Chapter – 1
Fundamental of Data Communications 1. Introduction
Data Communication is a system consisting of carries and related devices used to transport data from one point to another. Communication means to convey a message, an idea, a picture or speech that is received and understood clearly and correctly by the person for whom it is conveyed.
Ancient Methods of Communication and Their Demerits – Message were sent in olden times either through horse riders or by using pigeons. There was no surety that the messenger will be able to convey the message exactly in the same form as told to him verbally.
Electronic Methods of Communication – With the invention of telephone instrument and the communication satellites, the means of electronic communication has become very popular in India even though the cost of installation and maintenance of telephones is still very high and beyond the means of a common man.
Limitations of Telephonic Communication:
(a) Both the sender and the receiver of the message should be available at the same time and should speak the same language to understand.
(b) Telephone communication is not a secured means of communication because anyone can overhear the message.
(c) It is not suitable to send picture or any other type of message except a spoken message. (d) It is affected by the electrical interference or by the people digging roads etc.
(e) It is still quite costly to make a telephone call outside the city or the country.
Computerized Communication – Since the time computers have started playing an important role in the field of communications. The main reason for this is that computers can send data extremely fast. They can even transmit pictures and sound in a much secured manner. Further, PCs can send information on the existing telephone line.
Advantage of Computerized Communication –
(a) Telephonic calls, using Internet, can be made to any part of the world with the same expenses as a local telephone call made within the city.
(b) Pictures, sound and written matter can be sent within minutes and a confirmation about it reaching at the destination can be obtained immediately.
(c) Message can be sent in coded form so that they are not understood by anybody else except the person who is sending and the person who is receiving them.
(d) Message can be sent in any language from any place of world to any place. (e) Users need not take highly specialized training for sending or receiving message. 2. Communication Systems
A Communication system is the combination of hardware, software and data transfer links that make up a communication facility for transferring data in a cost effective and efficient manner. A communication system itself can be either analog or digital. The technique by which a digital signal is converted to its analog form is known as Modulation. The reverse process i.e. conversion of analog signal to digital signal is known as Demodulation. These processes of conversions carried out by a special device called Modem.
Advantage of Digital Transmission over Analog Transmission:–
(a) The voice data, music and images can be interspersed to make more efficient use of the same circuits and equipment.
(b) Much higher transmission rates are possible using telephone lines. (c) Digital transmission is much cheaper than analog transmission.
(d) Maintenance of a digital system is easier than maintenance of analog system.
(e) A digital signal can pass through an arbitrary number of regenerators in with no loss in signal and thus travel long distances with no information loss. In contrast, analog signal always suffer some information loss when amplified, and this loss is cumulative.
3. Signal And Data
Data in a communication system can be either digital data or analog data.
Digital Data – On the hockey playground, the referee blows a whistle and all the players in the field understand the message instantaneously. The whistle blown in short bursts of high pitched sound like PEE, PE or it may have a long burst PEEEEEEE. The first one is indication to the players to start the game and the second long whistle is to stop the match immediately. The message conveyed by the sound energy in short pulses is very clear to all the players. This is an example of Digital Data Transmission.
Analog Data – When we sit in a concert hall where many musical instrument being played by different players. For example say one player to playing sitar and other is playing tabla. This is an analog data communication. Both sitar and tabla are sending sound waves in the same sequence and there is a rhythm and harmony between the two. Any music system conveys the songs in the analog form.
Item Analog Transmission Digital Transmission Form It is in the form of continuous
variable of physical quantities It is in the form of discrete quantities and has binary digits
Cost of transmission Low High
Efficiency Low high
Maintenance cost of equipment
High Low
Effect of noise High Low
Attenuation High Low
Example TV transmission from DoorDarshan Data transmission from hard disk to memory
4. Channel Characteristics
A communication channel provides the medium to move electromagnetic energy from a source to one or more destination points. It is a pathway over which data are transferred between remote devices.
Characteristics: -
(a) It should be able to deliver maximum amount of electromagnetic energy from the transmitter to the receiver with minimum cost.
(b) It should not add much noise on the way so that the receiver is able to understand the message correctly.
(c) There should not be any restriction on the distances between the transmitter where the sender is located and the receiver where the signal is received.
Types of Communication Channels:
-There are two types of communication channel used in data communication. These are: (a) A public telephone system
(b) A commercial radio station
Both these channels are used for transfer of voice in analog form. The other type of channel is used for the transmission of the data between a PC and a printer. This carries digital data and transmits square waves. The digital signal between a PC and printer also gets attenuated if the distance of the printer is long.
Digital Channel Capacity:
-The capacity of a digital channel is the number of data bits a channel conveys in one second. The measurement is in bits per second (bps). It is also known as bit rate of channel. The bit rate of networking ranges from kilobits per second or Kbps to millions of bits per second. The duration of a binary digit determines the bit rate. The shorter duration of bit is the cause of the greater the bps rating of the signal.
Relationship between bit time and bit rate per second: -Bit time (milli
sec)
Bit rate per second(bps)
Bit time (milli sec)
Bit rate per second(bps)
3.3 300 .833 1200
.416 2400 .104 9600
.052 19200
Baud and Bit Rate: - Baud is a measure of the digital signaling rate in a channel. Bit rate is a measure of the digital bit values the channel conveys with each baud.
The only way to increase the digital bit rate is to decrease the bit time of the signal. But electrical characteristics of the material used for conveying the bits limit the reduction in the size of the bit time and thus fixing the maximum bit rate per second.
Maximum Data Rate of a Channel: - The maximum data rate of a noisy channel whose bandwidth is in Hertz (Hz), and whose signal-to-noise ratio, shown as S/N in decibels, is given by:
Maximum number of bits/sec = H1092 (1 + S/N) 5. Transmission Modes
There are three modes of data transmission. These are:
(a)
Simplex – Simplex communication imply a simple method of communication. In simplex communications mode, there is a one way communication transmission. Television transmission is a very good example of this type of communication.(b) Half-duplex - In half-duplex mode, both units communicate over the same medium, but only one unit can send at a time. While one is in send mode, the other unit is in receiving mode. It is like two polite people talking to each other—one talks, the other listens, but neither one talks at the same time.
(c) Full-duplex - In a half-duplex system, the line must be "turned around" each time the direction is reversed. This involves a special switching circuit and requires a small amount of time (approximately 150 milliseconds). With high speed capabilities of the computer, this turn-around
time is unacceptable in many instances. Also, some applications require simultaneous transmission in both directions. In such cases, a full-duplex system is used that allows information to flow simultaneously in both directions on the transmission path. Use of a full-duplex line improves efficiency as the line turn-around time required in a half-full-duplex arrangement is eliminated. It requires four wires.
