data communication unit1 as per pune university

Unit=1Data Communication Prof.Hitesh Mohapatra

Today computer is available in many offices and homes and therefore there is a need to share

data and programs among various computers. With the advancement of data communication facilities the communication between computers has increased and thus it has extended the power of computer beyond the computer room. Now a user sitting at one place can communicate with computers of any remote site through communication channel. The aim of this lesson is to introduce you the various aspects of computer network.

Data Communication

• We all are acquainted with some sorts of communication in our day to day life. For communication of information and messages we use telephone and postal communication systems.

• Similarly data and information from one computer system can be transmitted to other systems across geographical areas.

• Thus data transmission is the movement of information using some standard methods. These methods include electrical signals carried along a conductor, optical signals along an optical fibers and electromagnetic areas.

• Suppose a manager has to write several letters to various clients. First he has to use his PC and Word Processing package to prepare the letter, if the PC is connected to all the client's PC through networking, he can send the letters to all the clients within minutes.

• Thus irrespective of geographical areas, if PCs are connected through communication channel, the data and information, computer files and any other programs can be transmitted to other computer systems within seconds.

• The modern form of communication like e-mail and Internet is possible only because of computer networking.

Basic Elements of a Communication System

• The sender (source) who creates the message to be transmitted• A medium that carries the message• The receiver (sink) who receives the message

Communication Protocols

You may be wondering how computers send and receive data across communication links. The answer is data communication software. It is this software that enables us to communicate with other systems. The data communication software instructs computer systems and devices as to how exactly data is to be transferred from one place to another. The procedure of data transformation in the form of software is commonly known as protocol. The data transmission software or protocols perform the following functions for the efficient and error free transmission of data.

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1. Data sequencing: A long message to be transmitted is broken into smaller packets of fixed size for error free data transmission.2. Data Routing: It is the process of finding the most efficient route between source and destination before sending the data.3. Flow Control: All machines are not equally efficient in terms of speed. Hence the flow control regulates the process of sending data between fast sender and slow receiver.4. Error Control: Error detecting and recovering is the one of the main functions of communication software. It ensures that data are transmitted without any error.

Topology and Network Design:

A topology refers to the manner in which the cable is run to individual workstations on the network. The dictionary defines topology as: the configurations formed by the connections between devices on a local area network (LAN) or between two or more LANs

There are three basic network topologies (not counting variations thereon): the bus, the star, and the ring.

It is important to make a distinction between a topology and architecture. A topology is concerned with the physical arrangement of the network components. In contrast, an architecture addresses the components themselves and how a system is structured (cable access methods, lower level protocols, topology, etc.). An example of architecture is 10baseT Ethernet which typically uses the start topology.

Bus Topology

A bus topology connects each computer (node) to a single segment trunk. A ‘trunk’ is a communication line, typically coax cable, which is referred to as the ‘bus.’ The signal travels from one end of the bus to the other. A terminator is required at each end to absorb the signal so it does not reflect back across the bus.

In a bus topology, signals are broadcast to all stations. Each computer checks the address on the signal (data frame) as it passes along the bus. If the signal’s address matches that of the computer, the computer processes the signal. If the address doesn’t match, the computer takes no action and the signal travels on down the bus.

Only one computer can ‘talk’ on a network at a time. A media access method called CSMA/CD is used to handle the collisions that occur when two signals are placed on the wire at the same time.



The bus topology is passive. In other words, the computers on the bus simply ‘listen’ for a signal; they are not responsible for moving the signal along.

In a regular bus, each computer is attached to the cable segment (called a backbone) by means of a drop cable (a shorter cable connecting the computer to the backbone)

In a local bus, each computer is attached directly to the backbone in a daisy-chain configuration by means of a "T" connector. Peer-to-peer networks are often configured as a local bus.

Advantages of bus topology:

• Easy to implement and extend • Well suited for temporary networks that must be set up in a hurry • Typically the least cheapest topology to implement • Failure of one station does not affect others

Disadvantages of bus topology:

• Difficult to administer/troubleshoot • Limited cable length and number of stations • A cable break can disable the entire network; no redundancy • Maintenance costs may be higher in the long run • Performance degrades as additional computers are added

Star Topology

All of the stations in a star topology are connected to a central unit called a hub.

