Data Communication

1. Explain the communication model. A:

The fundamental purpose of the communication system is the exchange of data between two parties. The key elements of communication model are: A) source: The device which generates data to be transmitted. Ex: Telephone, computer. b) Transmitter: The data generated by the source is unable to transmit, and then the transmitter transforms the information in such a way to produce electromagnetic signals that can be transmitted across the transmission system. Ex: A modem transforms digital bit stream to analog bit stream that can be handled by the network. c) Transmission system: This can be a single transmission line or complex network connecting source and the destination. Ex: Telephone network. d) Receiver: The receiver accepts the signal from the transmission system and converts into the form that can be handled by a destination device. Ex: Modem accepts analog signals and converts into digital signals. e) Destination: Destination takes the incoming data from the receiver. Ex: Telephone or computer. The following diagram gives a simplified data communication model.

Consider input device is the computer. The user of system wants to transmit message (m) to the other system. 1

The system is connected to the modem and gives the input data to the transmitter as a sequence of voltage pulses(g(t)).The transmitter is directly connected to the medium and converts into a signal(s(t)) which is suitable for transmission. The transmitted signal s (t) is affected with number of impairments (like noise) before it reaches to the receiver. Thus the received signal r(t) may differ from s(t).And the receiver converts the received signal into(g’(t)).These voltage pulses are converted as the message m’ which is received by the destination. 2) What are the key tasks that must be performed in a data communication system? A:1)Transmission system utilization: This refers to the need of efficient use of transmission facilities that are typically shared among a number of communicating devices. 2) Interfacing: To communicate the device must interface with the transmission system. 3) Signal propagation: The properties of the signal such as form and intensity must be such that the signal is a) capable of being propagated through the transmission system b) interpretable as data at the receiver. 4) Synchronization: The receiver must be able to determine when a signal begins to arrive and when it ends. It must also know the duration of the each signal. 5) Exchange management: If data are to be exchanged in both directions the two parties must cooperative: telephone conversation. 6) Error detection and correction: In all communication system there is a chance of error transmitted signals may be distorted before they reach to the destination. 7) Flow control. This is required to assure that the source does not overwhelm the destination by sending data faster than they can be processed. 8) Addressing: When more than two devices share the transmission facility a source system must indicate the identity of the intended destination. 9) Routing: There exists various paths for transmission system. A specific route must be selected. 10) Recovery: Recovery techniques are needed in situations in which information exchange is taking place and some fault occurs like power failure. To resume the activity from the point or from the starting. 11) Message formatting: This is the agreement between the two parties for the form of data to be exchanged (i.e. binary codes) 12) Security: In the communication security is required that the intended parties should receive the data. 13) Network management: These are needed to configure the system and monitor the status react to failures. 3) Explain the three layer architecture. A: The communication tasks can be organized in three layers i.e. network access layer, transport layer and application layer. Network access layer: This layer is concerned with the exchange of the data between a computer and the network to which it is attached. Source should provide the destination address .The network routes the data to appropriate destination. Provides services like priority. Depending on the type of the network type of software will be used. Transport layer: Reliability will be provided regard less of the nature of the application that are communicating.ie data arrive at the destination application and that the data arrive in the same order in which they were sent. Application layer: This contains the logic needed to support the various user applications.Ex: Email, file transfer. 4) Explain the OSI model or seven layer architecture. A: The open system interconnection model was developed by international standard organization. A model for computer communiation.This model contains 7 layers. Each layer relies on the next lower layer to perform its functions. And each layer provides services to the next higher layer. Changes in one layer should not require changes in other layers. Following are the seven layers

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Physical layer: The physical layer coordinates the functions required to transmit a bit stream over a physical medium. It specifies electrical standard which relates about the signal levels, specifies mechanical standards which deals with the type of cables and connectors and also specifies functional interface and procedural characteristics to access physical medium. Data link layer: Provides the reliable transfer of data unit from one station to next. it accepts data unit from the third layer and adds some meaningful bits in the beginning(header) and at the end(trailer).This data unit with header and trailer is called as the frame. This layer is responsible for flow control, error control and synchronization. Flow control: The receiver should not overload with more data. Error control: Making a reliable transmission on unreliable channel by detecting the acknowledgement lost or frame transmission lost or retransmission. Error detection: The transmitted bits 1 receiving as 0 and 0 receiving as 1 is known as bit error. There fore we need a mechanism to detect the bit errors.

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Network layer: This layer is responsible for source to destination delivery of packet across multiple networks links. This provides services switching and routing. Also responsible for establishing, maintaining and terminating connections. This layer mainly designed for handling of routers and switches. Transport layer: The transport layer is responsible for end-to- end delivery of the entire message reliably .The basic function of this layer is to accept the data from the session layer dividing the data into small units known as packets and give to the network layer. Provides error recovery and flow control. Where as network layer performs end to end delivery of individual packet without knowing the relation to each other. The transport layer ensures that the whole message arrives in order. Transport layer is the first layer to have peer to peer communication between source and destination. It is an intermediate layer between applications and network so that it hides the network information from application so that applications can be independent of the network technologies used in the subnet. Session layer: It establishes maintains and synchronizes the interaction between communicating devices. a) Dialogue controller: It determines the transmitter and receiver of communication depending on flow of data and allows a two way simultaneous known as full duplex or on way alternate known as half duplex. b) Session management: It also performs the functions of establishing session maintaining the session and terminating the session. c) Data recovery: The session layer maintains the check point mechanism so that if a failure of network occurs with the help of these check points performing retransmission all the data since the last check point so that uninterrupted data transmission can be performed even if the network connections are being interrupted. Presentation layer: It defines the format of the data to be exchanged between applications. The functions of presentation layer are: a) Translation: it makes the data free of application format. The presentation layer defines the syntax used between application entities and provides for the selection and modification of representation used. b) Encryption/Decryption: To provide security in data transmission the original message is translated into another format known as encryption of data and at the receiver it is converted into original format known as decryption. c) Data compression: To reduce the size of the data transmission, the data is being compressed by using various techniques like zip files, rar files. At other side these are converted into normal format. Application layer: This enables the user to access the network. This layer contains management functions to support various distributed applications such as file transfer, terminal access remote computers and email are considered in this layer. Explain TCP/IP model or Explain TCP/IP Architecture. A: Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET). This is used by the global internet.TCP/IP is organized into five independent layers. They are: Physical layer, Network access layer, internet layer, host-host layer and application layer. Physical layer: It specifies the physical interface between computer and a transmission medium. This layer is responsible for transmitting the data on connected medium. It is concerned about specifying the characteristics of transmission medium, the nature of signals, data rate. Thus physical layer deals about mechanical, electrical and procedural standards. Network access layer. It is concerned with the exchange of data between an end system and the network to which it is connected. The sending computer must provide the network with the address of destination computer so that the network may route the data to appropriate destination connected on same network. It also provides some services like priority for medium access to transmit the data for example in the LAN Ethernet standard is used for packet switching x.25 is used. Internet Layer: The function of internet layer is to transmit the data between two computer two computers connected through different network. The basic function is routing of data from source to destination. For this 4

purpose Internet protocol which provides routing function across multiple networks also this protocol implemented in the routers. Host-Host Layer: Without concerning the type of application to perform reliable exchange of data from source application to destination application the functions are implemented in host-host layer. The protocol used here transmission control protocol which is most commonly used protocol for end-end reliable and transparent data transfer. Application layer: It provides the logic needed to support various user applications. For ex to transfer a file FTP is used, to log on to a remote computer TELNET is used similarly for transmitting the Email applications is used.

Terminology: Line Configuration: Point to point: Provides a direct link between two devices and those are the only devices sharing the medium. Multipoint: more than two devices share the same medium. Transmission modes: 1) Simplex: signals are transmitted in only one direction, one station is transmitter and the other is receiver. Half duplex o: both stations may transmit but only one at a time. Full duplex: both stations may transmit simultaneously.

Signals Analog signal Various in a smooth way over time Digital signal Maintains a constant level then changes to another constant level Periodic signal Pattern repeated over time Aperiodic signal Pattern not repeated over time The physical representation of Analog signal contains 3 characteristics. A) Amplitude: The maximum level of signal is known as amplitude measured in volts(V) or Amp(A) Amplitude Change

B) Frequency: No. of cycles generated per second known as frequency. The units are hertz. 5

Frequency Change

C) Phase: It indicates the delay between two signals and measured in radians degrees. Phase Change

A signal can be expressed in an equation with amplitude A, frequency f, Phase Φ(t) For a sine wave the signal expression can be S (t) =A sin (2Πft+ Φ) Bandwidth: A signal contains a set of frequency components. The difference between highest frequency and lowest frequency is known as bandwidth measured in hertz. For example a signal contain frequency components as S (t) =Asin2 (Πft) +A/3sin 2 (3Πft) +A/5sin2 (5Πft) Then the bandwidth is 5f-f=4f. Explain various transmission Impairments. A: With any communication system, the signal that is received may differ from the signal that is transmitted due to various transmission impairments. For analog signals, these impairments can reduce the signal quality and for digital signal bit errors may occur. The most important impairments are 1. Attenuation and attenuation distortion 2. Delay distortion 3. Noise. 1. Attenuation: The strength of the signal falls off with distance over any transmission medium. Attenuation is expressed as constant no of decibels. (db) per unit distance, for unguided media attenuation is more complex function of distance and also need to consider atmosphere conditions. Attenuation introduces 3 considerations for transmission. a) A received signal must have sufficient strength so that the signal can be detected by the receiver. b) The signal must maintain a level greater than noise level so that it can be received without error. c) Attenuation is the function of frequency. First two problems can be solved by using amplifiers and repeaters. The third problem is related to analog signal. To overcome this problem equalizer will be used which amplifies more the large frequencies and less small frequencies so that uniform attenuation occurs for a band of frequencies. Attenuation is measured wrt attenuation of 1000Hz.Assume that the power measured for a frequency ‘f’ is Pf and the power measured for 1000Hz signal P1000.Then the attenuation in decibels is 6

