MODULE III

Digital data transmission

The transmission of digitally encoded data between two points can be termed ad digital data transmission. Here the transmission of binary data occurs. It can be done in different strategies as:

Serial transmission

In serial transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices.

The advantage of serial over parallel transmission is that with only one communication channel, serial transmission reduces the cost of transmission over parallel

by roughly a factor of n. Since communication within devices is parallel, conversion devices are required at the interface between the sender and the line (parallel-to-serial) and between the line and the receiver (serial-to-parallel). Serial transmission occurs in one of three ways: asynchronous, synchronous, and isochronous.

Asynchronous transmission

Asynchronous transmission is so named because the timing of a signal is unimportant. Instead, information is received and translated by agreed upon patterns. As long as those patterns are followed, the receiving device can retrieve the information without regard to the rhythm in which it is sent. Patterns are based on grouping the bit stream into bytes. Each group, usually 8 bits, is sent along the link as a unit. The sending system handles each group independently, relaying it to the link whenever ready, without regard to a timer.

Without synchronization, the receiver cannot use timing to predict when the next group will arrive. To alert the receiver to the arrival of a new group, therefore, an extra bit is added to the beginning of each byte. This bit, usually a 0, is called the start bit. To let the receiver know that the byte is finished, 1 or more additional bits are appended to the end of the byte. These bits, usually 1 s, are called stop bits. By this method, each byte is increased in size to at least 10 bits, of which 8 bits is information and 2 bits or more are signals to the receiver. In addition, the transmission of each byte may then be followed by a gap of varying duration. This gap can be represented either by an idle channel or by a stream of additional stop bits.

The start and stop bits and the gap alert the receiver to the beginning and end of each byte and allow it to synchronize with the data stream. This mechanism is called asynchronous because, at the byte level, the sender and receiver do not have to be synchronized. But within each byte, the receiver must still be synchronized with the incoming bit stream. That is, some synchronization is required, but only for the duration of a single byte. The receiving device resynchronizes at the onset of each new byte. When the receiver detects a start bit, it sets a timer and begins counting bits as they come in. After n bits, the receiver looks for a stop bit. As soon as it detects the stop bit, it waits until it detects the next start bit.

In this example, the start bits are as, the stop bits are 1s, and the gap is represented by an idle line rather than by additional stop bits.

The addition of stop and start bits and the insertion of gaps into the bit stream make asynchronous transmission slower than forms of transmission that can operate without the addition of control information. But it is cheap and effective, two advantages that make it an attractive choice for situations such as low-speed communication. For example, the connection of a keyboard to a computer is a natural application for asynchronous transmission. A user types only one character at a time, types extremely slowly in data processing terms, and leaves unpredictable gaps of time between each character.

Synchronous transmission

In synchronous transmission, the bit stream is combined into longer "frames," which may contain multiple bytes. Each byte, however, is introduced onto the transmission link without a gap between it and the next one. It is left to the receiver to separate the bit stream into bytes for decoding purposes. In other words, data are transmitted as an unbroken string of 1s and Os, and the receiver separates that string into the bytes, or characters, it needs to reconstruct the information.

In the below figure, we have drawn in the divisions between bytes. In reality, those divisions do not exist; the sender puts its data onto the line as one long string. If

the sender wishes to send data in separate bursts, the gaps between bursts must be filled with a special sequence of Os and 1s that means idle. The receiver counts the bits as they arrive and groups them in 8-bit units.

Without gaps and start and stop bits, there is no built-in mechanism to help the receiving device adjust its bit synchronization midstream. Timing becomes very important, therefore, because the accuracy of the received information is completely dependent on the ability of the receiving device to keep an accurate count of the bits as they come in.

The advantage of synchronous transmission is speed. With no extra bits or gaps to introduce at the sending end and remove at the receiving end, and, by extension, with fewer bits to move across the link, synchronous transmission is faster than asynchronous transmission. For this reason, it is more useful for high-speed applications such as the transmission of data from one computer to another. Byte synchronization is accomplished in the data link layer.

Isochronous Transmission

In real-time audio and video, in which uneven delays between frames are not acceptable, synchronous transmission fails. For example, TV images are broadcast at the rate of 30 images per second; they must be viewed at the same rate. If each image is sent by using one or more frames, there should be no delays between frames. For this type of application, synchronization between characters is not enough; the entire stream of bits must be synchronized. The isochronous transmission guarantees that the data arrive at a fixed rate.