The hub offers a common connection for all stations on the network. Each station has its own direct cable connection to the hub. In most cases, this means more cable is required than for a bus topology. However, this makes adding or moving computers a relatively easy task; simply plug them into a cable outlet on the wall



If a cable is cut, it only affects the computer that was attached to it. This eliminates the single point of failure problem associated with the bus topology. (Unless, of course, the hub itself goes down.)

Star topologies are normally implemented using twisted pair cable, specifically unshielded twisted pair (UTP). The star topology is probably the most common form of network topology currently in use.

Advantages of star topology:

• Easy to add new stations • Easy to monitor and troubleshoot • Can accommodate different wiring

Disadvantages of ring topology:

• Failure of hub cripples attached stations • More cable required

Ring Topology

A ring topology consists of a set of stations connected serially by cable. In other words, it’s a circle or ring of computers. There are no terminated ends to the cable; the signal travels around the circle in a clockwise direction.

Note that while this topology functions logically as ring, it is physically wired as a star. The central connector is not called a hub but a Multistation Access Unit or MAU. (Don’t confuse a Token Ring MAU with a ‘Media Adapter Unit’ which is actually a transceiver.

Under the ring concept, a signal is transferred sequentially via a "token" from one station to the next. When a station wants to transmit, it "grabs" the token, attaches data and an address to it, and then sends it around the ring. The token travels along the ring until it reaches the destination address. The receiving computer acknowledges receipt with a return message to the sender. The sender then releases the token for use by another computer.

Each station on the ring has equal access but only one station can talk at a time.

In contrast to the ‘passive’ topology of the bus, the ring employs an ‘active’ topology. Each station repeats or ’boosts’ the signal before passing it on to the next station.

Rings are normally implemented using twisted pair or fiber-optic cable.

Advantages of ring topology:



• Growth of system has minimal impact on performance • All stations have equal access

Disadvantages of ring topology:

• Most expensive topology • Failure of one computer may impact others • Complex

Mesh Topology

In a mesh topology, each of the network node, computer and other devices, are interconnected with one another. Every node not only sends its own signals but also relays data from other nodes. In fact a true mesh topology is the one where every node is connected to every other node in the network. This type of topology is very expensive as there are many redundant connections, thus it is not mostly used in computer networks. It is commonly used in wireless networks. Flooding or routing technique is used in mesh topology.

Advantages of Mesh topology

1) Data can be transmitted from different devices simultaneously. This topology can withstand high traffic.2) Even if one of the components fails there is always an alternative present. So data transfer doesn’t get affected.3) Expansion and modification in topology can be done without disrupting other nodes.

Disadvantages of Mesh topology

1) There are high chances of redundancy in many of the network connections.2) Overall cost of this network is way too high as compared to other network topologies.3) Set-up and maintenance of this topology is very difficult. Even administration of the network is tough.Hybrid topologyA hybrid topology is a combination of any two or more network topologies in such a way that the resulting network does not have one of the standard forms. For example, a tree network connected to a tree network is still a tree network, but two star networks connected together exhibit hybrid network topologies. A hybrid topology is always produced when two different basic network topologies are connected.

The following factors should be considered when choosing a topology:

• Installation • Maintenance and troubleshooting • Expected growth • Distances


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• Infrastructure • Existing network

As a general rule, a bus topology is the cheapest to install, but may be more expensive to maintain because it does not provide for redundancy.

Transmission ModeA transmission mode is the manner in which data is sent over the underlying medium

Transmission modes can be divided into two fundamental categories:• Serial — one bit is sent at a time• Parallel — multiple bits are sent at the same time

Parallel TransmissionParallel transmission allows transfers of multiple data bits at the same time over separate media:

• used with a wired medium• the signals on all wires are synchronized so that a bit travels across each of the wires at precisely

the same time

The figure omits two important details:

• a parallel interface usually contains other wires that allow the sender and receiver to coordinate• to make installation and troubleshooting easy, the wires are placed in a single physical cable

A parallel mode of transmission has two chief advantages:(1) High speed--it can send N bits at the same time.(2) It can match the speed of the underlying hardware.