Nf = -10 log10 Pf / P1000 Attenuation distortion: will present less in digital signals because in digital signals most of the energy is in less frequency components than higher frequency component. because of attenuation only high frequency components will be distorted. Therefore the attenuation doesn’t contain much effect for digital signals. Delay distortion: It is phenomenon particularly for guided medium. The distortion is caused by velocity of the signal varies with frequency. For band limited signal the velocity will be more at the center of the frequency and less at the edges of the band. Thus various frequency components of a signal will arrive at the receiver at different times, which results in phase shift between different frequencies of signal. This affect is known as delay distortion and it is particularly observed for digital signal because of delay distortion some of the signal components of one bit position will spill over into other frequency components which causes bit error known as inter symbol interference, which restricts data rate over a channel. Equalizing techniques are used for delay distortion because the delay is also function of frequency. Noise: For any data transmission, the received signal will not consist of transmitted signal, modified because of various distortions in transmission system plus additional; unwanted signal that is inserted in between transmission and receiver. Thus the unwanted signal is known as noise. Noise is one of the major limiting factors in communication performance. Noise is divided into 4 categories 1) Thermal noise: It is due to thermal agitation of electrons and it is present in all electronic devices and transmission medium and it is a function of temperature. It is uniformly distributed across the frequency spectrum known as white noise. The amount of thermal noise found in a bandwidth of 1 Hz in any device is N0=KT watts/Hz Where N0 = Noise power density in watts/Hz K=boltzman’s constant= 1.38×10 -23 joules/Kelvin. T= temperature in Kelvin. The noise independent of frequency and the thermal noise present in bandwidth of B Hz can be expressed as N=K×T×B Problems Inter modulation Noise: When signals at different frequencies share the transmission medium, the result may be inter modulation noise. The effect of this noise is to produce the signals at a frequency that is the sum or difference the two original frequencies. Crosstalk: It is commonly observed in telephones it has been able to hear other conversation. It is an unwanted electrical coupling between signal paths. It can occur by electrical coupling between near by cables. The cross talk can also occur when unwanted signals are received by microwave antennas. Impulse noise: Impulse noise is non continuous consisting of irregular pulses or noise spike of short duration with very high amplitude. It is generated form external electromagnetic disturbances such as lighting and errors in communication system. Impulse noise causes minor disturbance in the analog data that is generating a short click sounds for audio without loosing original information. But impulse noise generates bit errors for digital data. Explain Channel capacity. A: The various impairments will limit the data rate for digital data. The maximum rate at which data can be transmitted over a given communication path is known as channel capacity and the units for channel capacity is bits per second. These are 4 concepts that we can relate each other. a) Data rate: The no of bits transmitted per second. 7

b) Band width: The bandwidth is constrained by transmitter and nature of transmission medium and expressed in cycles per second. c) Noise this is the level of noise over the communication path. d) Error rate: The rate at which bit errors are occurred. Since the greater the bandwidth, the greater the cost occurs for transmission of signals. Thus every channel transmits a limited bandwidth signal which in turn limits the data rate for digital data. Thus determining the maximum rate at which data can be transmitted is known as channel capacity.

Nyquist bandwidth: For a noise free channel the limitation on data rate depends on bandwidth of the signal. According to Nyquist, if the rate of signal transmission is 2B, then a signal with a bandwidth of the signal is B which is sufficient to carry the digital data. This technique is useful in digital-analog transmission. If the signals are binary signals then the data rate that can be obtained for a bandwidth of BHz is 2B bps. If each signal element represents multiple levels, then the channel capacity is C=2B log2m, Where m is no of discrete signal levels. For m=8 each signal carries 3 bits. Therefore channel capacity can be increased 3 times with same band width. But the drawback is complexity of the receiver will be increased. Shannon capacity formula • Consider data rate, noise and error rate • Faster data rate shortens each bit so burst of noise affects more bits — At given noise level, high data rate means higher error rate • Signal to noise ration (in decibels) • SNRdb=10 log 10 (signal/noise) • Capacity C=B log 2(1+SNR) • This is error free capacity Problems. Write the physical description, transmission characteristics and applications of various transmission media. A Guided transmission media:

The three guided media commonly used for data transmission are: • Twisted Pair • Coaxial cable • Optical fiber Twisted pair of cable: 8

Physical description: twisted pair consists of two insulated copper wires arranged in a regular spiral pattern separately insulated. Twisting of cables is required to decrease the crosstalk interference between adjacent pairs in a cable. On long distance link the twist length varies from 5 to 15 cm.The wires in a pair have thick ness of from 0.4 to 0.9mm.

Applications Most commonly used medium in the telephone network, between house and local exchange (local loop).Also used for local area networks within a building typically 10Mbps or 100Mbps. Transmission characteristics. Twisted pair may be used to transmit both analog and digital transmission. For analog signal amplifiers are required for every 5 to 6 km.For digital signal repeaters are required for every 2 or 3 km.the medium susceptible to interference and noise because of its easy coupling with electromagnetic fields. For point to point analog signaling, a bandwidth of up to about 1 MHz is possible. For long distance digital signaling data rate up to few Mbps are possible for very short distances data rates up to 1 Gbps can be possible. Twisted pair comes in two varieties: unshielded (UTP) and shielded UTP is ordinary telephone wire. This is least expensive of all the transmission media commonly used for LANs and easy to work and easy to install. Suffers from external EM interference STP Metal braid or sheathing that reduces interference. More expensive Harder to handle (thick, heavy). Provides better performance at higher data rates. Category 3 and category 5 UTP: CAT 3: — up to 16MHz — Voice grade found in most offices — Twist length of 7.5 cm to 10 cm CAT 4: Up to 20 MHz CAT 5: — up to 100MHz — Commonly pre-installed in new office buildings — Twist length 0.6 cm to 0.85 cm A key difference between cat3 and cat 5 is number of twists in the cable per unit distance. Cat 5 is much more tightly twisted with a typical twist length of 0.6 to 0.85cm, compared to 7.5 to 10cm for cat 3. Coaxial cable: 9

Physical description: Coaxial cable consists of two conductors, but is constructed differently to permit it to operate over wider range of frequencies.It consists of hollow outer cylindrical conductor that surrounds a single inner wire conductor.Th outer conductor is covered with a jacket or shield. A single coaxial cable has a dia meter of 1 to 2.5 cm.

Applications • Most versatile medium • Television distribution — Ariel to TV — Cable TV • Long distance telephone transmission — Can carry 10,000 voice calls simultaneously — Being replaced by fiber optic • Short distance computer systems links • Local area networks Transmission characteristics: Coaxial cable is used to transmit both analog and digital signals. • Analog — Amplifiers every few km — Closer if higher frequency — Up to 500MHz • Digital — Repeater every 1km — Closer for higher data rates Optical fiber: Physical description: An optical fiber is a thin flexible medium capable of guiding an optical ray. Various glasses and plastics can be used to make optical fibers. The optical fiber cable has a cylindrical shape and consists of three concentric sections: The core, the cladding and the jacket. The core is the inner most section and consists of one more fibers. Diameter of this is 8 to 100m.Each fiber is surrounded by its cladding that has optical properties different from core. The outer most layers surrounding one or bundle of cladded fibers is the jacket.

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Applications: • Long-haul trunks • Metropolitan trunks • Rural exchange trunks • Subscriber loops • LANs Following characteristics of optical fiber differ from twisted pair or coaxial cable. • Greater capacity — Data rates of hundreds of Gbps • Smaller size & weight • Lower attenuation • Electromagnetic isolation • Greater repeater spacing — 10s of km at least Transmission characteristics: • •

• •

Act as wave guide for 1014 to 1015 Hz — Portions of infrared and visible spectrum Light Emitting Diode (LED) — Cheaper — Wider operating temp range — Last longer Injection Laser Diode (ILD) — More efficient — Greater data rate Wavelength Division Multiplexing

Principle of optical fiber transmission:

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Rays at shallow angles are reflected and propagated along the fiber. Other rays are absorbed by the surrounding material. This form of propagation is called step index multimode. When the fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the core to the order of a wavelength only a single angle or mode can pass. This mode is called single mode. By varying the index of refraction of the core a third type of transmission known as graded-index multimode.

Wireless transmission: •

• •

2GHz to 40GHz — Microwave — Highly directional — Point to point — Satellite 30MHz to 1GHz — Omni directional — Broadcast radio 3 x 1011 to 2 x 1014 — Infrared — Local

Antennas An antenna can be defined as an Electrical conductor (or system of..) used to radiate electromagnetic energy or collect electromagnetic energy. 12

For transmission of a signal Radio frequency energy from transmitter Converted to electromagnetic energy by antenna and Radiated into surrounding environment. The same antenna can be used for reception. Electromagnetic energy impinging on antenna, Converted to radio frequency electrical energy. In two way communication same antenna can be used for both transmission and reception. Isotropic antenna is (theoretical) point in space. Radiates in all directions equally. Gives spherical radiation pattern. Parabolic reflective antenna: Parabola is locus of point equidistant from a line and a point not on that line. Fixed point is focus and Line is directrix.