6. Asynchronous and Synchronous Transmission
Asynchronous Mode: - Asynchronous mode refers to a series of events that take place which are not synchronized one after the other.
Asynchronous Transmission: - Asynchronous transmission is often referred to as start-stop transmission because of its nature, that is the sender can send a character at any time convenient and the receiver will accept it. Asynchronous communication lines remain in an idle state until the hardware on the line is ready to transmit. Since the line is idle, a series of bits have to be sent to the receiving node to notify it that there is more data coming. When data is finished, the node has to be notified that the transmission is complete and to go back to an idle state, hence the STOP bits are to be sent. This pattern continues for the duration of the time the link is operative. This is the characteristic of many terminals when on a terminal, the time spent between successive keystrokes would vary. Thus, in asynchronous transmission, data is transmitted character by character at irregular intervals.
Synchronous Transmission: - Synchronous devices need not use Start and Stop bits; so coordination between the two nodes, i.e. the sender and the receiver, is handled differently. In synchronous communications, there are two "channels" - one for data and another for link Synchronization. The channel for synchronization uses the integral clock in the hardware for link synchronization between the two nodes when one of the nodes is ready to transmit data, a unique combination of bits called a Sync Character is sent to the receiver. Since the first character will probably get trashed, a second one usually follows to ensure that synchronization is complete.
Comparison between Asynchronous and Synchronous Transmission: -
Synchronous communications tend to be more expensive than asynchronous ones as the hardware involved is more costly due to integral clocking mechanism that have to be used as well as more sophisticated engineering efforts.
Synchronous transmission is well suited to remote communication between a computer and such devices as buffered card readers and printers. It is also used for computer to computer communications.
The primary advantage of synchronous transmission is its efficiency. Not only does it eliminate the need for individual start-stop bits on each character, but much higher data rates can be used than with asynchronous transmission.
Asynchronous transmission is well suited to many keyboard type terminals. The advantage of this method is that it does not require any local storage at the terminal or the computer as transmission takes place character by character. Hence it is cheaper to implement.
Efficiency of Data Transmission in Synchronous and Asynchronous Modes: - Asynchronous data incorporates the use of extra framing bits to establish the start and ending (stop) of a data character word. A receiver responds to the data stream when it detects a start bit. A data character is decoded and defined after the stop bit is received and confirmed. Asynchronous data are easier to detect and synchronize, but the efficiency of data transmission is reduced by the addition of framing bits as overhead (no message data) bits.
A comparison of a single character using the two data types is as follows. For this purpose, the ASCII code of the letter E (1000101) is used. The order of transmission is to send the Least Significant Bit (LSB) first. The number of framing bits used for asynchronous data varies depending on the stations in the communication link. For example, suppose we use 1 start and 2 stop bits. This adds 3 more bits to the character 'word. Hence total 10 bits are required to send the letter E using asynchronous data. However, in' the case of synchronous transmission, only 7 bits are required for transmission of the character E.
The efficiency of transmission is defined as the ratio of the number of message bits to the total number of transmitted bits:
or % efficiency =
to talbitsd ata bits x 100
Cyclic Redundancy Check (CRC): - In synchronous communications, CRC is used to verify the integrity of the entire packet or block of data. Integrity of the packet means whether the complete packet of data is received in its correct form as it was sent at the sending end. In synchronous communications, parity checking is sufficient to ensure data integrity. In high-speed asynchronous communications, single bit corrections are not enough. As each packet is created, a CRC check is placed somewhere in the packet and is verified at the receiving end.
CRC is a calculation method used to check the accuracy of a digital transmission over a communication link. The sending computer uses one of several formulas to calculate a value from the
information contained in the data, and this value is appended to the message block before it is sent. The receiving computer performs the same calculation on the same data and should derive the same number. If the two CRCs do not match, indicating that a transmission error has occurred, the receiving computer asks the sending computer to retransmit the data.
This procedure is known as a redundancy check because each transmission includes extra or redundant error-checking values as well as the data itself. A CRC is generated by dividing the total number of bits in the block of data being sent by a predetermined binary number. The remainder is then added to the packet and the packet is transmitted. On the receiving end, the reverse mathematical operation is performed to verify the packet contents. If the computation is successful, the packet is passed to the next step. If it fails, the issuing node is notified and the entire packet is retransmitted.
Common CRC patterns are 12-bit (CRC 12), 16-bit (CRC-16 and CRC-CCITT), and 32-bit (CRC-32) Chapter – 2
Transmission Media
Introduction – Transmission media is the general term used to describe the data path that forms the physical channel between sender and the receiver
1.
Guided Media – The Guided Media refer to the media in which the signals are guided through a solid medium, such as copper wire, optical fiber etc. Examples of guided media are the following:(a)
Twisted-pair Wire – A twisted-pair cable consists of two insulated copper wires, typically about 1mm thick, here the wire are twisted together in a helical manner. Characteristics:
Twisted pair consists of two insulated copper wires. The thickness of coils is about 1 mm.
The wires are twisted together.
Twisted pair is commonly used in local telephone communication. For digital transmission over short distances up to 1 km.
Advantage:
Trained men power is available to repair and service the media.
In a telephone system signals can travel several kilometers without amplification, when twisted pair wires are used.
It is used for both i.e. analog transmission as well as digital transmission. It is least expensive.
If a portion of a twisted pair cable is damaged the network is not effected very badly. Disadvantage:
This cable has poor protection layer that’s why easily pick up noise. It is likely to break easily.
(b)
Co-axial Cable – A Co-axial cable consists of a stiff copper wire as the core, surrounded by an insulating material.Characteristics:
Co-axial cable consists of a copper wire as the core surrounded by an insulating material. It is available in two forms i.e. 50-ohm and 75-ohm.
50-ohm cable is used for digital transmission. 75-ohm cable is used for analog transmission. Advantage:
Co-axial cable is used to span the network to long distance at higher data rate (bit/s).
It is used for both digital and analog transmission. It has higher Band-width.
It is in-expensive as compare to fiber optics. Disadvantage:
In respect to twisted pair, it is expensive.
Failure of portion, may affect the whole network. Network cannot be extended above 1 km.
Bandwidth is not constant, varies according to length (Distance covers).
(c)
Fiber Optics – Fiber optics is the newest form of guided media. This media is superior in data handling and security characteristics. The Fiber Optic transmits light signals rather than electrical signal. Each Fiber has an inner core of glass or plastic and additional resourcesrequired at both side i.e. source and destination to convert electrical signal into light signal and vice versa.
In Optical fiber the outer jacket is made up of either polyvinyl chloride (PVC) or teblon. Inside the jacket, Kevlar which is a strong material used in the fabrication of bullet proof vests. Below the Kevlar another plastic coating to protect the fiber is at the centre of the cable and it consists of cladding and core.