Serial Transmission Serial transmission sends one bit at a time most communication systems use serial mode, because:



• serial networks can be extended over long distances at less cost • using only one physical wire means that there is never a timing problem caused by one wire being

slightly longer than another.

• Sender and receiver must contain a hardware that converts data from the parallel form used in the device to the serial form used on the wire

Simplex, Half-Duplex, and Full-Duplex Transmission

A communications channel can be classified as one of three types:

• Simplex: A simplex mechanism can only transfer data in a single direction. It is analogous to broadcast radio or television

• Full-Duplex: Full-duplex allows transmission in two directions simultaneously.

• Half-Duplex: A half-duplex mechanism involves a shared transmission medium. The shared medium can be used for communication in each direction but the communication cannot proceed simultaneously.

Asynchronous: In asynchronous transmission data is transferred character by character and each character (frame by frame i.e. Each character is an asynchronous frame in asynchronous transmission) and can be 5 to 8 bits long. The term “Asynchronous” means it is asynchronous at frame level. The bits are still synchronized at bit level during reception



• In a steady stream, interval between characters is uniform (length of stop element can be 1,1.5 or 2 stop bits - as programmed earlier)

• In idle state, receiver looks for transition 1 to 0 (start signal) • Then samples next five, seven or eight intervals (as programmed earlier) Timing only needs maintaining

within each frame (bit level). • Looks for parity (if programmed earlier) • Then looks for next 1 to 0 for next frame • Simple, Cheap.• Minimum hardware & software requirement to implement. • Overhead of 2 or 3 bits per frame (~20%) • Good for data with large gaps in between each frame (keyboard, low speed data..)

Start bit: It is prefixed to each byte and equals 0. Thus it ensures a transition from 1 to 0 at onset of

transmission of byte. The leading edge of start bit is used as a reference for generating clock pulses at required sampling instants. Thus each onset of a byte results in resynchronization of receiver clock.

Stop bit: To ensure that transition from 1 to 0 is always present at beginning of a byte it is necessary that

default state be 1. But there may be two bytes one immediately following the other and if last bit of first byte is 0, transition from 1 to 0 will not occur. Therefore a stop bit is suffixed to each byte equaling 1. Its duration is usually 1, 1.5,2 bits. Asynchronous transmission is simple and cheap but requires an overhead of 3 bits i.e. for 7 bit code 2 (start, stop bits)+1 parity bit implying 30% overhead. However % can be reduced by sending larger blocks of data but then timing errors between receiver and sender cannot be tolerated beyond [50/no. of bits in block] % (assuming sampling is done at middle of bit interval). It will not only result in incorrect sampling but also misaligned bit count i.e. a data bit can be mistaken for stop bit if receiver's clock is faster.For example:

• a user typing on a keyboard

• a user that clicks on a hyperlinkSynchronous:Synchronous transmission In Synchronous transmission a block of data in the form of bits stream is transferred without start / stop bits. The block can be of any arbitrary length. In order to establish synchronization with remote computer the transmitter transmits synch pulses initially. When the receiver locks to the transmitter’s clock frequency a block of data gets transmitted. The Characteristics are as follows • Block of data transmitted without start or stop bits • Initially synch pulses are transmitted (Clocks must be synchronized) Data Communications • Can use separate clock line (In that case synch pulses are not needed!) • Good over short distances



• Subject to impairments • Embed clock signal in data (Manchester encoding) • Carrier frequency (analog) is used • Need to indicate start and end of block • Use preamble and post amble (to leave sufficient space between blocks) • More efficient (lower overhead) than asynchronous transmission.

Bit Stuffing:

Suppose our flag bits are 01111110 (six 1's). So the transmitter will always insert an extra 0 bit after each occurrence of five 1's (except for flags). After detecting a starting flag the receiver monitors the bit stream. If pattern of five 1's appear, the sixth is examined and if it is 0 it is deleted else if it is 1 and next is 0 the combination is accepted as a flag. Similarly byte stuffing is used for byte oriented transmission. Here we use an escape sequence to prefix a byte similar to flag and 2 escape sequences if byte is itself a escape sequence.