Source placed at focus will produce waves reflected from parabola in parallel to axis. On reception; signal is concentrated at focus, where detector is placed Antenna Gain • Measure of directionality of antenna • Power output in particular direction compared with that produced by isotropic antenna • Measured in decibels (dB) • Results in loss in power in another direction • Effective area relates to size and shape — Related to gain Terrestrial Microwave Physical description: The most common type of microwave antenna is parabolic dish. Size about 3 m in diameter. The antenna focuses a narrow beam to achieve line of sight transmission to receiving antenna. Applications: • Long haul telecommunications. • Short point to point links between buildings, like closed circuit TV. • Cellular system. Transmission characteristics: Common frequencies used for transmission are in the range of 1 to 40 GHz. Higher the frequencies used give higher data rate. Satellite microwave: Physical description: A communication satellite is used to link two or more earth stations. The satellite receives on frequency band (uplink), amplifies and transmits to another frequency (down link). 13

Applications: • Television distribution. • Long distance telephone transmission • Private business networks. Transmission characteristics: The optimum frequency range for satellite communication is 1 GHz to 10 GHz. Below 1GHz the signal suffer with noise. Above 10 GHz the signal faces attenuation problem. Broadcast radio: Physical description: This is omni directional. This does not require dish shaped antenna. Applications: • FM radio • UHF and VHF television Transmission characteristics: The range 30MHz to 1 GHz is an effective one for broadcast communications. Source of impairment for radio waves is multi path interference. Infrared: • Modulate no coherent infrared light • Line of sight (or reflection) • Blocked by walls • e.g. TV remote control, IRD port

Encoding There are two type of data representation: digital and analog. There are two types of signal representations: digital and analog. So there will 4 encoding techniques: Digital data to digital signal; ex: computer. Digital data to analog signal; ex: Modem. Analog data to digital signal; ex: codec Analog data to Analog signal; ex: radio. Explain digital-to-digital encoding techniques with an example.

Digital-to-Digital

unipolar

polar

NRZ

NRZ-I

NRZ-L

bipolar

biphase

manchester

AMI

pseudoternary

Differential manchester 14

Unipolar Encoding: There is only one signal element either +ve or –ve is available to represent either binary one or binary zero.

Two problem are there with this method:1)DC component average amplitude of the unipolar signal is nonzero which create direct current component. 2) synchronization: when the signal is unvarying the receiver cannot determine the beginning and ending of each bit. NRZ-L and NRZ-I Encoding: (Non Return to Zero) NRZ-L: 0 = high level ; 1 = low level; NRZ-I:

0 = No transition at the beginning of interval; 1 = Transition at the beginning of interval

RZ Encoding : (Return to zero) This uses three values positive, negative and zero.Haghway through each bit interval, the signal returns to zero. 1 = positive to zero. 0 = negative to positive.

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Disadvantage is that it requires two signal changes to represent single bit which occupies more bandwidth. Manchester and Diff. Manchester Encoding Manchester: 1 = negative to positive. 0 = positive to negative. Diff. Manchester: A transition means binary 0 and no transition means binary 1.

Bipolar AMI Encoding: (Alternate mark inversion) Zero voltage represents 0. Binary 1 are represented by alternating positive and negative voltages.

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B8ZS Encoding: (bipolar 8 zeroes substitution) Same as bipolar AMI but with violations in the continues 8 zeroes. Changes are based on the polarity of the previous 1.

HDB3 Encoding: (high density bipolar-3 zeroes) Same as bipolar AMI, except that any string of four zeroes is replaced with change code. This is based on the polarity of the previous 1 and the number of 1s since the ls substitution

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Explain digital data to Analog signal encoding techniques. A: To transmit computer data through telephone network this encoding is required. Carrier signal is used to represent the data either by changing the amplitude or frequency or phase and these techniques are known as amplitude shift keying , frequency shift keying, phase shift keying. ASK: The two binary values are represented by two different amplitudes of carrier frequency on of the amplitude is zero and the other is one. Thus binary ‘1’ is represented by presence of the carrier and binary ‘0’ is represented by absence of carrier. The resulting signal is: S(t) = A cos (2Πfct) -------binary ‘1’ S(t) = 0 --------binary ‘0’ ASK is used to transmit digital data over optical fiber. FSK: The two binary values are represented by two different frequencies near the carrier frequency. The resulting signal is S(t) = A cos (2Πf1t) --------------------binary 1 S(t) = A cos (2Πf2t) --------------------binary 0 Where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts. In FSK the changes in the frequency is measured to detect binary data as 1 or 0. Where f1 = fc + ∆f f2 = fc - ∆f PSK:The phase of the carrier is shifted to represent the data. Binary ‘1’ is represented by sending a signal of opposite phase with respect to the previous one and binary ‘0’ is represented by sending a signal of same phase with respect to the preceding one. This is also known as differential PSK.The resulting signal is S(t) = A cos (2Πfct + Π) S(t) = A cos (2Πfct) The more efficient use of band width can be achieved, if each signal element represents more than one bit. For example instead of phase shift of 180, the phase shift is considered for 90 and obtaining 4 different signals, each signal represents 2 bits known as quadrature phase shift keying(QPSK),the signal is S(t) = A cos (2Πfct + Π/4) 45 S(t) = A cos (2Πfct + 3Π/4) 135 S(t) = A cos (2Πfct + 5Π/4) 225 S(t) = A cos (2Πfct + 7Π/4) 315 Modulation rate or baud rate: is the rate at which signal elements are generated.

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Performance: The bandwidth required for ASK, PSK is same as B = (1+r) R where ‘r’ is a constant based on filtering technique value lies between 0 and 1. For FSK B = 2∆f + (1+r) R Where 2∆f=f2-f1 For multi level signaling the bandwidth is calculated as B = (1+r) R/b where b=no.of bits in each signal. Explain pulse code modulation or Explain Analog data to digital signal encoding technique A: Analog data -digital signal PCM: Pulse code modulation is based on sampling theorem, derived by Nyquist.If a signal f(t) is sampled at different regular intervals of time and at a rate higher than twice the highest frequency of original signal. Then each sample has given a quantity whose amplitude depends on the value which is known as PAM (pulse amplitude modulation).Each analog samples must be assigned a binary value so that the binary code will be converted into digital signal. Then each sample is approximated by using a set of quantized levels known as quantization. For example if there are 16 quantization levels,4 bits are required to represent each quantization level. Thus each quantization level is represented with a set of bits. Thus PCM starts with continuous time, continuous amplitude from which a digital signal is produced. The digital signal consists of blocks of n bits where each n bit is the amplitude of PCM pulse. At the receiver the reverse process is performed to produce the analog signal. But because of quantizing the PAM pulse the original signal cannot be recovered exactly but only it is approximated. This effect is known as quantization error.

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Explain the Delta modulation encoding technique A: Delta Modulation In delta modulation, the analog input is approximated by a staircase function that moves up or down by one quantization level (δ), at each sampling interval. Then + δ is represented with binary 1 and – δ is represented with binary 0.

Delta Modulation - Operation

The transmitter of delta modulation contains comparator which compares the two sample values, an integrator which converts the binary value into approximated sample value, the output of the integrator is given to 21

input of comparator through 1 time delay unit so that by the time the feed back is taken, the next sample value will be available at the comparator input. Thus delta modulation performs analog input comparison with the most recent value of staircase function. If the value is increasing then a binary 1 otherwise binary 0 is generated. Explain Analog data to Analog signals encoding technique Modulation has been defined as the process of combining an input signal m (t) and a carrier at a frequency f to produce a signal s (t). There are 2 reasons for analog modulation of analog signals 1) A higher frequency needed for effective transmission so that the antenna size can be reduced. 2) The modulation allows transmission of multiple signals by using frequency multiplexing (combining). The various analog modulation techniques are Amplitude modulation (AM), Frequency modulation (FM) and Phase modulation (PM). Amplitude Modulation: The amplitude of the carrier varies with respect to the amplitude of the modulating signal. The modulated signal can be expressed as s(t) = [1+na x(t)] cos (2Πfct) Where cos (2Πfct) is carrier signal, x(t) is input signal, na Is known as modulating index. The band width required for AM signal is 2fm. Angle modulation: Frequency modulation and phase modulation are special cases of angle modulation. The modulated signal is expressed as S(t) = Ac cos(2Πfct + Φ(t) ) For phase modulation, the phase is proportional to modulating signal that is Φ (t)= np m(t) where np is phase modulation index. For frequency modulation, the derivative of the phase is proportional to modulating signal that is Φ (t) = nf m (t) Where nf is frequency modulation index.

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Thus in phase modulation according to the modulating signal the phase of the carrier signal will be varied.similarly for fm the frequency of the carrier will be varied according to the modulating signal. The bandwidth required for FM and PM is B= 2(β + 1) B Where β = np A for PM Β = nf A/2ΠB for FM , Where B is bandwidth of modulating signal

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Explain Asynchronous and synchronous transmission. A: The transmission of stream of bits from device to another device across a transmission link involves cooperation and agreement between the devices. One of the most fundamental requirements is synchronization. The receiver must know the data rate at which the bits are received so that it can sample the line at regular intervals to determine the value of each received bit. This to perform the sampling at regular intervals the timing interval of the clocks between transmitter and receiver should be same. To achieve synchronization two approaches are there: Asynchronous and synchronous. Asynchronous transmission: It is used to avoid the timing problem by not sending long, uninterrupted stream of bits; instead data is transmitted one character at a time, where each character contains 5-8 bits. Timing must be maintained with each character. When no character is transmitted the line between transmitter and receiver is in the idle state. The beginning of each character is signaled by a start bit with a value of binary 0 on the line. It is followed by 5-8 bits. Usually data bits are followed by a parity bit for error detection at the receiver. The final element is a stop bit which is binary 1. Asynchronous transmission is simple and cheap, but requires an overhead of 2 or 3 bits per character. To achieve the greater efficiency a different form of synchronization known as synchronous transmission is used.