Parallel transmission

Binary data, consisting of 1s and Os, may be organized into groups of n bits each. Computers produce and consume data in groups of bits much as we conceive of and use spoken language in the form of words rather than letters. By grouping, we can send data n bits at a time instead of 1. This is called parallel transmission.

The mechanism for parallel transmission is a conceptually simple one: Use n wires to send n bits at one time. That way each bit has its own wire, and all n bits of one group can be transmitted with each clock tick from one device to another.

The figure shows how parallel transmission works for n=8.Here 8 different bits have 8 different transmission lines. So all the 8 bits can be transmitted at the same time ie in a single clock pulse. Typically, the eight wires are bundled in a cable with a connector at each end.

The advantage of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor of n over serial transmission. But there is a significant disadvantage: cost. Parallel transmission requires n communication lines (wires in the example) just to transmit the data stream. Because this is expensive, parallel transmission is usually limited to short distances.

Applications

Transmission modes

Communication between two devices can be simplex, half-duplex, or full-duplex.

SimplexIn simplex mode, the communication is unidirectional,

as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive.

Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.

Half-DuplexIn half-duplex mode, each station can both transmit and

receive, but not at the same time. : When one device is sending, the other can only receive, and vice versa. The half-duplex mode is like a one-lane road with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems.

The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction.

Full-DuplexIn full-duplex mode (also called duplex), both stations

can transmit and receive simultaneously.The full-duplex mode is like a two way street with

traffic flowing in both directions at the same time. In full-duplex mode, signals going in one direction share the capacity of the link: with signals going in the other direction. This sharing can occur in two ways: Either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions.

One common example of full-duplex communication is the telephone network. When two people are communicating by a telephone line, both can talk and listen at the same time. The full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions.

Noise

Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise.

Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. Distortion means that the signal changes its form or shape. Distortion can occur in a composite signal made of different frequencies. Each signal component has its own propagation speed through a medium and, therefore, its own delay in arriving at the final destination. Differences in delay may create a difference in phase if the delay is not exactly the same as the period duration.

Noise may be defined as an unwanted amount of energy which tends to interfere with the proper reception and reproduction of transmitted signal. At the receiver the signals are reproduced in an unwanted form. In the radio receiver noise may produce ‘hiss’ type of sound in the output of the loud speaker. In the TV receiver it disturbs the picture output.

Noise is divided into two broad categories as: External and Internal Noise, based on the source of the noise is external or internal to the communication system.

External Noise

It is the type of noise which is generated external to the communication system. External noise cannot be analyzed quantitatively. Also it cannot be controlled as is the property of the geographical area. Major noises under external type are:

Atmospheric ExtraterrestrialIndustrial

Atmospheric Noise (or static noise)It is produced by lightning discharges in thunderstorm

and other electrical disturbances which occur in the atmosphere. The electric impulses are random in the nature, and so this energy is spread over the complete frequency spectrum used for radio communication. Atmospheric noise

contains radio signals which are distributed over a wide range of freq. So at any point the receiving antenna picks up not only the required signal but also the static from all the thunder storm as they are propagating like the required signals. The field strength of atmospheric noise varies inversely with the frequency. So noise produced is small in VHF and UHF bands. Atmospheric noise become less over 30 MHz.

Extraterrestrial NoiseHere the source of the noise is extraterrestrial. It

may be also divided into 2 depending on their source as:SolarCosmic

Solar noise is the electrical noise emitting from the sun. The sun is a big body at an extremely high temperature and it radiates energy in the form of noise over a very wide frequency spectrum which is occupied by radio communication. This is the noise due to sun in steady condition. But the condition of sun varies in some special days. In those days, electrical noise is produced that is in addition to the quiet noise from the sun. Although this additional noise is from a small portion of the sun, its effect is larger than that by the normal noise.

Cosmic noise is the noise due to the extraterrestrial bodies other than sun. Distant stars have high temperature, and can radiate noise as sun. The noise may also be received from our own galaxy and other virtual points such as quasars and pulsars. Though they emit high energy, they are far away from the earth, so their effect is diminished.

Industrial noiseIt is the man made noise which is produced by such

sources as automobile and aircraft ignition, electrical motors, leakage from high voltage transmission and other heavy electrical equipments. It is most intensive in industrial areas and dense populated urban areas. Industrial noise is in such areas is stronger than all other noises in the frequency range 1MHz to 600 MHz.