Example:Remote communication between computers and related devices like card reader and printers.

Bit rate and Baud rate: Bit Rate of a transmission system = number of bits transmitted per time unit .The baud rate of a

data communications system is the number of symbols per second transferred. A symbol may have more than two states, so it may represent more than one binary bit (a binary bit always represents exactly two states). Therefore the baud rate may not equal the bit rate, especially in the case of recent modems, which can have (for example) up to nine bits per symbol.

Bandwidth:A term used to describe the data-handling capacity of a communication service is bandwidth. And

width is the range of frequencies that is available for the transmission of data. A narrow range of frequencies in a communication system is analogous to a garden hose with a small diameter. The flow of information in such a system its data rate is restricted, just as is the flow of water in the narrow hose. Wider bandwidths permit more rapid information flow. The communication data transfer rate is measured in a unit called baud. Baud is identical to bits per second.

The bandwidth is the width of the frequency range that can be used for transmission over the channel. The bandwidth limits the maximal bit rate that can be obtained using a given channel.



Any signal can be treated as if it were made up of an infinite number of frequency components. We have seen the spectrum of a square wave already in which case the (significant) frequency components include much higher frequencies than the fundamental frequency. To transmit the waveform without significant distortion would therefore require a channel with a considerable bandwidth, together with a suitable phase change characteristic.A pulse train does not have to be received undistorted in order to make correct decisions about its binary states. In fact, in the case illustrated, so long as the fundamental component, at f=l/ (T) Hz, of the square wave corresponding to the bit stream ...01010101… can be transmitted, then correct decisions can be made about the binary states. It is possible, in theory at least, to transmit 2/T symbols per second over a channel of bandwidth 2/(T) Hz.

B = 2 x fBits per second = 2 x bandwidth

Classification of Signals:

Some important classifications of signals

Analog vs. Digital signals: as stated in the previous lecture, a signal with a magnitude that may take any real value in a specific range is called an analog signal while a signal with amplitude that takes only a finite number of values is called a digital signal.

Continuous-time vs. discrete-time signals: continuous-time signals may be analog or digital signals such that their magnitudes are defined for all values of t, while discrete-time signal are analog or digital signals with magnitudes that are defined at specific instants of time only and are undefined for other time instants.

Periodic vs. aperiodic signals: periodic signals are those that are constructed from a specific shape that repeats regularly after a specific amount of time T0, [i.e., a periodic signal f(t) with period T0

satisfies f(t) = f(t+nT0) for all integer values of n], while aperiodic signals do not repeat regularly. Deterministic vs. probabilistic signals: deterministic signals are those that can be computed

beforehand at any instant of time while a probabilistic signal is one that is random and cannot be determined beforehand.

Basic Terminology of Signal:

Amplitude:The maximum deviation from the average or

equilibrium value of any repeatedly changing quantity, such as the position of a vibrating object, pressure, velocity, voltage, current and many others. the amplitude of a sound wave is the maximum amount by which the instantaneous sound pressure differs from the ambient pressure.Channel capacity

The maximum rate at which data can be transmitted over a communication channel under given conditions is referred as the channel capacity.


+V

T

Time

Period = T

Bit Interval B = T/2B

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There are four parameters involved in the evaluation of channel capacity.• Data rate: The rate at which data can be transmitted. Measured in bits per second• Bandwidth: The bandwidth of the transmitted signal. Measured in cycles per second (Hz).Data Communications 7• Noise: The average level of unwanted signals over communication path.Expressed as the ratio between signal and noise.• Error rate: The rate at which error can occur. Then the channel capacity (in cycles per second) according to Shannon’s theorem is given by

C = B log2 (1+SNR)Where• C in Cycles per second and this is error free capacity• B is the bandwidth in Hertz.• SNR = 10 log10 (Signal power/Noise power)

Normally this theorem represents maximum channel capacity. Actual values maybe much less than as given by the formula. One reason for this is the SNR ratio. The SNR ratio assumes only white noise (thermal noise) where as other noise like impulse noise, attenuation noise and delay noise are not taken into account.