Characteristics:
Cost: - Fiber Optic cable is more expensive than any other cables. Installation: - Installation of this cable is difficult than any other cables.
Bandwidth: - Through fiber optics light signal is passed that’s why chances of attenuation will be lesser.
Fiber Optics cable provides higher Bandwidth than any other cables.
Maximum connecting points/node capacity: - If fiber optic is used as communication channel with Ethernet then up to 75 nodes can be easily connected/installed.
Mode of Transmission: - Fiber Optic supports half duplex mode of transmission. In half duplex mode of transmission, transmission is possible in both direction, but only one direction at a time.
Advantage:
This media is superior than any other media use to connect network resources physically. The Bandwidth of this media is higher than any other media.
This cable cannot be easily getting noisy. This media is lighter than any other media. Signal cannot be leakage.
Greater immunity to tapping. Disadvantage:
Fiber Optics required high skilled people to use. It doesn’t support two ways communication at a time. Cost of this cable is much higher than any other cable. Unidirectional light propagation.
Installation and maintenance is typical.
2.
Unguided Media – The media in which signal are not guided through a solid medium is called Unguided Media. For example Air is the media through which electromagnetic energy ca flow easily.There are several methods which are used to send electromagnetic energy through air:
(a)
Radio Waves – Electromagnetic waves ranging in frequency between 3 KHz and 1 GHz are calledradio waves.
Radio waves are omni-directional. When an antenna transmits radio waves, hey are propagated in all directional. This means that the sending and receiving antenna do not have to be aligned. Omni-directional properly has a disadvantage that radio waves transmitted by one antenna are susceptible interference by another antenna that may send signal using the same frequency or band.
Radio waves are those wave that propagate in the sky mode can travel long distance broadcasting.
Characteristics:
Have frequency between 10 KHz to 1 GHz. Radio waves are easy to generate.
Radio waves are omni directional. They can travel long distances. They can penetrate buildings easily. Advantage:
Due to low and medium frequency it can penetrates walls, means AM radio can receive a signals inside a building.
Disadvantage:
Due to low & medium frequency these can’t isolate a communication to just inside or outside a building.
Application of radio waves:
Due to omni-directional characteristics of radio wave, it is use in AM and FM radio, TV, Maritime radio, Codeless phone and paging.(b)
Microwaves – Electromagnetic waves having frequency between 1 GHz to 3 GHz are called microwaves.Microwaves are unidirectional when an antenna transmits microwaves, they can be narrowly focused. This means that the sending and receiving antenna need to be aligned. Microwave propagation is line of sight propagation.
There are two types of antenna used for microwave communication:
(i)
Parabolic dish antenna – It is based on the geometry of parabolic in which every line is parallel to the line of symmetry or line of sight and reflects off the curve at angles such that on the lines intersect at a common point called the focus.The parabolic dish works as a funnel, catching a wide range of waves and directing them to a common point. In this way, more of the signal is recovered than would be possible with a single-point receiver.
(ii)
Horn antenna – It looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem and deflected outward in a series of narrow parallel beams by the curved head. Received transmission is collected by the scooped shape of the horn and is deflected down into the same. Characteristics: Frequencies above 100 MHz.
Microwaves travel in straight line. Microwaves are in expansive as compare to fiber optics system.
Microwaves communication is widely used for telephones, television redistribution etc. Microwave system permit data transmission rate above about 16 GHz /sec.
Repeaters are used to extend the coverage area. Advantage:
A pair of antennas can be aligned without interfering with another pair of aligned antennas.
Disadvantage:
Very high frequency microwaves can’t penetrate walls, if receivers are inside the building.
Uses of certain portions of band in microwaves require permission from authorities. Application:
Microwaves are used for uni-cast communication such as cellular telephones satellite networks and wireless LANs.Types of Microwave communication system:
There are two types of microwave communication system:
1.
Terrestrial : -
Such system used directional parabolic antennas to send and receives signals.
The signals are highly focused and the physical part must be line to sight.
Relay towards are used to extend the signals. Frequency range between 21 to 23 GHz and 4 to 6 GHz.
Cost: - Short distance system can be inexpensive but long distance systems can be expensive.
Installation: - In Terrestrial microwave system line of sight maintain line of sight requirement can make installation difficult. Because antennas must be carefully aligned. Bandwidth capacity: - Data rates are from 1 to 10 m bit/sec.
2.
Satellite : -
Satellite microwave system transmits signals between directional parabolic antennas.
Such as also maintain line of sight. One antenna is on a satellite in geo-synchronous orbit (The orbit where the speed of the satellite matches the earth’s rotation speed), about 36000 kms above the equator.
This allows a ground station to aim its antenna at a fixed point in the sky.
In satellite communication microwave communication at 6 GHz are transmitted from a transmitter on earth to a satellite position in space.
The signal reaches the satellite and it become weak due to the distance of 36,000 km traveled. The transponder in a satellite amplifies the weak signal and sends them back to the earth at a frequency of 4 GHz. These signals are received at a receiving station on the earth.
Characteristics:
Frequency – Range: 4 GHz – 6 GHz and 11 GHz – 14 GHz
Cost – Building and launching such system is extremely expensive. Installation – Extremely difficult and technical.
Advantage:
Satellite communication is a single broadcast or relay station visible from any point of a very large area on the earth.
Satellites used for national transmission are visible from all ports of the country.
Transmission and receiving costs are independent from the distance between these stations.
It makes high quality communication. Maintenance cost is less.
Disadvantage:
Initial setup cost is very high.
Kepler’s Law: - It defines the period as a function of the distance of the satellite from the centre of the earth i.e.
According to Kepler’s Law, Period = C(distance)3/2
Where ‘C’ is a constant approximately equal to 1/100. ‘Period’ has a unit second.
(c)
Infrared and Millimetre Wave – In electromagnetic waves ranging in frequency between 3 GHz to 400 THz are called infrared.The infrared data association (IRDA) an association of sponsoring the use of infrared waves has established standards for communication between devices such as keyboards, mousse, PCs and printers. The recent standard defines a data rate of 4 MB/sec.
Infrared waves are those waves that propagate in the line of sight mode. Characteristics:
Used for short range communication.
The remote controls used on television and VCRS, DVDS use infrared communication. They are relatively directional, cheap, and easy to build but do not pass through solid
object.
No government license is needed to operate on infrared system like radio system. Infrared communication is used fro indoor wireless LAN.
Portable computers with infrared capability can be on the local LAN without having too physically connected to it.
Advantage:
Due to high frequency and short range communication, it prevents interference between one system and another.
Disadvantage:
High frequency infrared can’t penetrate walls, if receivers are inside the building.
It can’t be used outside the building because the sun’s ray contains infrared waves that can be interfering with communication.
