also helps to secure it as it is often difficult to physically secure all access to networks.
Data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an encoded representation would use, through use of specific encoding schemes. Compression is useful because it helps reduce the consumption of expensive resources, such as hard disk space or transmission bandwidth. On the downside, compressed data must be decompressed to be used, and this extra processing may be detrimental to some applications. For instance, a compression scheme for video may require expensive hardware for the video to be decompressed fast enough to be viewed as it is being decompressed (the option of decompressing the video in full before watching it may be inconvenient, and requires storage space for the decompressed video). The design of data compression schemes therefore involves trade-offs among various factors, including the degree of compression, the amount of distortion introduced (if using a loss compression scheme) and the computational resources required to compress and uncompress the data
H) INTEGRATION OF TRANSMISSION AND SWITCHING:
SWITCHING SYSTEM OPERATION : Path establishment -using extensive signaling
ô€ƒŠInformation interchange -using error free communication
ô€ƒŠFacilities -offering extensive facilities to subscribersô€ƒŠTariff computation -using extensive signaling
ô€ƒŠTearing down the path after information exchange is complete -using signaling
ô€ƒŠBilling -using computation facilities
ô€ƒŠMaintenance -using computation facilities and a few added equipment
ô€ƒŠPerformance measurement -using computation facilities and a few added equipments
I) EASY SYSTEM PERFORMANCE MONITORING:
Digital communications provide opportunities for performance monitoring, so the quality can be assessed, and problems can be quickly detected and remedied. One channel monitor tallies the time the received data are corrupted or absent, and normalize that time to the total elapsed time. This directly measures the unavailability of the communications. Although unavailability is a useful long-term measurement, it hides long Digital communications provide opportunities for performance monitoring, so the quality can be assessed, and problems can be quickly detected and remedied. One channel monitor tallies the time the received data are corrupted or absent, and normalize that time to the total elapsed time.
J) VOICE AND DATA INTEGRATION:
Basically, you start with a 4 KHz analog voice channel. Then you take”snapshot” of the voice signal’s amplitude every 1/8000th of a second (you have to sample at twice the maximum frequency to avoid a problem known as “aliasing”). Then you convert the measured amplitude to a number (the “quantization” process) that is represented by 8 bits. Thus, PCM requires 64 KBPS of digital bandwidth (8 KHz * 8 bits). This basic channel represents the first level of a digital hierarchy, known as a DS0.
A special type of Time-Division Multiplexer (TDM) called a “Channel Bank” takes 24 of these 64K DS0 channels and combines (multiplexes) them into a single aggregate rate of 1.544 MBPS. This rate is the combination of the channel data payload of 1.536 MBPS (64 KBPS * 24 Channels) + 8 KBPS of framing and synchronization bits. The 1.544 MBPS rate is known as the DS1 level in the digital hierarchy. Facilities that support this rate are usually referred to as “T-Spans” or “T1” circuits.
International standards were developed later. Although the basic hierarchal DS0 rate of 64 KBPS was preserved, the algorithm for converting the voice signal to a digital signal is different. Also, the International standard calls for 30 voice channels + a 64 KBPS synchronization channel + a 64 KBPS signalling channel. Therefore, these systems operate at a rate of 2.048 MPBS (1.920 MBPS + 64 KBPS + 64 KBPS). Facilities that support this rate are usually referred to as “E1” circuits.
Using a transmission line code known as Bipolar-Alternate Mark Inversion (AMI), a 1.544 MBPS T1 circuit requires 772 KHz of analog bandwidth. So, why go digital? I could use Frequency Division Multiplexing (FDM) and combine those same 24 channels into a 96 KHz (4 KHz * 24) analog pipe, right? While FDM saves bandwidth, noise is added as the signal travels through every amplifier and modulator. In a digital system, “ones” and “zeroes” go in, and “ones” and “zeroes” go out. Since major sources of analog noise are removed in digital systems, circuit lengths can be extended, and network topologies simplified through the reduction of the number of circuits required between any two telephone exchanges. Quality improves, operating costs decrease!