Synchronous transmission: With synchronous transmission a block of bits transmitted without start and stop bits. The block may be many bytes in length. To prevent the timing drift between transmitter and the receiver timing clock must be 23

synchronized. One way is to provide separate clock line between the transmitter and the receiver. There fore the receiver uses the clock of transmitter to read the data. This technique works well for short distances for long distances of transmission impairments will be there. Another method is keeping the clock information in the data signal known as self clocking encoding techniques like Manchester or differential Manchester containing clock information in the middle of the transition. Similarly for analog signal carrier frequency will be used to synchronize transmitter and receiver. With synchronous transmission another level of synchronization is required which allows the receiver to determine the beginning and ending of the data. To achieve this each block begins with a preamble bit pattern and ends with postamble bit pattern. In addition to the flag bits, control bits are also used to indicate the type of frame is being transmitting. Thus the data plus preamble postamble and control information is called frame.

The frame format of synchronous transmission starts with preamble called a flag which is 8 bits11111111. The flag 11111110 is also used as postamble.The receiver always look for flag pattern to indicate the start of the frame then followed by control field data field and finally the flag is repeated. For large blocks of the data synchronous transmission is more efficient because the overhead requires will being less. What types of errors? • •

•

An error occurs when a bit is altered between transmission and reception Single bit errors — One bit altered — Adjacent bits not affected — White noise Burst errors — Length B — Contiguous sequence of B bits in which first last and any number of intermediate bits in error — Impulse noise — Fading in wireless — Effect greater at higher data rates

Cyclic Redundancy Check(CRC) Explain modulo 2 arithmetic. A: T= n-bit frame to be transmitted. D= k- bit block of data, or message, the first k bits of T. F=(n-k) bit FCS, the last (n-k) bits of T P= pattern of n-k+1 bits; this is the predetermined divisor. We would like T/P to have no remainder. It should be clear that T=2n-k D + F i.e. by multiplying D by 2n-k ,we have in effect shifted it to the left by n-k bits and padded out the result with Zeroes. Adding F yields the concatenation of D and F, which is T. We want T to be exactly divisible by P. Suppose that we divide 2n-k D (2n-k D) / P = Q + R/P ---------------------------(Eq.1) 24

B’cos division is modulo 2, the remainder is always at least one bit shorter than the divisor. T=2n-k D + R Does this R satisfy our condition that T/P have no remainder? To see that it does consider T/P = (2n-k D + R) / P = (2n-k D)/ P + R/P Substituting Eq (1) we have T/P = Q + R/P + R/P However, any binary number added to itself modulo 2 yields zero. Thus T/P= Q + (R+R) / P = Q Ex:

Design a CRC logic for message M=1010001101 and P=110101 A: Digital logic The CRC can be implemented with the digital logic of shift registers and exclusive OR gates. The shift register is a string of 1 bit storage device. The circuit is implemented as follows: 1) Register contains n bits = length of FCS 2) There are upto ‘n’ Ex-OR gates 3) Presence or absence of term in the pattern P excluding xnth term Circuit Problem Line configurations: What is topology? The topology is the physical arrangement of stations on a transmission medium. 25

If there are only two links then it is point point . If there are more than two stations then it is multipoint topology. Write about interfacing with EIA-232/v.24 interface.

Mechanical specifications: It specifies 25 pin connector with specific arrangement of leads. This connector is the terminating plug for a cable running from DTE and DCE.The mechanical specifications are related to the physical connection of the DTE and DCE, the signals interchange circuits are bundled into a cable with a terminating connector of male or female at each end. Electrical specifications: It deals about the voltage levels and timing of voltage changes. Both DTE and DCE will have same code so that it must maintain same voltage levels, the electrical values are interpreted either as binary data or control signals. A voltage positive than +3v is represented as binary 0 and voltage more –ve than -3v is represented as binary 0. Functional specifications: The circuits can be grouped into the categories of data, control, timing and ground. There is one data circuit in each direction to perform full duplex operation. In addition there are 2 secondary data circuits that are useful to operate the device in a half duplex way so that data flow performs in only one direction at the same time. To perform half duplex communication, we need to transmit half or flow control message to a transmitting device. To accommodate this communication link contains a reverse channel, which operates at lower data rate than primary channel. Thus there are 4 data circuits transmitting data, received data secondary transmit data and secondary received data as shown below.

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Procedural specifications: It defines the sequence in which various circuits are used for particular application. The following procedure is specified by EIA-232 for dial up operation. 1.DTE turns on DTE Ready pin (20) to tell the modem it wants to begin a data exchange and transmits the phone number to DCE, so that modem A performs dialing operation. Pin: Pin(2)—Transmitted data Pin(20).----DTE ready 2.When the modem B receives the dialed pulses it alerts the DTE via ring indicator(22).Now DTE B turns on it’s DTE ready pin(20).Then modem B generates a carrier signal and also turns on DCE Ready(6) to indicate it’s readiness to receive the data. Pins: Ring indicator(22) DTE Ready(20) DCE ready(6). 3.When the modem Detects a carrier signal, it informs DTE a via pin(8) and also DCE sends carrier and indicates its readiness DCE ready signal(6). 4.Now the modem B indicates carrier is received by using pin(8). Thus a circuit is established between DTE a and DTE b through modem and telephone network. 5.The DTE a will generate RTS signal(4) and modem A responds with CTS(5).Now DTE a sends the data on transmitted data pin (2).Now mode4m A modulates the data pulses by using an analog carrier signal Pins: RTS (4)—Request to send CTS(5)---clear to send TD(2)---- transmitted data 6. Now modem B demodulates the carrier and generates digital data which are informed to DTE B via receive data pin(3). 27

Explain the concept of null modem. Null Modem: When DTEs are closely connected to each other directly, but the connections are such a way they are operating through a modem known as null modem. The following diagram gives the interconnection of circuits for null modem.

Explain the ISDN interface Integrated services digital network

The ISDN specifies 8 pin physical connector for digital interface. In ISDN terminology the physical connection is mad3e between terminal equipment and netwo5rk terminating equipment which is same as DTE and DCE respectively. Two pins are used to provide data transmission in each direction. These contact pins are used to connect twisted pair cables coming from NT and TE devices. The transmit and receive circuits are used to carry both data and control signals. The control information is transmitted in the form of messages. The specifications provide capability to transfer the power across the interface. The direction of power transfer depends on application of; it may also require providing power transfer from network to terminal. Electrical specification: It uses balanced transmission that is signals are carried on a line, such as twisted pair consisting of two conductors one for signal and another for ground known as balanced transmission and makes a complete circuit. For digital signals this technique is known as differential signaling that is the binary value depends on direction of voltage difference between the two lines. The advantage of balanced transmission is high data rate long distance communication is possible. Binary 1 is represented by absence o f signal and binary 0 is represented by both +ve and –ve voltage levels known as pseudo ternary technique. 28

What is flow control? Explain Stop and wait flow control. Flow control technique is for assuring that a transmitting entity does not overwhelm a receiving entity with data. In case flow control is not present the receiver buffer will fill up and overflow occurs while it is processing the data. Stop and wait Flow control: Source A transmits a frame. After the destination entity receives the frame it indicates it’s willingness to accept another frame by sending back an acknowledgement to the frame receives. The source must wait until it receives the acknowledgement before sending the next frame. Thus in this protocol, the source will stop the transmission and waits for acknowledgment. This is known as stop and wait flow control because the destination stops the flow of data by holding the acknowledgement. Channel efficiency for stop and wait flow control. To transmit the frame and receiving the acknowledgement, the time taken is Tframe + Tprop + Tproc + Tack + Tprop + T proc Where Tframe = frame transmission time Tprop = Propagation time from source to destination Tproc = Processing time from source to destination on incoming event Tack = acknowledgement transmission time Because and are very small and we can neglect them,then total time T= Tframe + 2. Tprop. Channel utilization u = Tframe /T U= 1/ 1+2a where a= Tprop / Tframe Tprop is propagation time, it specifies the time taken to propagate from source to destination depends on velocity of the signal ‘V’ and distance between two stations as Tprop = d/v Transmission time: It is the time taken to transmit all the bits of the frame by the station and it depends on frame length(l) and data rate ®. = L/R Then a= (d/v) / (L/R) A= dR/VL The disadvantage of stop and wait flow control is for every frame acknowledgement has to be transmitted which causes more number of acknowledgements and also the transmitter will not send the next frame, until it receives acknowledgement which decreases efficiency of channel utilization. Explain Sliding window flow control: In stop and wait flow control only one frame is transmitted at a time this causes inefficiency to improve the efficiency allowing multiple frames to be transmitted at the same time. Consider stations A and B connected via a full duplex link. Station B allocates buffer space for W frames. Thus B can accept W frames and A is allowed to send w frames without waiting for any acknowledgement. To keep the track of which frames have been acknowledged, each frame is given with a sequence number B acknowledges a frame by sending an acknowledgement that includes the sequence number of the next frame expected. This acknowledgement also indicates ‘B’ is ready to receive the next W frames with beginning of sequence number specified. This acknowledgement is also used for multiple frames. For example

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The station ‘A’ sends the frames continuously without waiting for acknowledgement until the window size becomes zero. Station A maintains a list of sequence numbers that it is allowed to se4nd and B maintains a list of sequence numbers that is prepared to receive. Each of these lists is known as window of frames. The sequence number is used for each frame with a maximum bit field. For example for 3 bit field the sequence number is 0-7 known as modulo 8.For a ‘n’ bit field the window size is 2n-1. When the transmitter sends the frames, the window will shrink based on no. of frames transmitted. Similarly the receiver window will shrink after receiving the no of frames. When the receiver sends an acknowledgement the receiver window will be enlarged. Similarly after receiving the acknowledgement the transmitter window will be enlarged. Thus by changing the window sizes and performing flow control is known as sliding window flow control.