Application:
It can be used for short range communication in a closed area using line of sight propagation.
(d)
Light Wave : - A modern application is to connect the LANs in two buildings via lasers mounted on their roof tops. Coherent optical signaling using lasers is inherently unidirectional; so each building needs its own laser and its own photo detector.Advantage:
The bandwidth is very high at very low cost. It is relatively easy to install.
It does not require any license. Disadvantage:
Laser beams cannot penetrate rain or thick fog, but they normally work well on sunny days.
Heat from the sun during the daytime causes convection currents to rise up from the roof of the building.
Fiber Optics Communication: - The huge capacity and digital efficiency of optical fibers have made them most appropriate for computer communication. Optical fibers are used to connect work-stations with central processor in a LAN. Main purposes of using optical fibers are to provide safe mechanism with high data rate.
Components: A fiber optics system has three components: - Light source
Transmission medium (channel) Detector
Light source – The two commonly use optical sources are Light Emitting Diodes (LEDs) and Injection Laser Diodes. The LEDs emit a lower level of light (-15 dbm power level) but concentrate it into a tighter cone pattern. The laser diodes emit light at -6 dbm. The pattern of light is shown below:
Transmission medium (channel) – The transmission medium is an ultra-thin fiber of glass.
Detector – The detector generates an electrical pulse when light falls on it. By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it as light pulses, and then reconverts the output to electrical signals art the receiving end.
Categorization into bands of electromagnetic spectrum of Radio wave & Microwave:
-(i) Very Low Frequency (VLF) – VLF have range between 3 to 30 Hz and we ground propagation method. Application of VLF is called long range radio navigation.
(ii)
Low Frequency (LF) – LF has range between 30 Hz to 300 KHz and use ground propagation method. LF is used in radio beacons and navigational locators.(iii)
Middle Frequency (MF) – MF has range between 300 KHz to 3 MHz and use sky propagation method. MF is used in AM (Amplitude Modulator).(iv)
High Frequency (HF) – HF has range between 3 MHz to 30 MHz and use ground sky propagation method. HF is used in citizens band (CB), ship/aircraft communication.(v)
Very High Frequency (VHF) – It has range between 30 MHz to 300 MHz and used both sky and line of sight propagation. VHF is used in VHF TV and FM radio.(vi) Ultra High Frequency (UHF) – UHF has range between 300 MHz to 3 GHz and use line of sight propagation method. UHF is used in UHF TV, cellular phones, paging, satellite.
(vii)
Super High Frequency (SHF) – SHF has range between 3 GHz to 30 GHz and use line of sight propagation method. SHF is used in satellite communication.(viii)
Extremely High Frequency (EHF) – EHF has range between 30 GHz to 300 GHz and use line of sight propagation method. It is used in satellite and radar.Chapter – 3 DATA MODEMS
Modulation: - Modification of one or more characteristic of a carrier wave by an information bearing signal is called ‘Modulation’.
Categorization of Modulation:
-1.
Amplitude Modulation – In Amplitude Modulation transmission, the carrier signal is modulated so that its amplitude varies with the changing amplitude of modulating signal. The frequency and phase of the carrier remains the same. Only the amplitude changes to follow the variation in information.Modulation creates a band width i.e. the twice the bandwidth of modulating signal and covers a range centered in a carrier frequency. However the signal components above and below the carrier frequency carry the exactly the same information. For this reason some implementation discarded one half of the signal and cut the bandwidth in half.
The total bandwidth required for amplitude modulation can be determined from the bandwidth of audio signal.
BAM = 2B
Where B – Bandwidth of Audio signal
BAM – Bandwidth of AM
The federal communication commission (FCC) allows 10 KHz for each amplitude modulation station (AM). AM stations allowed carrier frequency between 530 KHz to 1700 KHz.
Advantage:
Easy to implement
Used for both digital and analog
In the case of digital signal, two different voltage levels are used i.e. 0 or 1.
Disadvantage:
It is affected by the noise signal. That may add up with the information signal.
As the strength of the signal decreases in a channel with distance traveled it reaches a minimum level. Before signal strength goes down, it must be amplified.
2.
Frequency Modulation – In Frequency Modulation transmission, the frequency of carrier signal is modulated to follow the changing voltage levelof modulating signal. The peak amplitude and phase of carrier signal remains constant but as the amplitude of information system changes, the frequency of carrier changes corresponding.
The actual bandwidth is difficult to determine exactly but it can be solved shown that it is several lines that of analog signal.
BFM = 2(1 + β)B
Here β is a factor depends upon modulation technique.
The bandwidth of an audio signal broadcast in stereo is almost 15 KHz. The Fcc
allows 200 KHz for each station. FM stations are allowed carrier frequency anywhere between 88 MHz to 108 MHz.
Advantage:
Frequency modulated is least affected by the noise. Disadvantage:
Needs much higher bandwidth than amplitude modulation.
Use:
FM technique is used to convert digital signal into FM signals.
3.
Phase Modulation – In phase modulation transmission the phase of carrier signal is modulated to follow the changing voltage level of modulating signal. The peak of amplitudeand frequency of carrier signal remains constant but the amplitude of information signal changes the phase of carrier signal. In frequency modulation the instantaneous change in carrier frequency is proportional to the amplitude of modulating signal while in phase modulation; the instantaneous change in carrier frequency is proportional to the derivative of amplitude of modulating signal.
The actual bandwidth is difficult to determine exactly but it can be shown empirically that it is several times that of analog or modulating signal. Although the formula of same bandwidth for frequency modulation and phase modulation. The value of ‘β’ is lower in the case of phase modulation around 1 narrow band and 3 for wide band.
Advantage:
It provides the signal modulation that allows computers to communicate at higher data rates through telephone system.
Disadvantage:
Phase modulation requires two signals with a phase difference between them. Use: This technique is used to convert colour information in colour television broadcast. This technique is used to convert digital signals into phase modulated (PM) signal. Analog to Digital Conversion:
1. Pulse Code Modulation (PCM): - The common technique in which analog signal changes to digital data (Digitization) is called PCM. A PCM encoder has three
procedures:
(a) Analog signal is sampled (b) Sampled signal is quantized
(c) Quantized values are encountered as streams of bits.
(a)
Sampling – The analog signal sampled every Ts secondwhere Ts second is the sample interval or period. The
inverse of the sampling interval is called the sampling rate or sampling frequency and denoted by fs, where fs = 1/Ts. According to the Nyquist
theorem, the sampling rate must be at least 2 times the higher frequency contained in the signal.
For elaborating Nyquist theorem we remember some points: Sample a signal only & signal is band-limited.
Sampling rate must be at least 2 times the highest frequency, not the bandwidth.