Types of communication:Based on the requirements, the communications can be of different types:

• Point- to-point communication: In this type, communication takes place between two end points. For instance, in the case of voice communication using telephones, there is one calling party and one called party. Hence the communication is point-to-point.

• Point-to-multipoint communication: In this type of communication, there is one sender and multiple recipients. For example, in voice conferencing, one person will be talking but many others can listen. The message from the sender has to be multicast to many others.

Modulation Techniques:Modulation techniques are methods used to encode digital information in an analog world. The 3

basic modulation techniques are:

a. AM (amplitude modulation)b. FM (frequency modulation)c. PM (phase modulation)

All 3 modulation techniques employ a carrier signal. A carrier signal is a single frequency that is used to carry the intelligence (data). For digital, the intelligence is either a 1 or 0. When we modulate the carrier, we are changing its characteristics to correspond to either a 1 or 0.

AM - Amplitude Modulation:



Amplitude Modulation modifies the amplitude of the carrier to represent 1s or 0s. In the above example, a 1 is represented by the presence of the carrier for a predefined period of 3 cycles of carrier. Absence or no carrier indicates a 0.

Advantages:• Simple to design.

Disadvantages:• Noise spikes on transmission medium interfere with the carrier signal.• Loss of connection is read as 0s

FM - Frequency Modulation:

Frequency Modulation modifies the frequency of the carrier to represent the 1s or 0s. In the above example, a 0 is represented by the original carrier frequency and a 1 by a much higher frequency ( the cycles are spaced closer together).

Advantages:• Immunity to noise on transmission medium.• Always a signal present. Loss of signal easily detected

Disadvantages:• Requires 2 frequencies• Detection circuit needs to recognize both frequencies when signal is lost.



PM - Phase Modulation:

Phase Modulation modifies the phase of the carrier to represent a 1 or 0.

The carrier phase is switched at every occurrence of a 1 bit but remains unaffected for a 0 bit. The phase of the signal is measured relative to the phase of the preceding bit. The bits are timed to coincide with a specific number of carrier cycles (3 in this example = 1 bit).

Advantage:• Only 1 frequency used• Easy to detect loss of carrier

Disadvantages:• Complex circuitry required to generate and detect phase changes.

Quadrature Amplitude Modulation (QAM):

Quadrature Amplitude Modulation or QAM is a form of modulation which is widely used for

modulating data signals onto a carrier used for radio communications. It is widely used because it offers advantages over other forms of data modulation such as PSK, although many forms of data

modulation operate alongside each other.Quadrature Amplitude Modulation, QAM is a signal in which two carriers shifted in phase by

90 degrees are modulated and the resultant output consists of both amplitude and phase variations. In view of the fact that both amplitude and phase variations are present it may also be

considered as a mixture of amplitude and phase modulation.

QAM advantages and disadvantages:

Although QAM appears to increase the efficiency of transmission for radio communications

systems by utilising both amplitude and phase variations, it has a number of drawbacks. The first is that it is more susceptible to noise because the states are closer together so that a lower level



of noise is needed to move the signal to a different decision point. Receivers for use with phase or frequency modulation are both able to use limiting amplifiers that are able to remove any

amplitude noise and thereby improve the noise reliance. This is not the case with QAM.The second limitation is also associated with the amplitude component of the signal. When

a phase or frequency modulated signal is amplified in a radio transmitter, there is no need to use linear amplifiers, whereas when using QAM that contains an amplitude component, linearity must

be maintained. Unfortunately linear amplifiers are less efficient and consume more power, and this makes them less attractive for mobile applications.

Digital-to-Analog Modulation:



Phase shift keying (PSK): The phase of the carrier is discretely varied in relation either to a reference phase

or to the phase of the immediately preceding signal element, in accordance with data being transmitted. Phase of carrier signal is shifted to represent ‘0’, '1'.

QPSK:4-PSK– PSK that uses phase shifts of 90º=π/2 rad ⇒ 4 different signals generated, each representing 2 bits.• Advantage: higher data rate than in PSK (2 bits per bit interval), while bandwidth occupancy remains the same • 4-PSK can easily be extended to 8-PSK, i.e. n-PSK .• However, higher rate PSK schemes are limited by the ability of equipment to distinguish small differences in phase.