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Error control Explain stop and wait ARQ method. It is based on stop and wait flow control technique. The source station transmits a frame and must wait for an acknowledgement and no other data frames are sent until the destination reply arises at the source station. There two type’s errors may occur. First, the frame that arrives to the destination may be damaged. The receiver detects this by using error detection techniques. To solve this problem, the source station contains the timer. After a frame is transmitted the source station waits for an acknowledgement if no acknowledgement is received by the time the timer expires the same frame is sent again and this method requires the transmitter maintain a copy of transmitted frame until an acknowledgement is receives for that frame. The second is damaged acknowledgement. Station A sends a frame the frame is received by station B which responds with an acknowledgement. The ack is damaged in transit and is not recognized by A. Therefore again the time out condition occurs and retransmit the same frame, but it causes duplicate frame.Therfore the destination discards the duplicate frame and sends the ack. To detect the duplicate frames the frames are alternatively labeled either with 0 or 1.An ack 0 acknowledges of frame1 and it is ready to receive frame 0. 31

Explain Go-Back-N ARQ. The error control is based on sliding window flow control. In this method the station may send a series of frames sequentially numbered with some maximum value. The number unacknowledged frames outstanding are determined by window size using sliding window flow control technique. If no errors occur, the destination will acknowledge by using ready to receive (RR) for the incoming frames. If the destination detects an error in frame it sends a negative acknowledgement reject (REJ) for that frame. The destination will discard that frame and also future frames until the error frame is correctly received. Thus the source station, when it receives reject it must retransmit the frame in error plus all the succeeding frames that are transmitted by the time the reject acknowledgement is received. Thus in Go- Back –N ARQ if the frames are received out of order those being discarded and sending the negative acknowledgement(REJ) so that the source will retransmit all the frames from rejected frame.

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Damaged Frame: • Receiver detects error in frame i • Receiver sends rejection-i • Transmitter gets rejection-i • Transmitter retransmits frame i and all subsequent Lost Frame (1) • Frame i lost • Transmitter sends i+1 • Receiver gets frame i+1 out of sequence • Receiver send reject i • Transmitter goes back to frame i and retransmits Lost Frame (2) • Frame i lost and no additional frame sent • Receiver gets nothing and returns neither acknowledgement nor rejection • Transmitter times out and sends acknowledgement frame with P bit set to 1 • Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i ) • Transmitter then retransmits frame i Damaged Acknowledgement • Receiver gets frame i and send acknowledgement (i+1) which is lost • Acknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter times out on frame i • If transmitter times out, it sends acknowledgement with P bit set as before • This can be repeated a number of times before a reset procedure is initiated Damaged Rejection • As for lost frame (2) Explain Selective –Reject ARQ. 33

A: With selective reject ARQ, the only frames transmitted are those that received a –ve acknowledgement called SREJ or those frames that are timed out. Selective –reject is more efficient than Go- Back –N ARQ because it minimizes the no of retransmissions, but the protocol is more complex because the receiver has to maintain the buffer to store the frames after sending SREJ and till it receives the frame with correct order. It also requires logic for re inserting the frames in proper sequence. The transmitter also requires a complex logic because it has to search a particular frame and sending the frame out of sequence. Because of these limitations Go- Back –N ARQ is more widely used than selective Reject ARQ. Another problem with selective reject is window size limitation and maximum window size is 2n-1.So that the overlapping between and receiving window does not occur.

What are the basic characteristics of HDLC? The most important data link control protocol is HDELC.IT is a basic protocol for other data link control protocols. Basic characteristics: HDLC defines 3 types of stations, 2 link configurations and 3 data transfer modes of operation. The 3 stations are: 1) Primary station: Responsible for controlling the operation of link. The frames issued by the primary stations are called commands. 2) Secondary station: This operates under the control of primary station. The frames issued by a secondary station are called responses. 3) Combined station: Contains the combined features of primary and secondary. Issues commands and responses. The two types of link configurations are 1) Unbalanced configuration: It consists of one primary and one or more secondary stations but supports both half duplex and full duplex. 2) Balanced configuration: Both the stations are combined stations and support both half duplex and full duplex. There are 3 types of data transfer modes: 34

1) Normal response mode: It is used with an unbalanced configuration, the primary may initiate data transfer to secondary, but secondary will transmit the data only in response to a command from primary. 2) Asynchronous balance mode: Used with balanced mode configuration, either combined station may initiate data transmission without waiting for explicit permission from the other station. 3) Asynchronous response mode: Used with unbalanced configuration, the secondary may initiate data transmission without explicit permission from the primary. The primary still responsible for initialization of the line, error recovery and logical disconnection.

Explain the HDLC frame structure.

The HDLC frame contains the flag, address control field before the information known as header. Then the FCS flag fields are following the data known as trailer. FLAG The 8 bit pattern "01111110" which signals the beginning and end of an HDLC frame. If a piece of data within the frame to be transmitted contains a series of 5 or more 1's, the transmitting station must insert a 0 to distinguish this set of 1's in the data from the flags at the beginning and end of the frame. This technique of inserting bits is called bit-stuffing. These bits are detected and removed upon receipt. If a pattern of five 1's is followed by a 1 and then a 0, it marks the end of the frame. If a pattern of five ones is followed by two more 1's, it is a signal from the transmitting station to abort. ADDRESS The address field always contains the address of a secondary station. When the primary station transmits, it is the receiving secondary station's address, if it is a secondary responding, it is it's own address CONTROL The format of the control field varies with the data it contains. There are three categories of HDLC frames: • • •

Information (I-frame) - Carries data Supervisory (S-frame) - Carries commands and responses Unnumbered (U-frame) - Carries additional command sequences

N(S)-Sending frame sequence number 35

N(R) - Receiving frame sequence number. All the control fields contains polling and final bit. In the command frames the p bit is set to 1 as a polling response for HDLC entity. In response frames the F bit is set to 1 indicate the response frame transmitted as a result of a command. The basic control field S and I frames uses 3-bit sequence number. Information field: Information field is present only in I-frames and some U-frames contains integral multiple of bytes of information with some maximum value. FCS: It is an error detecting code calculated from the remaining bits of the frame except flags. The normal code is 16 bit CRC and it is also used for 32 bit CRC. Explain the operation of HDLC frames

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Operations: It consists of exchange of I-frames, s_frames and U-frames between two stations. As shown in the diagrams the operation involves 3 phases. On any one side the data link is initialized and the initialization phase contains by using six set mode commands. One of the commands is transmitted as NRM, ARM or ABM is used. It also specifies either 3 bit sequence no or 7 bit sequence number. If other side accepts this request then HDLC module on the other side transmits UN numbered acknowledgement frame back to the initiating side. If the request is rejected DM frame is transmitted. After initializing the information frames are transmitted then the connection is released by signaling through disc frame.

Data transfer: Both the sides user data can be transmitted by using I frames stating with sequence no zero either using modulo 8 or modulo 128 depending on 3 bit or 7 bit sequence number and places the sequence number in n(s).N(R) is acknowledgement for the I frames received. S_frames are also used for flow control and error control. The receiver ready (RR) acknowledges the last I-frame received by indicating the next frame expected. Receive not ready (RNR) acknowledges an I-Frame as RR, but requesting temporarily suspend the transmission of frames. REJ initiates Go- Back –N ARQ, so that retransmission will perform from the last frame. SREJ is used for retransmission of just single lost frame. Disconnect: Either HDLC module can initiate a disconnect by issuing a DISC frame. The remote entity accepts the disconnect and sends the reply UA frame indicating the connection is terminated. Example of operation of HDLC: The diagram 1 shows link setup and disconnect. The HDLC protocol on one side generates SABM command to the other side and starts a timer. The other side returns a UA response and initializing the connection. 37

In the diagram 2 full duplex exchanges of I-frames are given. The sequence no of I-frame is incremented if other side no transmission is performed. It uses piggy back mechanism to acknowledge the frames. In the diagram 3 busy conditions is specified. The station A temporarily stops the transmission by indicating RNR.Then the Station B sends RR with P bit to know its ready ness. After 2 attempts A indicates its readiness to receive the frame 4 as a response RR4, F. In diagram 4 Reject Recovery is used to overcome the frame lost. Station B receives the frame 5 without receiving 4.Then it sends Rej4 and discards the frame 5 so that station A retransmits both frame 4 and 5. Time out recovery at station A timeout condition is occurred then it indicates by using RR P bit =1 then the station B sends the response as RR3 so that station A transmits frame 3.

END OF II UNIT

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III UNIT Define multiplexing and explain frequency division multiplexing. In general two communicating stations will not utilize the full capacity of the link. For efficiency it should be shared by multiple stations known as multiplexing.

As shown in the diagram there are n inputs to a multiplexer which is connected by a single data link to a demultiplexer .The link is able to carry n separate channels of data. Thus multiplexer combines the data from n input lines and transmits over a high capacity data link. The demultiplexer accepts the multiplexed data and separates them according to channel and given to appropriate line. FDM:

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FDM is used with analog signals. A no of signals are carried simultaneously on the same medium by allocating each signal with different frequency. A number signals can be carried simultaneously if each signal is modulated onto a different carrier frequency and carrier frequency are sufficiently separated that the bandwidths of the signals do not sufficiently overlap. To prevent the interference between channels the channels are separated by guard bands which are un used portion of the frequency spectrum. The composite signal transmitted across the medium is analog. However the input can be either digital data or analog data. In case of digital data the input signals are passed through modem to convert into analog.The disadvantages of FDM system are: 1) Cross talk which because of overlapping of adjacent signals 2) Because of non linearity of transmission system, the combines signals may be generated of different independent signals known as inter modulation noise. Applications of FDM System: 1) In the long distance telephone systems as analog carrier system AT & T developed an FDM standard.

12 voice channels each of 4 kHz are combined to produce a group .5 groups are combined to form supergroup.10 super groups are combined to form master group.6 master groups are combined to form a jumbo group. Explain synchronous time division multiplexing. Synchronous time division multiplexing is possible when the data rate of the medium exceeds the data rate of digital signals to be transmitted. Multiple digital signals can be carried on a single transmission path by interleaving of each signal. The interleaving can be at the bit level or byte level or larger quantities. For example for the 6 inputs of multiplexer with 9.6 kbps, then the capacity of the line at least 6 * 9.6= 57.6kbps.