Internal Noise

Internal noise is that noise which is generated within the communication system or receiver. It can be treated

quantitatively and can be reduced by proper design. The internal noise present in a system of bandwidth B is same at any frequency in the frequency spectrum. Internal noise may be classified as :

Thermal NoiseIt also known as white noise or Johnson noise. It is

generated in a resister or the resistive component due to rapid and random motion of molecules, atoms and electrons. The temperature of the body denotes its kinetic energy and KE become zero at absolute zero. The noise power produced in a resistor is proportional to absolute temperature and the bandwidth over which the noise is measured. Therefore the expression for maximum noise power output of a resistor is given as Pn = kTB.

If an ordinary resistor at standard temperature ie 17oC is not connected to any voltage source, an electronic voltmeter can show some voltage across it. It is caused by random movement of electrons with in the resistor which constitutes a current. The rate of arrival of electrons at either end of the resistor varies randomly, so does the potential difference between the two ends. The rms value of noise voltage of the resistor is √4kTBR where T is the temperature, B the bandwidth, R the resistance.

Shot noiseThe most important of all other source of noise is shot

effect which leads to a shot noise in all amplifying device and virtually all active devices. It is caused by random variation in the arrival of electrons at the output electrode of amplifying devices and appears as a randomly varying noise current superimposed on the output. When amplified it makes a sound as a shower of lead shot were falling on a metal sheet.

In bipolar transistors, it is mainly a result of random drift of the discrete current carriers, across the junction. Shot noise behaves in a similar manner as thermal noise, apart from the fact that it has a different source.

For diode, the shot noise current is given by

in = √2eipB where,

in is the noise currente is the charge of electron ip is the direct diode currentB is the bandwidth of the system.

Transit-time noiseThe time taken by the electron to travel from emitter

to collector is the transit time. If this delay becomes significant to the time period of the signal, the noise input admittance of the transistor increases. The minute current induced in the input of the device by random fluctuation in the output current become of great importance at such frequencies and creates a random noise. As this noise more for high frequency it is also called High Frequency Noise.

Flicker noiseIn semiconductor the flicker noise is generated because

of the fluctuation in the carrier density. Fluctuation in the carrier density causes the fluctuation in the conductivity of the material. The power of the fluctuation noise is inversely proportional to frequency. So theses noise is dominant at low frequencies. It may be completely avoided above 500 kHz.

Resistance noiseThe Resistance noise in the transistor is mainly due to

base, emitter, and collector internal resistance. In most of the cases the main reason is the base resistance.

Signal to noise RatioIt is defined as the ratio of the signal power to noise

power. It is used to compare the noise and signal power at the same point to ensure that noise is not excessive.

S / n = signal power / noise power = (Vs

2/R) / (Vn2/R)

= Vs2 / Vn

2

At any point a high value of S/N is required.

Noise figureNoise figure or noise factor is the ratio of S/N

supplied to the input terminals of the receiver to the S/N supplied to the output terminal.

F = (input S/N) / (output S/N)A practical generate some noise and S/N ration

deteriorates as one move towards the output. The ideal value of noise figure is 1 which indicates that no noise is created its own. But practical receivers will create a noise

on its own and noise figure exceeds 1. The value of noise figure is used in the comparison of two kinds of equipments in evaluating their performance.

Switching

Switching is the technique to connect devices to make a one-to-one connection when we have multiple devices, using a switched network. The other methods like a point to point connection or using a central master slave connection are impractical and wasteful when using in a large network.

A switched network consists of a series of interlinked nodes, called switches. Switches are devices capable of creating temporary connections between two or more devices linked to the switch. In a switched network, some of these nodes are connected to the end systems (computers or telephones, for example). Others are used only for routing.

Here the end systems are connected thru switches only. Only when the switches are ON such that there is a path between any 2 devices, they can communicate ie the witches makes a temporary connection.

Traditionally, three methods of switching have been important: circuit switching, packet switching, and message switching.

Circuit Switching A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link.

Here different terminals are connected to one another thru the switches in between. By moving the levers of the switches any phone on the left can be connected to one on the right. Before starting communication, the stations must make a reservation for the resources to be used during the communication. These resources, such as channels (bandwidth in FDM and time slots in TDM), switch buffers, switch processing time, and switch input/output ports, must remain dedicated during the entire duration of data transfer until the teardown phase. Data transferred between the two stations are not packetized. The data are a continuous flow sent by the source station and received by the destination station, although there may be periods of silence.