Wireless communications system use has exploded, with dramatic growth in Cellular voice and data technologies. Of particular interest are the merger of AT&T and McCaw Cellular (Cellular 1) and the development of the Cellular Digital Packet Data (CDPD) standards. Additional frequency allocations have recently occurred for the development of wireless Personal Communications Systems (PCS), in an unprecedented spectrum auction by the Federal Communications Commission (FCC).
Slow, but steady increases are seen in the use of Integrated Services Digital Networks (ISDN); providing higher speed digital access capabilities to the residence and businesses. New methods of integrating voice and data, as well as Local Area and Wide Area networks, are under development. These new “cell-based” transmission technologies are known as Switched Multimegabit Data Service (SMDS), Asynchronous Transfer Mode (ATM), and Broadband ISDN.
K) QOS MONITORING;
Digital communications provide opportunities for performance monitoring, so the quality can be assessed, and problems can be quickly detected and remedied. One channel monitor tallies the time the received data are corrupted or absent, and normalize that time to the total elapsed time. This directly measures the unavailability of the communications. Although unavailability is a useful long-term measurement, it hides long but In frequent channel disturbances. For example, suppose a channel monitor is set to alarm when the unavailability exceeds 500 x 10-6. If that channel is error-free for one year and then the channel is completely lost, the unavailability monitor will not alarm until four hours later.
A second monitor can be used to alarm when the channel is not available for a certain continuous time, say one second. The unavailability alarm responds to gradual degradations in bit-error rate. The duration alarm responds more quickly to outright channel failures .A sample report from such a monitor follows. It reports the 256 most-recent errors, the average unavailability for the time of the report, and the longest-duration channel outage.
L) INTEGRATION OF SERVICES LEADING TO ISDN
ISDN is based on concepts developed for telephony. Therefore, evolutionary changes. Transition from the present network to ISDN may require about one decade. End-to-end digital connectivity to be obtained digital transmission, TDM switching and or SDM switching. Present ITU standards part of new standards In early development of ISDN interim measures needed for interfacing with present networks
Supports a wide using range of voice and non-voice applications Switched and non-switched connections Circuit switching and packet switching Based on 64 Kbps channels Intelligence for providing service features, maintenance and management integrated Layered protocol used Flexibility for implementation at specific national situations
M) SIGNAL REGENERATION, OPERATION AT LOW SNR, SUPERIOR PERFORMANCE:
FIG NO: 06 SIGNAL DEGRADATION AND REGENERATION
Signal-to-noise ratio is defined as the power ratio between a signal (meaningful information) and the background noise (unwanted signal):
where P is average power. Both signal and noise power must be measured at the same or equivalent points in a system, and within the same system bandwidth. If the signal and the noise are measured across the same impedance then the SNR can be obtained by calculating the square of the amplitude ratio:
where A is root mean square (RMS) amplitude (for example, RMS voltage). Because many signals have a very wide dynamic range, SNRs are often expressed using the logarithmic decibel scale. In decibels, the SNR is defined as
which may equivalently be written using amplitude ratios as
The concepts of signal-to-noise ratio and dynamic range are closely related. Dynamic range measures the ratio between the strongest un-distorted signal on a channel and the minimum discernable signal, which for most purposes is the noise level. SNR measures the ratio between an arbitrary signal level (not necessarily the most powerful signal possible) and noise. Measuring signal-to-noise ratios requires the selection of a representative or reference signal. SNR is usually taken to indicate an average signal-to-noise ratio, as it is possible that (near) instantaneous signal-to-noise ratios will be considerably different. The concept can be understood as normalizing the noise level to 1 (0 dB) and measuring how far the signal ‘stands out’.
All real measurements are disturbed by noise. This includes electronic noise, but can also include external events that affect the measured phenomenon – wind, vibrations, gravitational attraction of the moon, variations of temperature, variations of humidity, etc. depending on what is measured and of the sensitivity of the device. It is often possible to reduce the noise by controlling the environment. Otherwise, when the characteristics of the noise are known and are different from the signals, it is possible to filter it or to process the signal. When the signal is constant or periodic and the noise is random, it is possible to enhance the SNR by averaging the measurement.