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As shown in the diagram a no of signals m (t) are to be multiplexed on the same transmission medium, the signals carry digital data are generally digital signals. Then the incoming data from each source is buffered, the buffer is either 1 bit or 1 character in length. The buffer is scanned sequentially to generate composite data stream m (t). The scan operation is rapid so that each buffer is emptied before more data arrive. Thus the data rate of M (t) may be at least equal to sum of the data rates of mi (t).Then the digital signal m (t) is transmitted directly or passed through modem to convert into analog signals, in either case the transmission is synchronous. The data is organized into frames; each frame contains a group of cyclic time slots. In each frame one or more time slots are dedicated to each data source called as channel. The slot length equals to transmitter buffer length. For asynchronous sources character interleaving technique is used that is each time slot contains one character of data but without having stop and start bits, which are inserted by receiver. The bit interleaving technique is used by synchronous sources. At the receiver the interleaved data is routed to appropriate destination buffer. For each input source mi (t) there is an output source which will receive the input data at the same rate at which it was generated. Synchronous TDM is called synchronous not because of synchronous transmission but because the time slots are pre assigned to the sources and they are fixed. Whether source contains data to send or not therefore synchronous TDM causes to waste the capacity in case of the time slots are empty. TDM link control: The TDM frames does not contain headers and trailers because the control mechanism provided by data link layer is not provided in TDM because the flow control, error control are the responsibilities of data link layer, but not the responsibility of multiplexer. The data rate on the multiplexed line is fixed and they are designed to operate at the same rate, even in case of time slots are empty. Therefore no flow control is required. Similarly no error control is required for TDM, because no retransmission of frame ever occurs. In TDM the responsibility is of data link layer. As shown in the diagram, there are 2 data sources transmitting HDLC frames. One contains 3 bytes and the other contains 4 bytes. Because the byte interleaving the HDLC frames are shuffled together for transmission over the multiplexed lines. The destinations HDLC will wait until end of the frame is detected, then process the frame at the same time. 41

Framing: The basic requirement for framing is we require assuring some means of frame synchronization so that the data will not be lost. To achieve this 1 bit is added as TDM frame synchronization. The other method is alternating bit pattern is also transmitted to synchronize the receiver and sender. Pulse stuffing: The most typical problem in TDM is synchronizing the various data sources. If each station has separate clock any variation among the clocks will looses the synchronization. Also the data rate of input data streams are not related by a rational number. To solve these prooble4ms a pulse stuffing is commonly used. In this case the outgoing data rate of the multiplexer is higher than the sum of the maximum incoming data rates. These pulses are inserted at fixed locations and they are identified and removed by de multiplexer.

Explain statistical multiplexing. An alternate to synchronous TDM is statistical TDM.In this time slots will be allocated dynamically on demand. Like synchronous TDM it also contains number of input line and a high speed multiplexed line on the other side. Each line contains a buffer, but the umber of slots available on the frame is less than the number of 42

sources. The function of the multiplexer is scan the input buffer and collecting the data until a frame is filled and then send the frame. At the receiver the multiplexer receives the frame and distributes the slots of the data to the appropriate output buffer.

Statistical multiplexer never sends empty slots. However the receiver will not be knowing in advance that the data belongs to which source in any particular slot. Explain asymmetric digital subscriber line. In the implementation of high speed wide area public digital network, the critical part is the link between subscriber and the network which is digital subscriber line.Instaling a new cable for digital subscriber line requires more money instead the network designers had an idea of usage of existing twisted pair telephone cable only a bandwidth of 4 KHz but the twisted pair link provides 1 MHz band width. Thus ADSL is a family of new modern technologies designed to provide high speed digital data transmission on an ordinary telephone line. ADSL design: ADSL provides more capacity for downstream from exchange to customers place than upstream from customer to exchange. Initially ADSL was designed for video transmission on twisted pair but because of high demand of high speed internet access, the ADSL technology is used for transmission of internet data. Since the user requires more capacity for down streams than upstream transmissions because of the short email messages where as downstream contains large amount of data which includes images and video. ADSL uses FDM to share the 1 MHz capacity of twisted pair it contains three elements Reserve the lowest 20 KHz for voice known as plain old telephone service (POTS) But only 0-4Khz bandwidth is used and to avoid the cross talk additional bandwidth is reserved. Use either echo cancellation or FDM to allocate two bands known as upstream and downstream. In FDM an upstream use a frequency band 25 to 200 KHz and down stream is 250 to 100 KHz. In echo cancellation variable band width is provided for upstream so that the left over band width is used for downstream. Advantages of echo cancellation: Higher the frequency attenuation is also more. Therefore with echo cancellation the down stream is also represented with lower frequency spectrum. So that it will be affected for attenuation. Flexibility is more because of variable bandwidth. Disadvantages. Complexity is more because at the both the sides it requires echo cancellation mechanism. Discrete multi tone: DMT uses multiple carrier signals at different frequencies sending some of the bits on each channel so that high data rate is possible as shown in the diagram. The digital data is transmitted by modulating on different carriers for example in ADSL 256 sub channels are used with each 4 KHz carrying 60 KBPS and achieving a data rate of 15.36 Mbps Explain circuit switching: For transmission of data beyond the local area the communication is achieved from transmitting the data from source to destination through intermediate switching nodes. 43

There are two ways of switching circuit switching and packet switching Circuit switching: Circuit switching means the dedicated path between communicating devices. This involves three phases: circuit establishment, data transfer and circuit disconnect. Circuit establishment: Before any are to be transmitted an end to end circuit is dedicated till the completion of the data transfer. Data transfer: Now the information can be transmitted from source to destination without any further delay because the path is established already. Circuits disconnect: After some period of data transfer the connection is terminated by the action of any one of the stations. Circuit switching is developed to handle voice traffic, but it also used for data traffic. The example for circuit switching is telephone network. The general components in telephones are:

Subscribers: The devices that are attached to the network like interconnection of telephones to the public networks. Subscriber line: the link between subscriber and network and their responsibility is establishing circuits. Exchanges: The switching center in the network and their responsibility is establishing circuits. Trunks: The branches between exchanges trunks carry multiple voice signals either by using FDM or TDM. Circuit switching is inefficient for transmission of data, because the channel capacity is dedicated for the duration of connection even if no data is transmitted. Space division switching: Space division switching was originally developed for analog environment, can also be used for both analog and digital signals. As its name implies a space division switch contains the signal path are physically separated from one another. Each connection requires the establishment of a physical path through the switch which is used for transfer of signals between the two end points. The switch contains a cross point or a logical gate that is enabled or disabled. Let us consider 10 full duplex input and 10 output lines. Each station is connected to one of the line and communication is possible between two stations by enabling the appropriate cross bar.

The limitations of the crossbar switching are: 44

1)

The no of cross points increases with square of the no of stations which are attached and also causes to increase the cost of the switch. 2) The loss of cross point will stop the communication between two devices. 3) Cross points are inefficiently utilized. To overcome these limitations multistage switches are used. Multistage space division switching reduces the no of cross points for example consider 3 stage switches with N×N input and out put lines, instead of constructing single N×N cross bar switch we can have smaller cross bars by considering in the first stage each cross bar has N inputs. There fore the number of cross bars required in the first stage is N/n cross points. The second stage has k cross points each with N/n × N/n input and out put lines. The third stage is repeater of the first stage, but reversed from left to right. K value depends on design consideration and the no of cross points= 2kn+ k (N/n) 2

Time division switching: Use digital time division techniques to set up and maintain virtual circuits.The modern switching is time division switching which involves transmitting low speed bit stream by sharing high speed stream, providing time slots which are manipulated by control logic to route the data from input to output. Explain the control signaling functions: • • • • • • • • •

Audible communication with subscriber with the dial tone, ringing tone, busy signal and so on Transmission of dialed number to switching office that will attempt to complete the connection. transmission of information between switches indicating that Call can not be completed transmission of information between switches indicating that Call ended indication A Signal to make telephone ring Transmission of information used for Billing Transmission of status info of Equipment and trunk in the network Transmission of information used in Diagnosing and isolating system failures. Control of special equipment such as channel equipment.

Explain the signaling techniques for circuit switched networks. Traditional control signaling is in channel signaling. the same channel is used to carry control and data signals. Two forms of in channel signaling are in use. 1)in band and out of band. In band: this uses not only same physical path but also uses the same frequency as the voice signal are carried. This is simplest technique. It is necessary for call information signals and may be used for other control signals.Inband can be used over any type of subscriber line interface. Can go anywhere a voice signal can. Impossible to set up a call on a faulty speech path Out of band signaling: this uses the advantage of the fact that voice signals do not use the full 4 KHz band width allotted to them. Narrow signal band within 4 kHz used for control. Transmits control signals using the same facilities as the voice signal but a different part of the frequency band. Unlike in band out of band 45

signaling provides continuous supervision for duration of a connection. Can be sent whether or not voice signals are present. Slower signal rate (narrow bandwidth).Need extra electronics Drawbacks of In Channel Signaling • • •

Limited transfer rate Delay between entering address (dialing) and connection Overcome by use of common channel signaling

Common channel signaling: One control signal channel can carry signals for a number of subscriber channels. Common control channel for these subscriber lines. Transmits control signals over signaling channels that are dedicated to control signals and are common to a number of voice channels. This reduces call setup time compared with in channel methods. It is also more adaptable to evolving functional needs. Two modes of operation are used in common channel signaling. Associated mode: The common channel closely tracks along the entire length the interswitch trunk groups that are served between end points. The control signals are on different channels from the subscriber signals and inside the switch the control signals are routed directly to a control signal processor. Disassociated mode: This is more complex but more powerful. In this additional nodes will be arranged known as signal transfer points. Now there are two separate networks, with links between them so that the control portion of the network can do the control over the switching nodes that are servicing the subscriber calls.