Circuit switching today can use either of two technologies: the space-division switch or the time-division switch.

Space division switchIn space-division switching, the paths in the circuit

are separated from one another spatially. This technology was originally designed for use in analog networks but is used currently in both analog and digital networks. It has evolved through a long history of many designs. Crossbar switch

A crossbar switch connects n inputs to m outputs in a grid, using electronic microswitches (transistors) at each crosspoint. The major limitation of this design is the number of crosspoints required. To connect n inputs to m outputs using a crossbar switch requires n x m crosspoints. For example, to connect 1000 inputs to 1000 outputs requires a switch with 1,000,000 crosspoints. A crossbar with this number of crosspoints is impractical. Such a switch is also inefficient because statistics show that, in practice, fewer than 25 percent of the crosspoints are in use at any given time. The rest are idle.

Multistage switchThe solution to the limitations of the crossbar switch

is the multistage switch, which combines crossbar switches in several (normally three) stages. In a single crossbar switch, only one row or column (one path) is active for any connection. So we need N x N crosspoints. If we can allow multiple paths inside the switch, we can decrease the number of crosspoints. Each crosspoint in the middle stage can be

accessed by multiple crosspoints in the first or third stage.

The design of multistage switch depends on number of stages and number of switches in each stage. Normally middle stage has fewer switches. In the below figure, there are 3 stages to connect between 15 inputs and 15 outputs.

Each of the first stage switches have an output to each of the intermediate switches ie 2 outputs. Each 3rd stage switches have inputs from each middle stage ie 2 inputs each. Each middle stages switches must have inputs from each 1st stage switches and output to each 3rd stage switches.

If we are using a crossbar switch to implement a 15X15 line, 225 crosspoints are needed. Here in multistage switch the crosspoints required are:

Three first stages each have to make 2 output from 5 inputs ie 10 (5 x 2) cross pints. So total 30 crosspoints.

Two middle stage switches each have to make 3 outputs from 3 inputs ie 9 (3 X 3) crosspoints. So total 18 crosspoints.

Three 3rd stage switches each have to make 5 outputs from 2 inputs ie 10 (2 X 5) crosspoints. So total 30 crosspoints.

So total number of crosspoints needed is 78. It means multistage switch reduces the number of crosspoints required.

Multistage switch provide several options for connecting each pair of linked devices. The above figure

shows a path from input line 4 to output line 8 thru switches 1, 2, 2 of each stage respectively. Some other path is also possible say thru the switches 1, 1, 2 to reach line 8 from line 4.

The design of multistage switch has a drawback that it results in blocking during heavy traffic. It is the condition when an input cannot be connected to output because no path is available at that moment. All the paths are occupied. In crossbar switch (single stage switch), blocking will not occur because for every pair of input and output devices a dedicated path is available. As the number of stages increases possible blocking increases.

Time division switchIt uses a time division MUXing to achieve switching.

There are 2 popular approaches:

Time slot interchangeHere the common time division MUXing is employed. But

here the data are output on the same order as they are sent. So a time slot interchange device is added between the sender and the receiver. It changes the order of the frames sent to the desired sequence.

Here the TSI reorder the frames sent by 4 senders. If TSI was not there, A will go to terminal 1, B to terminal 2, and so on as in normal TDM. TSI is the device to make the proper order. It consists of a RAM and a control unit. The RAM fills up with the incoming data from time slots in the order received. But the frames are sent out in an order on the decision of the control unit.

TDM BusHere the input and output line are connected using a

high speed bus thru switches. Each input switch is closed

during one of the 4 time slots. At that time only one of the output switches is opened according the decision of the control unit.

A combination of the space division and time division switching also possible. It takes the advantages of both the systems. Advantage of space division switching is that it is instantaneous. But its large requirement of crosspoint is not a good feature. Time division switching need not any crosspoints, but the processing each connections creates delay. Multistage switches of this sort can be designed as time-space-time (TST), time-space-space-time (TSST) space-time-time-space (STTS) etc.

Application :1) Telephone network :

During the last decade, the telephone network has undergone many technical changes. The network is now digital as well as analog.