N) REMOTE PROCESSING: analog systems are difficult to store because of noise and distortion while digital signals can easily stored on media like magnetic tapes ,disc etc. thus compared to analog digital can easily be transposed . so remote processing of digital can be done easily. Systems and methods for remote digital signal processing of multiple signals are disclosed. The system generally includes a mobile communication device with a first microphone for receiving a first acoustic signal and a second microphone for receiving a second acoustic signal. The first acoustic signal and the second acoustic signal are transmitted to a processing station for processing using a wireless protocol.
MANUFACTURING: It is being simpler and cheaper for manufacturing of devices as compared to analog due to the invention of high speed computers and ICs
VERSABILITY: Digital system can be reprogrammed for other applications( where programmable DSP chips are used) .Moreover ,digital system can be ported to different hardware.
REPEATABILITY: Digital systems can be easily duplicated. These system does not depend upon component tolerance and temperature
IMPLEMENTATION OF ALGORITHMS: implementation of mathematical communicational processing can be done easily in case of digital communication systems.
EASY UPGRADATION: Because of use of software, digital communication system can be easily upgraded as compared to analog systems.
COMPATIBILITY: In digital communication system all applications need standard hardware .hence operation of digital communication and processing is mainly dependent upon software .Hence universal compatibility is possible as compared to analog.
LESS SENSITIVE TO CHANGE IN ENVIRONMENTAL CONDTIONS: Digital communication system is less effected to environmental conditions like temperature etc. as compared to analog systems.
RELIABLE COMMUNICATION: The above points from A to U clearly depicts that digital communication system is more reliable than analog communication system
IV FUTURE OF DIGITAL COMMUNICATION SYSTEM:
A high-profile report into the future of the UK’s ability to compete in the vital area of creative, digital and information technology has urged the government to take urgent steps to keep the country competitive. The report comes from a Task Force – a group of academics and business leaders – set up to examine the issue and co-chaired by the University of Surrey’s Vice Chancellor Professor Chris Snowden.
The UK’s future economic prosperity relies, in part, on the ability of government, industry and universities to spark rapid growth in its Creative, Digital and Information Technology businesses, according to The Fuse a report published by the Council for Industry and Higher Education (CIHE) today (Wednesday 8 September).
The digital market is set to exceed $3 trillion revenue in the next four years, and entertainment and media $1.7 trillion. In the wake of this growth new industries have emerged that are simultaneously creative, digital and IT focused. With technology and content industries currently contributing £102 billion in gross value added to the economy and the Coalition Government’s first Comprehensive Spending Review just weeks away, The Fuse argues it is vital that the UK claims a leading position in this fiercely competitive, fast-paced global market.
The landmark report presents a series of urgent recommendations from CIHE’s Creative, Digital and Information Technology industries Task Force – a group of influential figures from industry and academia co-chaired by Rona Fairleads, Chairman and CEO of the Financial Times Group, and Professor Christopher Snowden, Vice Chancellor of the University of Surrey.
Its main findings are:
â€¢ Coalition Government and devolved administrations should acknowledge CDIT (creative, digital and IT industries) as a strategic priority alongside STEM
â€¢ CDIT employers challenged to work more closely with universities on graduate employability and design of courses
â€¢ ICT curriculum criticized for failing to teach fundamental computing principles
Professor Christopher Snowden said: “The CDIT industries already play a very important part in the economy, with the UK a leading contributor to this global industry. This report captures the dynamic and vibrant nature of the businesses and the important contribution of higher education both in terms of developing skills and contributing to the growth of this sector. Most importantly it identifies the support needed to build on the successes in the UK and extend the contribution of these industries to the economy, ensuring future prosperity and growth.”
The report’s editor Dr David Docherty, CEO of the CIHE and Chair of the Digital TV Group, said: “We believe that the UK has a window of opportunity in which to establish itself in the highly competitive, multi-trillion dollar CDIT market or be left trailing behind countries such as China, the US, Japan and Australia.
“We have to compete hard for our share of this revenue. To do this the UK Government must recognize CDIT industries as a national priority in the same way as it has science, engineering and manufacturing. UK universities and businesses, meanwhile, need to learn from and replicate the initiatives and innovation environments which brought the world Google, Amazon and Face book.”
Dr Mike Short, Vice President, Research and Development, O2, added: “CDIT industries together should be the horizontal platform for growth and competitiveness for the UK in
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