Explain packet switching techniques and advantages. Data are transmitted in short packets. If the source has a message to send the message is broken into a series of packets. Each packet contains a portion user’s data plus control information. This packet switching has number advantages over circuit switching: 1) Line efficiency is greater, because single node to node link can be dynamically shared by many packets over time. Packets are queued up and transmitted as rapidly as possible over the link. 2) A packet switching network can perform data rate conversion .i.e. two stations of different rates can exchange packets because each connects to its node at its proper data rate. 46

3) When traffic becomes heavy in circuit switching network, the network refuses to accept additional connection requests until the load on the network decreases. On a packet switching network, packets are still accepted but delivery delay increases. 4) Priorities can be used. Thus if a node has a number packets queued for transmission it can transmit the higher priority packets first. Switching techniques: To transmit the series of packets two approaches are used. Datagram and virtual circuit. Datagram: In datagram each packet is treated independently with no reference to packets that have gone before.

Each node chooses the next node on a packet’s path by taking information from neighboring nodes on traffic and failures and so on. So all the packets of the same destination do not follow the same route. And they may arrive out of sequence at the exit point. Virtual circuit: A preplanned route is established before any packets are sent. Once the route is established, all the packets between a pair of communicating parties follow this same route through the network. This route is fixed for duration of logical connection. It is somewhat similar to a circuit in a circuit switching. Each packet contains virtual circuit identifier as well as data. Each node in the pre established route knows where to direct such packets. No routing decisions are required. A packet is buffered at each node and queued for output line, while the other on other virtual circuits may share the use of the line. The network may provide services like sequencing and error control .One advantage of datagram is call setup phase is avoided.

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Explain the effect of packet size on transmission time. Let us assume that there is a virtual circuit from station X through nodes ‘a’ and ‘b’ to station Y.The message to be sent comprises 40 octets and each packet contains 3 octets of control information, which is placed at the beginning of each packet and is referred as header. The entire packet is transmitted from X to ‘a’ and then ‘a’ to ‘b’ and ‘b’ to Y.Ignoring switching time the total transmission time is 129 octet time. Let us break the message up into two packets each containing 20 octets plus 3 bytes header. In this case as soon as node gets first packet from X without waiting for the second packet, transmitted to ‘b’.Because of this overlap the transmission time drops to 92 octet time. By message into five packets the total transmission time becomes 77 octet-time for transmission. However this process of using more and smaller packets results in increase in delay.

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Compare circuit switching and packet switching.

Circuit switching

Datagram packet switching

Virtual circuit packet switching

Dedicated transmission path

No dedicated path

No dedicated path

Continuous transmission of data

Transmission packets

Transmission of packets

Fast enough for interactive

Fast enough for interactive

Fast enough for interactive

Messages are not stored

Packets may be stored until delivered Packets stored until delivered

The path is established for entireRoute established for each packet conversation

Route established conversation

for

entire

Call setup delay; transmission delay

Call setup delay transmission delay

and

packet

negligiblePacket transmission delay

Busy signal signal if called party isSender may be notified if packet notSender notified of connection denial busy delivered Overload may block call setup; noOverload increase packet delay Overload may block call setup; delay for established calls. increases packet delay 49

Electromechanical or computerizedSmall switching nodes Small switching nodes switching nodes User responsible for message loss Network may be responsible forNetwork may be responsible for individual packets packet sequences Usually no speed or code conversion Speed and code conversion Speed and code conversion Fixed bandwidth

Dynamic use of bandwidth

Dynamic use of bandwidth

No over head bits after call setup

Overhead in each packet

Overhead in each packet

ATM Explain the protocol architecture of ATM Asynchronous transfer mode allows multiple logical connections to be multiplexed over a single interface. The information flow on each logical connection is organized into fixed size packets called cells. ATM is a stream lined protocol with minimal error and flow control capabilities.

The physical layer involves the specifications of a transmission medium and a signal encoding scheme. The data rates specified at the physical layer range from 25.6Mbps to 622.08Mbps. Two layers of the protocol architecture relate to ATM functions. There is an ATM layer common to all services that provides packet transfer capabilities, And an ATM adaptation layer that is service dependent. The AAL maps higher layer information into ATM cells to be transported over an ATM network, then collects information from ATM cells for delivery to higher layers. The protocol reference model involves three separate planes. User plane: Provides for user information transfer along with the associated control (flow control and error control). Control plane: Performs call control and connection control functions. Management plane: Includes plane management which performs management functions related to a system as a whole and provides coordination between all the planes and layer management which performs management function relating to resources and parameters residing in its protocol entities. Explain the ATM logical connection: Logical connection in ATM are referred as Virtual Channel Connection (VCC) A VCC is set up between two end users through the network and a variable rate full duplex flow of fixed size cells is exchanged over the connection. VCC is also used for user to network exchange and network to network exchange. 50

ATM deals with the concept Virtual path connection. VPC is a bundle of VCCs that have the same end points. Advantages of virtual path: Simplified network architecture: Network transport functions can be separated into those related to an individual logical connection and those related to a group of logical connections. Increased network performance and reliability: The network deals with fewer aggregated entities. Reduced processing and short connection setup time: Much of the work is done when the virtual path is setup. By reserving capacity on a virtual path connection, new virtual channel connections can be established by executing simple control functions at the points of the virtual path connection. No call processing is required at transit nodes. Enhanced network services: The virtual path is used internal to the network but is also visible to the end users. Thus the user may define closed networks of virtual channel bundles. Call establishment using virtual path:

Explain the ATM cell format ATM cell consists of a 5 octet header and 48 octet information fields. The use of small cell may reduce queuing delay for a high priority cell, because it waits less if it arrives slightly behind a lower priority cell that has gained access to a resource. Second it appears that fixed size cells can be switched more efficiently which is important for the very high data rates of ATM.

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Header format. GFC: The generic flow control field does not appear in the cell header internal to the network, but only at the user network interface. Virtual path identifier: Constitutes a routing field for the network. It is 8 bits at the user network interface and 12 bits at the network interface. Virtual channel identifier is used for routing to and from the end user. Pay load type this field indicates the type of information in the information field. A value of 0 in the first bit indicates user information. Second bit indicates the congestion has been experienced or not and third bit with SDU (service data unit ---0 or 1) type. Cell loss priority: This is used to provide guidance to the network in the event of congestion.0 indicates relatively higher priority. GFC: Used in UNI .To support GFC mechanism two sets of procedures are used: controlled and uncontrolled transmission. Every connection is identified as either subject to flow control or not. The controlled equipment called terminal equipment(TE),initializes two variables: TRANSMIT is a flag initialized to SET(1), and GO_CNTR, which is credit counter, is initialized to 0.A third variable GO_VALUE, is either initialized to 1 or set to some larger value at configuration time. The rules for transmission by controlled device are as follows: 1. If TRANSMIT= 1, cells on uncontrolled connections may be sent at any time. If TRANSMIT = 0, no cells may be sent on either controlled or uncontrolled connections. 2. If a HALT signal is received from the controlling equipment, TRANSMIT is set to 0 and remains at zero until a NO_HALT signal is received, at which time TRANSMIT is set to 1. 3. If TRANSMIT = 1 and there is no cell to transmit on any uncontrolled connections, then a) If GO_CNTR > 0, then the TE may send a cell on a controlled connection. The TE marks that cell as a cell on a controlled connection and decrements GO_CNTR. b) If GO_CNTR = 0 then the TE may not send a cell on a controlled connection. 4. The TE sets GO_CNTR to GO_VALUE upon receiving a SET signal; a null signal has no effect on GO_CNTR. Explain ATM Header error control: Each ATM cell contains 8 bit header error control which is calculated based on the remaining 32 bits of the header. The operation used at the receiver is

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The receiver will be in default single bit error correction mode as each cell is received.HEC calculation and comparison is performed as long as no errors are detected. The receiver will be in error correction mode. When an error is detected the receiver will correct the error if it is a single bit error or will detect that a multiple bit error has occurred, in both the cases the receiver will move to detection mode and in this mode no attempt is made to correct the errors by assuming a sequence of errors can be occurred which is not possible to correct the error by HEC.Therefore the receiver remains in error detection mode as long as error cells are received. When a header is examined and found not to be in error, then the receiver will move to error correction mode. Explain ATM services. An ATM network is designed to transfer many different types of traffic including real time flows like audio and video and burst TCP flows. Each traffic flow is a stream of 53 byte cell and transmitting through a virtual channel. The various services defined by ATM are: 1. Real Time services: a) Constant bit rate (CBR) b) Real time variable bit rate (RT-VBR) 53