The telephone network is made of three major components: local loops, trunks, and switching offices. The telephone network has several levels of switching offices such as end offices, tandem offices, and regional offices.

Local LoopsOne component of the telephone network is the local

loop, a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office. The local loop, when used for voice, has a bandwidth of 4000 Hz (4 kHz). It is interesting to examine the telephone number associated with each local loop. The first three digits of a local telephone number define the office, and the next four digits define the local loop number.

TrunksTrunks are transmission media that handle the

communication between offices. A trunk normally handles hundreds or thousands of connections through multiplexing. Transmission is usually through optical fibers or satellite links.

Switching OfficesTo avoid having a permanent physical link between any

two subscribers, the telephone company has switches located in a switching office. A switch connects several local loops or trunks and allows a connection between different subscribers.

Accessing the switching station at the end office is accomplished thru dialing. In rotary or pulse dialing digital signal was sent to the end office for each number dialed. Today tone dialing is used in which the user sends two small burst of analogue signals called dual tone. The frequency of the signal sent is based on the number dialed. So from the freq of the sent signals, the dialed number can be identified.

Packet switchingIn packet switching data are transmitted in discrete

units called packets. The packet contains not only data but also a header with control information. The packets are sent over the network node to node.

At the source the data is divided into packets using the packetizer. Then the address of the sender and the receiver is added. Then it makes sure that the packet is of proper size. If it is too large, it is fragmented in a fragmentation module. At any intermediate node, ie a switch is responsible for routing the packet. At the destination, it verifies the destination address of the packet. It also checks whether the packet is corrupted or not. If the packet is a fragment, then a reassembling is done at the destination.

There are two popular approaches for packet switching. Datagram approachVirtual circuit approach

Datagram approachHere each packet is treated independently of others and

each packet is routed independently. A packet in this approach is called datagram.

Here 4 packets are sent from A to X. All the 4 packets are belonging to the same message but is taking different

paths to reach the destination X. But here there is a chance for the packets to reach the destination out of order. It is responsibility of the upper layer to make it in the proper order.

In datagram approach the decision of the routing is done at the packet switch only and no call setup is done at the beginning. Each of the switches maintains a routing table. The destination addresses and the corresponding forwarding output ports are recorded in the tables. The routing tables are dynamic and are updated periodically. Every packet in a datagram network carries a header that contains, among other information, the destination address of the packet. When the switch receives the packet, this destination address is examined; the routing table is consulted to find the corresponding port through which the packet should be forwarded.

Datagram approach has the advantage that it does not need any call setup at the beginning. The datagram networks are sometimes referred to as connectionless networks ie each packet is treated independent; there is no relationship between the packets of the same message while transmitting. Also no connection is established between the sender and receiver at the beginning. Internet chooses this type of data transmission because internet is made up of a number of heterogeneous networks, and it is almost impossible to create a connection from source to destination without knowing the nature of the network.

Virtual circuit approachIn virtual ckt approach, the relationship between the

packets of a message is preserved. All the packets of the same message are routed in the same path. For that a path is crested at the beginning. Wide area network uses this approach.

In a virtual-circuit network, two types of addressing are involved: global and local. A source or a destination needs to have a global address-an address that can be unique in the scope of the network or internationally if the network is part of an international network. Local address (Virtual-Circuit Identifier) is the identifier that is actually used for data transfer is called the virtual-circuit identifier A VEI, unlike a global address, is a small number that has only switch scope; it is used by a frame between two switches. When a frame arrives at a switch, it has a VCI; when it leaves, it has a different VCI. A VCI does not need to be a large number since each switch can use its own unique set of VCIs.

The data communication using virtual ckt approach is accomplished thru 3 phases. A setup phase to create the path, a data transfer phase where the data transmission is occurred. A teardown phase where the path created is deleted after the communication is completed.

Setup phase:In the setup phase, a switch creates an entry for a

virtual circuit. Two steps are required: the setup request and the acknowledgment. Each switch maintains a table, the switching table. It stores the details of the incoming and outgoing port and VCI for the packets. Its structure is :

Incoming OutgoingPort VCI Port VCI

1 14 3 222 33 3 16

Consider the first raw. Here the values indicate that, if a packet with VCI 14 is arriving at the 1st port, then it

will be directed out thru the 3rd port with an outgoing VCI 22. This switching table is for that switch only. The outgoing VCI of this switch is the incoming VCI of the next switch connected thru that outgoing port.