BPSK – MODULATION:Consider a sinusoidal carrier. If it is modulated by a bi-polar bit stream according to the scheme

illustrated in Figure 1, its polarity will be reversed every time the bit stream changes polarity. This, for a sine wave, is equivalent to a phase reversal (shift). Is multiplier output is a BPSK 1 signal.The information about the bit stream is contained in the changes of phase of the transmitted signal.



A synchronous demodulator would be sensitive to these phase reversals. Figure 2: a BPSK signal. A snap-shot of a BPSK signal in the time domain is shown in Figure 2 (Lower trace). The upper trace is the binary message sequence. There is something special about the waveform of Figure 2. The wave shape is ‘symmetrical’ at each phase transition. This is because the bit rate is a sub-multiple of the carrier frequency ω/(2π). In addition, the message transitions have been timed to occur at a zero-crossing of the carrier. Whilst this is referred to as ‘special’, it is not uncommon in practice. It offers the advantage of simplifying the bit clock recovery from a received signal. Once the carrier has been acquired then the bit clock can be derived by division. But what does it do to the bandwidth? Band limiting

The basic BPSK generated by the simplified arrangement illustrated in Figure 1 will have a bandwidth in excess of that considered acceptable for efficient communications. Band limiting can be performed either at baseband or at carrier frequency.Demodulation:

Demodulation of this signal is possible with a demodulator of the synchronous, producttype. But there will be a phase ambiguity between the sent and received signals. One way of overcoming this is to use a digital line code which is impervious to phase ambiguity - this is differential phase shift keying (DPSK). Multiplexing

By Multiplexing different message signals can share a single transmission media (The media can be guided or unguided). All they need is they should either differ in their frequency slot or wavelength slot or in time slot.

Frequency domain multiplexing (FDM)

In this each message signal is modulated by different radio frequency signals called rf carriers. At the receiving end filters are used to separate the individual message signals. Then they are demodulated (removing the rf carrier) to retrieve back the original messages.

The Radio /TV broadcasting are the best examples for frequency domain multiplexing. Several individual stations broadcast their programs in their own allotted frequency band sharing the same unguided media. The receiver tunes his set according to his choice. The cable TV network is another example of Frequency domain multiplexing employing guided media.



Question:Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz Design an appropriate configuration, using FDM.Solution:The satellite channel is analog. We divide it into four channels, each channel having a 250-kHz bandwidth. Each digital channel of 1 Mbps is modulated such that each 4 bits is modulated to 1 Hz. One solution is 16-QAM modulation. Figure below shows one possible configuration.

Wavelength division multiplexing (WDM) Wavelength division multiplexing is a type of FDM scheme used in fiber optical communications where various wavelengths of infrared light are combined over strands of fiber. Optical communication with few exceptions are digital since light transmitters and receivers are usually poorly suited for analog modulation.

Prisms in wavelength-division multiplexing and de-multiplexing

• Combining and splitting of light sources are easily handled by a prism• A prism bends a beam of light based on the angle of incidence and the frequency• Using this technique, a MUX can be made to combine several input beams of light, each containing a narrow band of frequencies, into one beam of wider band of frequencies• A DEMUX can be made to reverse the process



Time domain multiplexing (TDM)

A type of multiplexing where two or more channels of information are transmitted over the same media by allocating a different time interval ("slot" or "slice") for the transmission of each channel. The channels take turns to use the media. Some kind of periodic synchronizing signal or distinguishing identifier is usually required so that the receiver can tell which channel is which. A typical practical setup combines a set of low-bit-rate streams, each with a fixed and pre-defined bit rate, into a single high-speed bit stream that can be transmitted over a single channel. The main reason to use TDM is to take advantage of existing transmission lines. It would be very expensive if each low-bit-rate stream were assigned a costly physical channel (say, an entire fiber optic line) that extended over a long distance.

Interleaving:

TDM can be visualized as two fast rotating switches, one on the MUX side and the other on the DEMUX side. The switches are synchronized and rotate at the same speed but in opposite directions. On the MUX side, as the switch opens in front of a connection, that connection has the opportunity to send a unit onto the path. This process is called interleaving.


data communication unit1 as per pune university

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