If the analog signal is low-pass, the bandwidth & the highest frequency are the same value.
If the analog signal is band pass, the bandwidth value is lower than the value of the maximum frequency.
(b)
Quantization - The result of sampling is a series of pulse with amplitude values between the maximum to minimum amplitudes of the signal. The set of amplitude can be infinite with non-integral values b/w the two limits. These valuescan’t be used in the encoding process. Steps in Quantization: -
We assume that the original analog signal has instantaneous amplitude between Vmin& Vmax.
We divide the range into 1 zones, each of height∆
(delta) i.e.∆
= 2 Vmin -Vmax
We assign quantized values of 0 to 1-10 to the mid-point of each zone. We approximate the value of the sample amplitude to the quantized values.
For quantization & encoding, we take a sampled signal and the sample amplitude are between -20 v and + 20 v.
We decided to have eight levels. This means that
∆
= SV(c)
Encoding – After each sample is quantized & no. of bits per sample is decided, each sample can be changed to an n-bit codeword i.e. no. of bits nn = log2L, where L = Quantization level & Bitrate = Sampling rate x no. of bits per sample = fs x nn
In PCM decoder, we first use circuitry to convert the code words into a pulse that holds the amplitude. After the
staircase signal is completed, it passes through a low-pass filter to smooth the staircase signal into an analog signal. The filter has the same cut off frequency as the original signal at the sender. If the signal has been sampled at or
greater than the Nyquist sampling rate & if there are enough quantization levels, the original signal will be recreated. The maximum and minimum values of the original signal can be achieved by using amplification.
3.
Delta Modulation (DM) – DM finds the change from previous sample. There are no code words; bits are sent one after another.Modulator – The Modulator is used at the sender site to create a stream of bits from an analog signal. The process records the small positive or negative changes, called Delta. If the delta is positive, the process records a 1.
if it is negative the process records a 0. However the process needs a base against which the analog signal is compared. The modulator builds a second signal that resembles a staircase. Finding the change I then reduced to comparing the input
signal with the gradually made staircase signal. Demodulator – The Demodulator takes
the digital data & using the staircase maker & the delay unit, creates the analog signal. The created analog signal needs to pass through a low-pass filter for smoothing.
Digital to analog Conversion: - Digital to
analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data. This is also called Shift Keying.
Relationship between Data rate & signal rate – Data or bit rate is the no. of bits per second. Signal or baud rate is the no. of signal elements per second.
In the analog transmission of digital data, the baud rate is less than or equal to the bit rate. S = N x 1/r baud, where S = signal rate, N = data rate, r = no. of data elements carried in one signal = log2l where l is the type of signal element.
Shift Keying: -
Amplitude Shift Keying (ASK) – In amplitude shift keying the amplitude of the carrier signal is varied to create signal elements. Both frequency & phase remain constant while the amplitude changes.Levels of ASK:
-(d) Level o – This is also called Binary ASK or on-off keying & peak amplitude of one signal level is 0 & the other is the same as the amplitude of the carrier frequency.
Bandwidth for ASK, B = (1 + d) X S Where d = value between 0 & 1
S = Signal rate
The formula shows that the required bandwidth has a minimum value of s & a maximum value of 25.
(e)
Multilevel ASK (MASK) – Multilevel ASKis that in which more than two levels. We can use 4, 8, 16 or more different amplitude for the signal & modulate the data using 2, 3, 4 or more bits at a time.
Frequency Shift Keying (FSK) – In FSK, the frequency of the carrier signal is varied to represent data. The frequency of the modulated signal is constant for the duration of one signal element, but changes for the next signal element if the data element changes. Both peak element if the data element changes. Both peak amplitude & phase remain constant for all signal elements.(a)
Binary FSK (BFSK) – Binary FSK is considering two frequencies f1 & f2. We use the firstcarrier if the data element is 0; we use the second if the data element is 1.
The middle of one bandwidth is f1 & the other is f2. Both f1 & f2 are
∆
f apart from the midpoint between the twobands, so the difference between the two frequencies is 2
∆
f. Bandwidth for FSK, B = (1 + d) X 3 + 2∆
f(b)
Multilevel FSK (MFSK) – Multilevel FSK is that in which more than two frequencies used. To send 2 bits at a time, we can use four frequencies and so on. Bandwidth for FSK for multilevel,B = (1 + d) x S + (L – 1) 2
∆
f
Phase Shift Keying (PSK) – In PSK, the phase of the carrier is varied to represent two or more different signal elements. Both peak amplitude & frequency remain constant as the phase changes. Today, PSK is more common than ASK & FSK.(a)
Binary PSK (BPSK) – The simplest PSK is binary PSK, in which we have only two signal elements, one with a phase of 00 and the other with a phase of1800.
Bandwidth of PSK, B = (l+ d) S
(b) Quadrature PSK (QPSK) – The simplicity of BPSK enticed designers to use 2 bits at a time in each signal element, thereby decreasing the baud rate & eventually the required bandwidth. The scheme is called quadrature PSK because it uses two separate means out of phase. The incoming bits are first passed through a serial to parallel conversion that sends bit to one modulator and the next bit to the other modulator. If the duration of each bit in the incoming signal is T, the duration of each bit sent to the corresponding BPSK signal is 2T. This means that the bit to each BPSK signal has one half the frequency of the signal.
DPSK –
Encoding Technique and CODEC: -
MODEMS: - It is a device converts digital signal generated by computer into an analog signal to be carried by public access telephone line. It is also the device converts the analog signals received over a telephone line into digital signal usable by the computer. A modem derives its meaning from modulation and demodulation i.e. a signal modulator and signal demodulator. A modulator converts digital signals into analog and demodulator converts analog signal into digital signal. Modems are classified into many categories. Modem speed range from 300bps to 56kbps. The tasks which modem can perform are:
Automatically dial another modem using touch tone or pulse dialing. Auto answer.
Disconnect a telephone connection when data transfer has completed. Automatically speed negotiation between two modems.
Converts bits into the form suitable for the LAN. Transfer data reliable.
Converts received signal back into bits.
Modem Commands – When a computer wants to make a connection using telephone no. as parameter using tone dialing. The modem then replies with an ok response i.e. it tries to make connection with remote modem if it is not able to make connection it sends message in the form of code.
3 – for no carrier 7 – for busy
6 - for no dial tone, etc.
If it gets connected then it returns a connect code as it sends +++ and then wait for a command from host computer. In this case command is hang up the connection (ATH). The modem will then return an OK message when it has successfully created a connection.
Classification of Modems – Modem can be of the following types:
1. Landline Modems – Landline modems are those modems which connect to the public switched telephone network (PSTN). To connect to PSTN, this modem has a jack known as RJ-11 jack or regular phone jack.