Setup request :a. Source A sends a setup frame to switch 1.b. Switch 1 receives the setup request frame. It

knows that a frame going from A to B goes out through port 3. The switch, in the setup phase, acts as a packet switch; it has a routing table which is different from the switching table to give information about the port. The switch creates an entry for the virtual ckt in the switching table. The switch assigns the incoming port (1) and chooses an available incoming VCI (14) and the outgoing port (3). It does not yet know the outgoing VCI, which will be found during the acknowledgment step. The switch then forwards the frame through port 3 to switch 2.

c. Switch 2 receives the setup request frame. The same events happen here as at switch 1; three columns of the table are completed: in this case, incoming port (l), incoming VCI (66), and outgoing port (2).

d. Switch 3 receives the setup request frame. Again, three columns are completed: incoming port (2), incoming VCI (22), and outgoing port (3).

e. Destination B receives the setup frame, and if it is ready to receive frames from A, it assigns a VCI to the incoming frames that come from A, in this case 77. This VCI lets the destination know that the frames come from A, and not other sources.

Acknowledgement : A special frame, called the acknowledgment frame,

completes the entries in the switching tables.a. The destination sends an acknowledgment to

switch 3. The acknowledgment carries the global source and destination addresses so the switch knows which entry in the table is to be completed. The frame also carries VCI 77, chosen by the destination as the incoming VCI for frames from A. Switch 3 uses this VCI to complete the outgoing VCI column for this entry. Note that 77 is the incoming VCI for destination B, but outgoing VCI for switch3.

b. Switch 3 sends an acknowledgment to switch 2 that contains its incoming VCI in the table, chosen in the previous step. Switch 2 uses this as the outgoing VCI in the table.

c. Switch 2 sends an acknowledgment to switch 1 that contains its incoming VCI in the table, chosen in the previous step. Switch 1 uses this as the outgoing VCI in the table.

d. Finally switch 1 sends an acknowledgment to source A that contains its incoming VCI in the table, chosen in the previous step.

e. The source uses this as the outgoing VCI for the data frames to be sent to destination B.

Data transfer phase :

To transfer a frame from a source to its destination, all switches need to have a table entry for this virtual circuit. When a packet is arrived at a particular port with a particular VCI, the switch searches the switching table for the entry with that port and VCI and gets the outgoing port and outgoing VCI. Then the packet is assigned the new VCI and is sent out thru the outgoing port. The data transfer phase is active until the source sends all its frames to the destination. The procedure at the switch is the same for each frame of a message. The process creates a virtual circuit, not a real circuit, between the source and destination.

Teardown phase : In this phase, source A, after sending all frames to B,

sends a special frame called a teardown request. Destination B responds with a teardown confirmation frame. All switches delete the corresponding entry from their tables.

Application :1) WAN (Virtual ckt approach)2) Internet (datagram approach)

Internet chooses the connectionless approach Of packet switching. It uses the universal addresses defined in the network layer to route packets from the source to the destination. The identifier used to uniquely identify the devices connected to the internet is internet protocol address or IP address. In the current version it is a 32 bit address. IP address is unique and if a system has 2 connections to the internet via 2 networks, it has 2 IP addresses.

Packet sizeOne important design issue is the packet size to be

used in the network. There is a significant relationship between packet size and transmission time. Consider a transmission of packet is from the node A to B thru the nodes X, Y. Let the size of the message is 30 bits. Suppose the msg is sent as a single packet of size 33 bits ie 30 bits data and 3 bits for header. Then the packet first moves from A to X, then from X to Y, then from Y to B. the total time taken will be 99 bit time( 33 X 3). Suppose now that we break up the message into two packets, each containing 15 bits of the message and, of course, 3 bits each of header or control information. In this case, node X can begin

transmitting the first packet as soon as it has arrived from A, without waiting for the second packet. Because of this overlap in transmission, the total transmission time drops to 72 bit-times. By breaking the message up into 5 packets, each intermediate node can begin transmission even sooner and the savings in time is greater, with a total of 63 octet-times.

On dividing the message into packets, the amount of retransmission on any error can be reduced. When we are sending a 30 bit msg as single packet, and there happens an error in any bi, then we have to retransmit that packet ie 33 bits. But if the msg is divided to 2 packets of 15 bits, then on any error in a packet, we have to retransmit only that packet ie 18 bits.

