2.a.Q.What Is Switching? Ex-plainthe three Switching techniques with diagrams.Ans. A network is a set of connected devices. Whenever we have multiple devices, wehave the problem of how to connect them to make one-to-one communication possible.One solution is to make connection by meshtopology or startopology etc.These methods,however, are impractical and wasteful when applied to very large networks.The numberand length of the links require too much infrastructure to be cost-efficient.A bettersolutionis switching. 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 areconnected to the end systems (computers or telephones, for example).Others are usedonly for routing.Example:

We can divide today's networks into three broad categories: circuit-switched networks,packet-switched networks, and message-switched. Packet-switched networks can further be divided into two subcategories-virtual-circuit networks and datagram networks as shown below

CIRCUIT-SWITCHED NETWORKS : 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. Each link is normally divided into n channels by using FDM or TDM.In circuit switching, the resources need to be reserved during the setup phase; the esources remain dedicated for the entire duration of data transfer until the teardown phase.Three PhasesThe actual communication in a circuit-switched network requires three phases: connectionsetup, data transfer, and connection teardown.>Setup PhaseBefore the two parties (or multiple parties in a conference call) can communicate, adedicated circuit (combination of channels in links) needs to be established. The end systemsare normally connected through dedicated lines to the switches, so connection setupmeans creating dedicated channels between the switches.>Data Transfer PhaseAfter the establishment of the dedicated circuit (channels), the two parties can transfer data.>Teardown PhaseWhen one of the parties needs to disconnect, a signal is sent to each switch to release the resources.

DATAGRAM NETWORKS:In data communications, we need to send messages from one end system to another. Ifthe message is going to pass through a packet-switched network, it needs to be dividedinto packets of fixed or variable size. The size of the packet is determined by the networkand the governing protocol.In packet switching, there is no resource allocation for a packet. This means thatthere is no reserved bandwidth on the links, and there is no scheduled processing timefor each packet. Resources are allocated on demand. The allocation is done on a firstcome,first-served basis. When a switch receives a packet, no matter what is the sourceor destination, the packet must wait if there are other packets being processed. As withother systems in our daily life, this lack of reservation may create delay. For example, ifwe do not have a reservation at a restaurant, we might have to wait. __________________________

-------------------------------------VIRTUAL-CIRCUIT NETWORKS: A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both.1. As in a circuit-switched network, there are setup and teardown phases in additionto the data transfer phase.2. Resources can be allocated during the setup phase, as in a circuit-switched network,or on demand, as in a datagram network.3. As in a datagram network, data are packetized and each packet carries an address in the header. However, the address in the header has local jurisdiction (it defines what should be the next switch and the channel on which the packet is being canied), not end-to-end jurisdiction. The reader may ask how the intermediate switches knowwhere to send the packet if there is no final destination address carried by a packet.The answer will be clear when we discuss virtual-circuit identifiers in the next section.4. As in a circuit-switched network, all packets follow the same path established duringthe connection.5. A virtual-circuit network is normally implemented in the data link layer, while a circuit-switched network is implemented in the physical layer and a datagram network in the network layer. But this may change in the future.

2.b.Q. Explain the Token ring network ( IEEE 802·5 ) & FDDI.Ans: Token Ring: In the token-passing method, the stations in a network are organized in a logical ring.In other words, for each station, there is a predecessor and a successor. The predecessor is the station which is logically before the station in the ring; the successor is the station which is after the station in the ring. The current station is the one that is accessing the channel now. The right to this access has been passed from the predecessor to the current station. The right will be passed to the successor when the current station has no more data to send.But how is the right to access the channel passed from one station to another? In this method, a special packet called a token circulates through the ring. The possession of the token gives the station the right to access the channel and send its data. When a station has some data to send, it waits until it receives the token from its predecessor.It then holds the token and sends its data. When the station has no more data to send, it releases the token, passing it to the next logical station in the ring. A Token Ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The Token Ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 Token Ring technology provides for data transfer rates of either 4 or 16 megabits per second. Very briefly, here is how it works:

1. Empty information frames are continuously circulated on the ring.2. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0

to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.

3. The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0.

4. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.

5. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.

The token scheme can also be used with bus topology LANs.The standard for the Token Ring protocol is Institute of Electrical and Electronics Engineers (IEEE) 802.5. The Fiber Distributed-Data Interface (FDDI) also uses a Token Ring protocol.

FDDI : Token management is needed for this access method. Stations must be limited in the time they can have possession of the token. The token must be monitored to ensure it has not been lost or destroyed. For example, if a station that is holding the token fails,the token will disappear from the network. Another function of token management is to assign priorities to the stations and to the types of data being transmitted. And finally, token management is needed to make low-priority stations release the token to highpriority stations.

In the physical ring topology, when a station sends the token to its successor, the token cannot be seen by other stations; the successor is the next one in line. This means that the token does not have to have the address of the next successor. The problem with this topology is that if one of the links-the medium between two adjacent stations fails , the whole system fails.The dual ring topology uses a second (auxiliary) ring which operates in the reverse direction compared with the main ring. The second ring is for emergencies only (such as a spare tire for

a car). If one of the links in the main ring fails, the system automatically combines the two rings to form a temporary ring. After the failed link is restored, the auxiliary ring becomes idle again. Note that for this topology to work, each station needs to have two transmitter ports and two receiver ports. The high-speed Token Ring networks called FDDI (Fiber Distributed Data Interface) and CDDI (Copper Distributed DataInterface) use this topology.

FDDI (Fiber Distributed Data Interface) is a set of ANSI and ISO standards for data transmission on fiber optic lines in a local area network (LAN) that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the Token Ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users. FDDI is frequently used on the backbone for a wide area network (WAN). The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles).