Landline modem can be of following types:
(a) Internal Modems – Internal modems are installed within the computer as interface cards.
(b) External Modems – External Modems are installed as a separate hardware device, outside the computer. They are more expensive than the internal modems.
(c) PCMCIA Modems – PCMCIA Modems are credit-card sized modems used in laptop computers. PCMCIA stands for Personal Computer Memory Card International Association.
(d) Voice/data/fax Modems – Voice/data/fax Modems which are use for transferring files, sending and receiving faxes and voice mail using associated software.
2. Wireless Modems – Wireless modem are based on webs. Using wireless modem, one can connect to a network while being mobile. Like Landline Modems wireless modems do not plug into jack. There are very few manufacturers of wireless modem.
3. LAN Modems – LAN Modems allows share remote access to LAN resources. LAN modems are of various types. Depending upon the number of parts, network architecture, memory requirements, security, etc.
Modem Standard – There are two modem standards:
1. Bell Modems – The first commercially available modems were developed by Bell Telephone Company. In the early 70’s they defined the development of the technology and provided the
standards. Some major Bell modems include the 103/113 series, 202 series, 212 series, 201 series, 208 series and 209 series.
2. ITU-T Modems – Many of today’s modems are based on the standard published by ITU-T. V.21, V.22, V.23, V.22bis, V.32, V.32bis, V.33 and V.34 modems are ITU-T modems.
Modem Protocols –
1.
X.25 Protocol – X.25 is an end to end protocol. It acts as an interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE).X.25 is a switching protocol that defines the interface between a synchronous packet-switching host computer and analog dedicated circuits or dial-up switched virtual circuits in the voice-grade public data network.
X.25 allows a variety of devices that are designated as data terminal equipment (DTE) to talk to the public data network (PDN).
2. Triple-X Protocol – X.3, X.28 and X.29 protocols are collectively known as Triple-X protocols. Triple-X protocols are used to connect a dumb terminal to an X.25 network. A dumb terminal is any terminal that does not understand X.25 protocol. X.3 defines a packet assembler/disassembler (PAD). X.28 defines the rules for communication between a dumb terminal and a PAD. X.29 defines relationship between a PAD and a remote terminal.
Protocol Used by Modem for Transferring Files – Some of the protocols used by modem for transferring files are described in the following sub-sections:
XMODEM – XMODEM is a file transfer protocol used in telephone-line communication between PCs. XMODEM protocol requires that one terminal or computer be set up as the sender and other be set up as the receiver. A block of data sent under XMODEM protocol will have the following format:
Start of Header Block Number 1’s Complement of
Block Number Characters128 Data Checksum bits Characteristics:
• It is easy to implement with a small computer.
• It requires manual setup for each file to be transferred.
• The error detection technique is unsophisticated and unable to detect reliably the most common type of transmission error, which is noise burst that can last of the order of 10 milliseconds. • It is a half-duplex protocol.
YMODEM – It is similar to XMODEM, but with some differences. These differences are the following: • A data unit is of 1024 bytes.
• Two CANs are sent to abort a transmission. • ITU-T CRC-16 is used for error checking. • Multiple files can be sent simultaneously.
ZMODEM – It combines the features of both XMODEM and YMODEM.
Kermit – It is an also a file transfer protocol like XMODEM. It allows the transmission of control characters as text.Establishing a Connection – Connection can be of the following three types:
Direct Connection between PCs – One PC can call another PC. Modems at both ends of the connection talk to one another.
Connection to a Mainframe Computer – A PC connects a mainframe computer.
Connection to an on-line service – On-line service consist of one or more central computers linked by telephone lines to other small computers spread across the country or world.
Chapter – 4
Multichannel Data communication
Multiplexing: - Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.
Format of multiplexed system:-
Multiplexer is that which combines transmission stream into a single stream i.e. many to one method.
Demultiplexer is that which separates the stream back into its component transmission i.e. one-to-many method & directs them to their corresponding lines. Link refers to the physical path channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many channels.
Categories / techniques of multiplexing:
-(a)
Frequency-division multiplexing : - FDM is an analog technique that can be applied when the band-width of a link is greater than the combined band-widths of the signals to be transmitted. In FDM, signals generated by each sending device modulate. Different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link. Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. These bandwidthranges are the channels through which the various signals travel, channels can be separated by strips of unused bandwidth, guard bands to prevent signals from overlapping.
Application of FDM:-
FDM is used in AM & FM radio broadcasting with band from 530 to 1700 KHz & 88 to 108 MHz respectively.
FDM is used in television broadcasting
FDM is used in first generation cellular telephones.
(b)
Wavelength division multiplexing : - WDM is an analog technique that is designed to use the high data-rate capability of fiber-optic cable. The optical-fiber data rate is higher than the data rate of metallic transmission cable. Using a fiver-optic cable fox one single line wastes the available bandwidth. Multiplexing allows us to combine several lines into one. In WDM, the multiplexing & demultiplexing involve optical signals transmitted through fiber-optic channels.In WDM technology, we want to combine multiple sources light into one single light at the multiplexer & do the reverse at the demultiplexer. The combining & splitting of light source are easily handled by a prism. A prism bends
a beam of light based on the angle of incidence & the frequency. Using this technique, a multiplexer can be made to combine several input beams of light, each containing a narrow band of frequencies into one output beam of a wider band of frequencies. A demultiplexer can also be made to reverse the process.
Dense WDM can multiplex a very large number of channels by spacing channels very close to one another. It achieves even greater efficiency.
Application of WDM:
- WDM is used in SONET
network in which multiple optical fiber lines are multiplexed & demultiplexed.
(c)
Time–division multiplexing : - TDM is a digitalmultiplexing technique for combining several low rate channels into one high-rate one.
Two schemes for TDM:
-1.
Synchronous TDM: - In synchronous TDM, the data flow of each input connection is divided intounits, where each input occupies one input time slot. A unit can be 1 bit, 1charecter of 1 block of data. Each input unit becomes one output unit & occupies one output time slot. However the duration of an output time slot is n times shorter. Than the duration of an input time slot. If an input time slot is Ts, the output time slot is
T/n & where n is the no. of connections. In other wards, a unit in the output connection has a shorter duration; it travels faster.
In synchronous TDM, the data rate of the link is n times slots are grouped into frames. A frame consists of one complete cycle of time slots, with one slot dedicated to each sending device. In a system with n input lines each flame has n slot, with each slot allocated to carrying data from a specific input line.
2.
Statistical TDM: - In statistical TDM, slot are dynamically allocated to improve bandwidth efficiency. Only when a input line has a slot’sworth of data to send is it given a slot in the output frame. In statistical TDM, the no. of slots in each frame is less than the no of input lines. The multiplexer checks each input line in round robin fashion: it allocates a slot for an input line if the line has data to send
otherwise it skips the line & checks the next line. In statistical TDM, no solt is left empty as long as there are data to be sent by any input line.