But on increasing the number of packets, the number of headers will be also increased. When we are sending a 30 bit msg as single packet, only one header is needed of size 3 bits. SO the total amount of data to be sent is 33 bits. If we are dividing it to 2 packets of 15 bits then 2 headers are needed 1 for each of 3 bits. So the total amount of data to be sent will be, 15+3+15+3 = 36. Delay

The delays that are related to switching are :

Propagation delay: It is the time it takes a signal to propagate from one

node to the next. This time is generally negligible. The speed of electromagnetic signals through a wire medium, for example, is typically 2 X 108 m/s.

Transmission time: The time it takes for a transmitter to send out a block

of data. For example, it takes 1 s to transmit a 10,000-bit block of data onto a 10-kbps line.

Node delay: The time it takes for a node to perform the necessary

processing as it switches data.

For circuit switching, there is a certain amount of delay before the message can be sent. First, a call request signal is sent through the network in order to set up a connection to the destination. If the destination station is not busy, a call-accepted signal returns. A processing delay is incurred at each node during the call request; this time is spent at each node setting up the route of the connection. On the return, this processing is not needed

because the connection is already set up; once it is set up, the message is sent as a single block, with no noticeable delay at the switching nodes.

Virtual-circuit packet switching appears quite similar to circuit switching. A virtual circuit is requested using a call-request packet, which incurs a delay at each node. The virtual circuit is accepted with a call-accept packet. In contrast to the circuit-switching case, the call acceptance also experiences node delays, even though the virtual circuit route is now established the reason is that this packet is queued at each node and must wait its turn for retransmission. Once the virtual circuit is established, the message is transmitted in packets. It should be clear that this phase of the operation can be no faster than circuit switching, for comparable networks; this is because circuit switching is an essentially transparent process, providing a constant data rate across the network. Packet switching involves some delay at each node in the path; worse, this delay is variable and will increase with increased load.

Datagram packet switching does not require a call setup. Thus, for short messages, it will be faster than virtual-circuit packet switching and perhaps circuit switching. However, because each individual datagram is routed independently, the processing for each datagram at each node may be longer than for virtual-circuit packets. Thus, for long messages, the virtual-circuit technique may be superior.

Comparison of circuit and packet switching techniques

Circuit switching Datagram packet switching Virtual-circuit

packet switching

Dedicated transmission path No dedicated path No dedicated path

Continuous transmission ofdata

Transmission of packets

Transmission of packets

Messages are not stored

Packets may be stored untildelivered

Packets stored untildelivered

The path is established for

Route established for each

Route established for entire

entire conversation packet conversation

Call setup delay: negligibletransmission delay

Packet transmission delay

Call setup delay; packettransmission delay

Busy signal if called partybusy

Sender may be notified ifpacket not delivered

Sender notified ofconnection denial

Usually no speed or codeconversion

Speed and codeconversion

Speed and codeconversion

Fixed bandwidthtransmission

Dynamic use of bandwidth

Dynamic use of bandwidth

No overhead bits after callsetup

Overhead bits in eachpacket

Overhead bits in eachpacket

Message switchingIn computer communication transmission generally occurs

in relatively short bursts separated by long time of no transmission. Typically a line connecting 2 computers may be in use only 1 percent of the total time allocated. It is uneconomical when the users must be charged for entire time of allocation.

Message switching offers the probability of greatly improved economy. Consider a connection between nodes A and D as :

Here the communication between A and D is represented by a series of links AB, BC, and CD. In ckt switching there is no communication until complete connection is made

between A and D. In message switching no such complete connection at any time is needed. Instead A can send the message to B even if any of the links is not available, and it can store the message there. The message is kept there until BC is not required for some prior transmission. When BC became available, the msg at B will be transmitted to C. Again after storage and possible delay to wait for available open time, the msg will go from C to D.

The system has a disadvantage that there is a delay between the time of transmission and time of reception. But if using the msg switching, the long distance, fast, expensive lines between computers will be idle for a short time.

When a line BC is not transmitting any data from A to D, then this line can be used to relay a msg between any other computers. Generally in total transmission from A to D only one of the lines will be released to handle messages between other computers. As message switching involves storing the data and forwarding subsequently it is also termed as store and forward switching.

Message switching technique is applied in E Mail and telex transmission. In fact most E Mail message is carried using message switching, with the links between message switches provided by packet or circuit switched network.