2. Non Real Time Service a) Non Real time variable bit rate (NRT-VBR) b) Available bit rate (ABR) c) Unspecified bit rate (UBR). Real time services: The most important distinction among the applications is time constraint applications known as real time applications. For ex flow of audio and video information should be continuous. To support these applications ATM provides 2 services. a) Constant bit rate: CBR is a simple service used by the applications which requires a fixed data rate and available continuously for entire connection period. It’s commonly used in uncompressed audio and video information. Examples are video conference for telephony service, audio video distribution. b) Real time variable bit rate (RT-VBR): Is also used for time sensitive applications which require less delay for data transmission. The application transmit with variable data rate will use this service. For ex in a compressed video, the data rate will be changing from one frame to another frame and it is more flexible than CBR.Thus this service is suitable for transmission of real time compressed audio and video. c) Non real time variable bit rate: NRT-VBR is mainly used in critical response time requirement like airline reservations, banking transactions (ATM machines) and process monitoring. d) Available bit rate (ABR): A reliable end to end protocols TCP is used for variable size data applications. For those applications ABR service is used which guarantees minimum cell rate (MCR) and if network resources are available it also provides peak cell rate (PCR).This service is suitable for internet applications to provide bursty data rate. e) Unspecified bit rate (UBR): Without any guarantee of data delivery and also some time the MCR becomes zero and this service is suitable for text/data/image transfers. It is also used in messaging system. Explain AAL layer with various protocols ATM adaptation layer supports information transfer protocols not based on ATM.The functions of AALK layer are: a) Handling of transmission errors. b) Segmentation and reassembly (SAR) to enable larger blocks of data to be carried in ATM cells. c) Handling of the lost cells during the transmission d) Flow and timing control. ATM layer maps variety of applications on to the ATM layer provides the traffic management capabilities on top of the ATM layer. AAL layer is organized into two sub layers called Convergent sub layer and segmentation and reassembly layer. The CS sub layer provides the functions based on the applications, each AAL user attaches the applications at an address called service access point (SAP).This sub layer depends on the service required by application known as service dependent. SAR sub layer is responsible for making the data received from CS into cells and at the other side, unpacking the information from the received cells. As shown in the diagram the general protocol architecture the higher layers generates the data which is given to CS sublayer.The CS sub layer adds header and trailer makes as CS PDU (protocol data unit) and is then passed to SAR sub layer, which makes into no. of blocks, each block contains header and trailer added by SAR and makes as 48 bytes of SAR PDU and given to ATM layer which adds 5 bytes of ATM header and makes 53 bytes ATM cells. AAL supports different protocols AAL1, AAL2, AAL3/4, and AAL5. AAL type1:It is designed for CBR and the responsibility of SAR protocol is take either 47 bytes or 46 bytes bytes of CS PDU,then adding either 1 bytes or 2 bytes header respectively making as 48 bytes SAR PDU as:

P

SN

SNP

Parity

1bit

3 bits

3 bits

1 bit

47 bytes cs payload

54

P

SN

SNP

Parity

Pointer

1bit

3 bits

3 bits

1 bit

8 bits

46 bytes cs payload

P bit indicates the cell is a pointer cell or a non pointer cell.’0’ means non pointer cell. SN Sequence number: Every cell contains unique number used to identify either lost or wrongly inserted cell. SNP sequence number protection: It is CRC calculated based on SN value. It is used for detecting the errors in sequence number. Parity: Even parity is used to detect bit errors in the header. Pointer: For a message oriented applications, the end of the message is indicated by pointer. AAL 2: It is also same as AAL type 1 protocol, but used for compressed audio video transmission for VBR variable bit rate. It also doesn’t contain any protocol for CS sublayer.It divides the data and given to SAR protocol which adds both header and trailer as shown in the diagram. Header of 2 bytes and Trailer of 2 bytes.

SN

IT

44 bytes cs payload

LI

CRC

SN Sequence number: Every cell contains a unique no known as sequence number. IT Information time: Indicates the cell is a beginning of the message, middle of the message or end of the message. LI length indicator: Indicates the length of the data. CRC cyclic redundancy check used for detecting the errors. AAL ¾ protocols: The characteristics of AAL3/4 are 1) It supports both connection oriented and connection less. 2) The service may be message mode or stream mode. 3) It allows multiplexing of different application cells transmitting of same logical connections. AAL3/4 protocol contains protocol for both CS sub layer and SAR sublayer.A block of data from higher layer is taken by CS layer and adding 4 bytes of data header and 4 bytes of trailer, then making as 44 bytes of data and given to SAR sublayer, then the SAR sub layer adds 2 bytes header and 2 bytes trailer making as 48 bytes of data and given to ATM layer and making as 53 bytes ATM cells. The CS protocol contains header and trailer. The header contains the following fields: CPI

BTAG

BA size

1byte

1byte

2bytes

CS payload

PAD

1byte

ETAG

Length

1byte

2bytes

CPI Common Part Indicator: Indicates the interpretation of remaining fields as bits or bytes. Btag: beginning tag: A number associated for each data transmission. the same value appears in the Btag field in the header and Etag field in the trailer. BA size: Buffer Allocation size Indicates the maximum buffer size required for specifying to destination. PAD: At CS layer the padding is performed to make the data as a multiple of 4 bytes. The trailer of CS PDU contains 1byte un used. ETAG: Which contains same number of BTAG. Length: the length of the PDU payload is indicated without padding bits. At the SAR sublayer, the header contains: ST

SN

MID

3bits

3bits

10bits

SAR payload

LI

CRC

6bits

10 bits

55

Segment Time: Which indicates the cell is beginning of the message, middle of the message or end of the message. Sequence Number: SN specifies a unique number for each cell used to recognize the lost cells or misinserted cells. Multiplexing identification: A unique identifier which is given same number for all the cells of the same application so that different application cells can be transmitted on a single logical connection. The SAR PDU trailer contains LI :Indicates the number of bytes from the SAR PDU so that occupy the segmentation unit of SAR PDU and this number will be between 4 and 44 bytes in multiples of 4 bytes. CRC: 10 bit CRC for entire SAR PDU used to calculate bit errors. AAL 5: The most recent protocol is AAL 5 protocol. Designed to provide stream lined transport facility for higher layer with connection oriented service. It means the higher layers should be responsible for flow and error control and no multiplexing is performed. It also doesn’t contain any sub layer by adding 8 bytes of trailer after padding the received data. The CS sub layer divides the data into 48 bytes and given to SAR sub layer, without adding any header and trailer given to ATM layer. Which converts into 53 bytes of ATM cells. The CS layer contains: CS payload

PAD

UU

CPI

Length

CRC

1byte

1byte

2bytes

4bytes

User to User information: It is used by higher layers to provide the information. Common part indicator: Indicates the interpretation of remaining fields in the trailer. Length: Indicates the length of PDU payload CRC: used to dete3ct the bit errors Explain ATM cell transmission or explain the data link layer of ATM. Since ATM cells are transmitted at high data rates 25 mbps to 155 mbps we need to specify the transmission structure to carry this pay load. 2 approaches are used cell based physical layer and SDH physical layer. Cell based physical layer:

In cell based physical layer, each cell is transmitted separately, without any framing is used. Since no frame is used some method is required for synchronization, which can be used on the basis of HEC by using cell delineation algorithm, it can be obtained. The procedure is: 1) Initially the receiver will be in the hunt state, the cell delineation is performed for bit by bit to determine whether the HEC coding law is observed. If so it is assumed that one header has been found that it enters into pre sync state. 56

2) 2) In Pre sync state cell structure is assumed then the cell delineation is performed cell by cell and it is not following the HEC law for one cell, it returns to hunt state otherwise it enters into sync state after observing for δ consecutive cells with correct HEC. 3) In the Sync state, the HEC used for error detection and correction. SDH based physical layer:

Imposes structure on ATM cell stream, the framing is imposed by using STM-1 frame. The pay load may start after certain offset which is indicated by pointer in the overhead of the frame. The payload contains 9 bytes over head portion and remainder contains ATM cells with a maximum of 2340 bytes. So that the cell may cross a boundary. The H4 byte is used to indicate number of bytes to the first cell boundary. The advantages are: 1) It can be used to carry either ATM payloads or STM based payloads. 2) The connections can be allowed through circuits switching so that both ATM switching and circuits switching is possible. 3) Using SDH multiplexing techniques, several ATM streams can be combined. Explain X.25 X.25 is the packet switching protocol used in the wide area network.This known as subscriber network interface protocol. X.25 protocol is organized into three layers: the physical layer a, link level and packet level. The physical level deals with the physical interface between the attached station and the link that attaches that station to the node. The link level standard is referred as LAPB(Link Access Protocol-Balanced). The packet level provides virtual circuit service, which is external virtual circuit.

User data are passed down to X.25 level 3,which appends control information as a header, creating packet. This control information serves several purposes, including, • Header • Identifies virtual circuit • Provides sequence numbers for flow and error control The entire X.25 packet is then passed down to the LAPB entity which appends control information at the front and back of the packet, forming LAPB frame.The operation of X.25 is similar to HDLC operation. 57

Explain Frame Relay. This is designed more efficient transmission than X.25. The following are key features of X.25: • Call control packets, in band signaling • Multiplexing of virtual circuits at layer 3 • Layer 2 and 3 include flow and error control • Considerable overhead • Not appropriate for modern digital systems with high reliability The key differences between frame relaying and conventional X.25 packet switching service are: • Call control carried in separate logical connection • Multiplexing and switching at layer 2 — Eliminates one layer of processing • No hop by hop error or flow control • End to end flow and error control (if used) are done by higher layer • Single user data frame sent from source to destination and ACK (from higher layer) sent back Advantages and disadvantages of frame relay: • Lost link by link error and flow control • Streamlined communications process — Lower delay — Higher throughput • ITU-T recommend frame relay above 2Mbps Frame relay protocol architecture:

There are two separate planes of operation: control plane and user plane. Control plane: • Between subscriber and network • Separate logical channel used — Similar to common channel signaling for circuit switching services • Data link layer — LAPD (Q.921) — Reliable data link control 58

— Error and flow control — Between user (TE) and network (NT) — Used for exchange of Q.933 control signal User plane: • End to end functionality • Transfer of info between ends • LAPF (Link Access Procedure for Frame Mode Bearer Services) Q.922 — Frame delimiting, alignment and transparency — Frame mux and demux using addressing field — Ensure frame is integral number of octets (zero bit insertion/extraction) — Ensure frame is neither too long nor short — Detection of transmission errors — Congestion control functions User Data Transfer: Let us consider the frame format of frame relay:

DLCI serves the same function as the connection identifier. EA: If EA=0 no extension of the address field. Otherwise extension of the address field is there. There is no control field in the frame. This having following implications. • • •

One frame type — User data — No control frame No in band signaling No sequence numbers — No flow nor error control

END OF UNIT-III

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