Difference between synchronous &statistical TDM:
- An output slot in synchronous TDM is totally occupied by data where as in statistical TDM, a slot need to carry data as well as address of the destination.
In statistical TDM, a block of data is usually many bytes while the address is just a few bytes where as in synchronous TDM, there is no such situation.
The frame in statistical TDM need not be synchronized means there is no need of synchronization bits where as in synchronous TDM must be synchronized.
In statistical TDM, the capacity of the link is normally less than the sum of the capacity of cache channel where as in synchronous TDM, the capacity of the link is equal to the sum of the capacity of channel.
Access technique: - Access technique is basically divided into three groups.
(a)
Random Access: - In random access ofcontention methods, no station is superior to another station & name is assigned the control over the another. No station permits of does not permit, another station to send.
Method of random access technique: - ALHO
CSMA (carrier sense multiple access)
CSMA/CD (Carrier sense multiple Access with collision detection) CSMA/CA ( Carrier sense multiple access with collision Avoidance )
(b)
Controlled Access: - In controlled access, the stations consult one another to find which station has the right to send. A station can’t send unless it has been authorized by other stations.Method of controlled access technique: - reservation
polling
token passing
(c)
Channelization: - Channelization is a multiple access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations.Method of channelization:
-(a)
Frequency Division Multiple Access (FDMA): - In FDMA, the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data in other wards, each band is reserved for a specific station, & it belongs to the station all the time. Each station also uses a band pass filter to confine the transmitter frequencies to prevent station interferences, the allocated bands are separated from one another by small guare bands.FDMA specifies a predetermined frequency band for the enter period of communication. The means that stream data can easily be used with FDMA.
FDMA is an access method in the data link layer. In each station tells its physical layer to make a band pall signal from the data passed to it. The signals must be created in the allocated band there is no physical multiplexer at the physical layer. The signals created at each station are automatically band pass filtered. They are mixed when they are sent to the common channel.
(b)
Time Division Multiple Access (TDMA): - In TDMA, the station share the bandwidth of the channel in time. Each station is allocated to a time slot during which it can send data. Each station transmits its data, in is assigned time slot.TDMA lies in achieving synchronization between the different stations. Cache station needs to know the beginning of its slot & the location of its slot this may be difficult because of propagation delays introduced in the system if the station are spread over a large area. To compensate for the delays, inserting guard times synchronization is accomplished by having some synchronization bits at the beginning of each slot.
TDMA is an access method in the data link. The data link layer in each station tells its physical layer to user the allocated time slot there is no physical multiplexer at the physical layer.
(c)
Code Division Multiple Access (COMA): - In CDMA, one channel carries all transmission simultaneously but having no timesharing.CDMA simply means communication with different codes. If a station needs to send a 0 bit, it encodes it as -1 ; if it needs to send a 1 bit, it encodes it as +1 & when a station is idles, it senda no signal which is interpreted as a ‘0’.
Properties of orthogonal sequence: -
Each sequence is made of N elements, where N is the no. of stations.
If we multiply a sequence by a no; every element in the sequence is multiplied by that element. This is called multiplication of a sequence by a scalar.i.e. - [ +1+1-1-1 ] = [ +2+2-2-2 ]
If we multiply two equal sequence element by element add the results, we get N, where N is the no. of elements in the each sequence. This is called the inner product of two equal sequences. i.e. - [+1+1-1-1] . [+1+1-1-1] = 1+1+1+1 = 4.
If we multiply two different sequence, element by element & add the results. We get 0. This is called inner product of two different sequences. i.e.: - [+1+1-1-1] . [+1+1+1+1] = 1+1-1-1 = 0
Adding two sequences means adding the corresponding elements. The result is another sequence. i.e. - [+1+1-1-1] + [+1+1+1+1] = [+2+200].
Spread Spectrum: - Spread spectrum combines signals from different sources to bit into a latger bandwidth. It is designed to be used in wireless communication (LANs & WANs).
To achieve bandwidth efficiency, spread spectrum techniques add redundancy; they spread the original spectrum needed for each station. If the required bandwidth for each station is B, spread spectrum expands it to Bss, such that Bss>>B. The expanded bandwidth allows the source to corp. its message, in a protective envelope for a mare secure transmission.
Two principles for achieving goals of spread spectrum: -
The bandwidth allocated to each station needs to be. By for larger than what is needed. This allows redundancy.
The expanding of the original bandwidth B to the bandwidth Bss must be done by a process that is independent of the original signal. In other worked. The spreading process occurs after the signal is created by the source.
Two techniques to spread the bandwidth:
-(a)
Frequency Hopping Spread Spectrum (FHSS): - The FHSS technique uses M different carrier frequencies that are modulated by the source signal. At one moment, the signal modulates one carrier, frequency, at the next moment, the signal modulates another carrier frequency, although the modulation is done using one carrier frequency at a time, M frequencies are used in the long sun. The bandwidth occupied by a source after spreading is BFHSS >> B.Layout for FHSS:
-Pseudorandom code generator, called Pseudo random noise (PN), creates a K-bit pattern for every happing period Th. He frequency table uses the pattern to find the frequency to be used for this hopping period & passes it to the frequency synthesizes. The frequency synthesizer creates a carrier signal of that frequency & the source signal modulates the carrier signal.
(b)
Direct Sequence Spread Spectrum (DSSS): - The DSSS technique expands the bandwidth of the original signal in DSSS. We replace each data. Bit with n bits is assigned a code of n bits, called chips, where the chip sale is n times that of thedata bit.
Layout of DSSS:
-Digital Hierarchy: - Telephone companies implement TDM through a hierarchy of
digital signal, called digital signal service of Digital Hierarchy.
United States, ANSI standard & Europe IIU-I standard is called the synchronous optical network (SONET) & Synchronous. Digital Hierarchy (SDH) respectively. SONET/ SDH are a synchronous n/w using synchronous TDM multiplexing. All clocks in the system are locked to a master clock.
Architecture of SONET/STD:
-(a)
Signals : - SONET defines a hierarchy of electrical signaling levels called synchronous transport signals (STSD) each STS levels (STS-1 to STS-192) supports a certain data rate, specified in megabits per sec. the corresponding optical signals are called optical carriers (OCS). SOH Specifies a similar system called a synchronous transport module (STM). STM is intended to be compatible with existing European hierarchies such as E-lines & with STS levels.SONET/SDH rates: -STS OC Rate (Mbps) STM STS – 1 OC – 1 S1 – 840 STS – 3 OC – 3 155.520 STM – 1 STS – 9 OC – 9 466.560 STM – 3 STS – 12 OC – 12 622.080 STM – 4 STS – 18 OC – 18 933.120 STM – 6 STS – 24 OC – 24 1244.160 STM – 8 STS – 36 OC – 36 1866.230 STM – 12 STS – 48 OC – 48 2488.320 STM – 16 STS – 96 OC – 96 4976.640 STM – 32 STS - 192 OC - 192 9983.280 STM - 64
(b)
Devices : - Basic devices for SONET transmission:(i)
STS Multiplexer/Demultiplexer : - They mark the beginning points & end points of a SONET link. They provide the interface between an electrical tributary network & the optical network. An STS Multiplexer multiplexes signals from multiple electrical sources & creates the corresponding OC signal. An STS Demultiplexer demultiplexes an optical signal into corresponding electrical signals.(ii)
Regenerator : - It extends the length of the links. A regenerator is a repeater that takes a received optical signal, demodulates it into the corresponding electrical signal, regenerates the electric signal & finally modulates the electric signal into its correspondent optical signal. A SONET regenerator replaces some of the existing information with new information.(iii)
Add/Drop Multiplexer : - It allows insertion and extraction of signal.(iv)
Terminals : - It is a device that uses the services of a SONET network.(c)
Connection s: - The devices are connected using section lines & paths.(i)
Sections : - A section is the optical link connecting two neighbour devices like Multiplexer to Multiplexer, multiplexer to regenerator or regenerator to regenerator.(ii)
Lines : - A line is the portion of the network between two Multiplexer like STS Multiplexer to Add/Drop Multiplexer, two add/drop Multiplexer, or two STS Multiplexer.(iii)
Path : - A path is the end to end portion of the network between STS Multiplexers. Network using SONET equipment:-Chapter – 5 Network fundamentals
AN OVERVIEW OF NETWORKING
A network is a group of computers connected in some fashion in order to share resources. A group of computers in a network provide greater storage capacity and processing power than that by standalone independent machines. In addition to computers, a network also consists of peripheral devices with carriers and data communication devices used for the purpose of exchanging data and information.
By using computer networks, the cost of data transfer can be made cheaper than other conven-tional means like telegrams etc. as computers can send data at a very fast speed. Thus, computers enable us to reduce both cost and time in transferring data. In a network, computers of different make can be connected together and users can work together in a group. Software packages have been developed for group working in Data Base Management (DBMS) and graphical artworks. Also, data from different departments located at distant places can be transferred to and stored on a central computer. This data can then be accessed by the computers located in different departments. The data at the central computer is updated and accessed by all users. This prevents any bottlenecks in the smooth functioning of the organization because all the users will get the latest information (for example, inventory) stored in the central computer.
Communication Switching Techniques: - In a WAN two devices are not connected directly but a network of switching nodes provides a transfer path between the two devices. The process of transferring data block from one node to another is called data switching. There are following types of switching techniques: -
Circuit Switching – In Circuit Switching, there is a dedicated communication path between the sending and receiving
devices.
A circuit switched network is made of a set of switches connected by physical links in which each link is divided into n-channels.
Circuit switching takes place at the physical layer. In this, data are continuous flow sent by the source
station & received by the destination station, although there may be periods of silence. There is no addressing involved during data transfer.
Switching at the physical layer in the traditional telephone network uses the circuit switching approach.
Workstation – It is a basically a PC or printer or other sharable resources. Workstation is also called a terminal or data access point of a network.
The dedicated path is a sequence of links between switching nodes. Circuit Switching involves three steps:
(a) Circuit Establishment (b) Signal Transfer (c) Circuit Transfer
Circuit Switching is mainly used for voice based network. It is not effective for data communication.
Message Switching – In message switching, it is not necessary to establish a dedicated path between a sending and receiving devices. In message switching, the sending device spends destination address to the message and passes to the network. The message is then passes through the network from one node to another until reaches to its destination. Each switching nodes receives the message stored it and then transmit it to the next node. Examples of message are email etc. Packet Switching – Packet Switching combines the advantage of message and circuit switching but it is functionally similar to message switching. There are two approaches to packet switching.
In data communication, the message is going to pass through a packet switched network & it needs to be divided into
packets of fixed or variable size.
In packet switching, there is no resource allocation for a packet. This means that there is no reserved bandwidth on the links & there is no schedule processing time for each packet. Resources are allocated on demand. The
allocation is done one a first come, first served basis.
In packet switched network, each packet is treated independently of all others. Even if a packet is part of multi-packet transmission, the network treats it as though it existed alone. Packet switching is normally done at the network layer.
[Node – Node is a service provides of a network to particular region. The workstations are connected with the node.]
DATAGRAM
A datagram is a packet that is sent over a network using a connectionless service, i.e. a network where the delivery of data does not depend on the maintenance of connections between the communicating computers. In the next Chapter, you will learn that a protocol called User Datagram Protocol (UDP) handles such connectionless services. These services do not guarantee that the datagrams will be delivered without error, without duplication or loss and in the same serial order in which they were sent. They only guarantee a "best effort" delivery of datagram.
VIRTUAL CIRCUIT
In a circuit-switching network, making a connection actually means a physical path is established from the source to the destination through the network. In a virtual circuit network, when a circuit is established, what really happens is that the route is chosen from source to destination, and all the switches (that is routers) along the way make table they can route any packets on that virtual circuit. They also have the opportunity to reserve resource for the new circuit.
When a packet comes along, the switch inspects the packet's header to find out which virtual circuit it belongs to. Then it looks up that virtual circuit in its table to determine which communication line to send. They are also known as switched virtual circuit.
CONNECTIONLESS AND CONNEC TION ORIENTED COMMUNICATION
In a connection oriented service, a logical connection is established between the two communicating computers. The TCP protocol (discussed in a later Chapter) is used in such a communication. It guarantees error free delivery of messages without loss or duplication. The packets are received in the same serial order in which they were sent. The connection is established by a three-way handshake: In this method, before the sending device can send data to the receiving device, the former must determine the availability of the latter and a network pathway must be discovered on which the data can be sent. This is known as connection establishment. It normally involves the following steps:
The sender sends a connection request packet to the receiver.
The receiver, if available, returns a confirmation packet (acknowledgement) to the sender. The sender then returns an acknowledgement of this confirmation packet.
Once the connection is established, the packets are sent in order, and their acknowledgements from the receiving device are also received in that same order. After the communicating computers finish off with the sending and receiving, the connection is terminated. Connection termination also involves full confirmation between both the communicating devices, as in connection establishment.
In a connectionless communication, there is no maintenance of connection between the two devices. Each data packet (preferably called as 'datagram) takes its own path and reaches the destination. There is also no guarantee that datagrams will be received error-free and in the same order in which they are being sent. Also no connection establishment and termination are required.