2.c.Q. What are the differences between unicast, multicast and broadcast addresses?Ans: Unicasting : In unicast communication, there is one source and one destination. The relationship between the source and the destination is one-to-one. In this type of communication, both the source and destination addresses, in the IP datagram, are the unicast addresses assigned to the hosts (or host interfaces, to be more exact). Note that in unicasting, when a router receives a packet, it forwards the packet through only one of its interfaces (the one belonging to the optimum path) as defined in the routing table. The router may discard the packet if it cannot find the destination address in its routing table.Multicasting: In multicast communication, there is one source and a group of destinations. The relationshipis one-to-many. In this type of communication, the source address is a unicast address, but the destination address is a group address, which defines one or more destinations.The group address identifies the members of the group.Broadcasting:In broadcast communication, the relationship between the source and the destination isone-to-all. There is only one source, but all the other hosts are the destinations. The Internet does not explicitly support broadcasting because of the huge amount of traffic it would create and because of the bandwidth it would need. Imagine the traffic generated in the Internet if one person wanted to send a message to everyone else connected to the Internet.Addressing : Each station on an Ethernet network (such as a PC, workstation, or printer) has its own network interface card (NIC). The NIC fits inside the station and provides the station with a 6-byte physical address. The Ethernet address is 6 bytes(48 bits), nonnally written in hexadecimal notation, with a colon between the bytes.06:01 :02:01:2C:4B6 bytes =12 hex digits =48 bitsUnicast, Multicast, and Broadcast: Addresses A source address is always a unicast address-the frame comes from only one station. The destination address, however, can be unicast, multicast, or broadcast.1>If the least significant bit of the first byte in a destination address is 0, the address is unicast; otherwise, it is multicast.2>A unicast destination address defines only one recipient; the relationship between the sender and the receiver is one-to-one. A multicast destination address defines a group of addresses; the relationship between the sender and the receivers is one-to-many.The broadcast address is a special case of the multicast address; the recipients are all the stations on the LAN. A broadcast destination address is forty-eight Is.

3.a.Q. According to Nyquist, what frequency is necessary to support a bit rate of 3 kbps using (i) one bit per signal component (i i) three bits per signal component?Ans:3kbps=3000bits/second.(i) one bit per signal component

3000bits/second=3000 signal component/secondFrequency = 3000hz=3khz

(i i) three bits per signal component3000bits/second=(3000/3) signal component/second=1000 signal component/secondFrequency = 1000hz=1khz

3.b.Q. Given the following information, find the maximum bandwidth for each signalsource:

(i) FDM multiplexing(ii) Total available bandwidth - 7900 Hz(iii) Three signal sources(iv) A 200-Hz guard band between each signal source.

Ans:{7900-(200X2)}Hz=7500Hz(7500/3)Hz=2500Hz

2500Hz 200Hz 2500Hz 200Hz 2500Hz

4.Q. Explain the following terms with examples: 3x5a) FSKb) AMc) PSK.

Ans: a)Frequency Shift Keying:In frequency shift keying, the frequency of the carrier signal is varied to represent data.The frequency of the modulated signal is constant for the duration of one signal element,but changes for the next signal element if the data element changes. Both peak amplitude and phase remain constant for all signal elements.Binary FSK (BFSK):One way to think about binary FSK (or BFSK) is to consider two carrier frequencies. InFigure, we have selected two carrier frequencies,f1 and f2. We use the first carrier if the data element is 0; we use the second if the data element is 1.Normally the carrierfrequencies are very high, and the difference between them is very small.

As Figure shows, the middle of one bandwidth is f1 and the middle of the other is f2. Both f1 and f2 are

apart from the midpoint between the two bands. The difference between the two frequencies is

Bandwidth for BFSK Figure also shows the bandwidth of FSK. Again the carrier signals are only simple sine waves, but the modulation creates a nonperiodic composite signal with continuous frequencies. We can think of FSK as two ASK signals, each with its own carrier frequency Cil or f2). If the difference between the two

frequencies is , then the required bandwidth is

B=(l+d)xS+

What should be the minimum value of ? In Figure, we have chosen a value greater than (l + d)S. It can be

shown that the minimum value should be at least S for the proper operation of modulation and demodulation.

b) Amplitude Shift Keying: In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements. Both frequency and phase remain constant while the amplitude changes.Binary ASK (BASK)Although we can have several levels (kinds) of signal elements, each with a different amplitude, ASK is normally implemented using only two levels. This is referred to as binary amplitude shift keying or on-off keying (OOK). The peak amplitude of one signallevel is 0; the other is the same as the amplitude of the carrier frequency. Figure gives a conceptual view of binary ASK.

Bandwidth for ASK Figure also shows the bandwidth for ASK. Although the carrier signal is only one simple sine wave, the process of modulation produces a nonperiodic composite signal. However, there is normally another factor involved, called d, which depends on the modulation and filtering process. The value of d is between 0 and 1. This means that the bandwidth can be expressed as shown, where S is the signal rate and the Bis the bandwidth.

B =(1 +d) x SThe formula shows that the required bandwidth has a minimum value of 5 and a maximum value of 25.

c) Phase Shift Keying:In phase shift keying, the phase of the carrier is varied to represent two or more different signal elements. Both peak amplitude and frequency remain constant as the phase changes. Today, PSK is more common than ASK or FSK. However, we will see that QAM, which combines ASK and PSK, is the dominant method of digital to analog modulation.Binary PSK (BPSK)The simplest PSK is binary PSK, in which we have only two signal elements, one with a phase of 0°, and the other with a phase of 180°. Figure 5.9 gives a conceptual view of PSK. Binary PSK is as simple as binary ASK with one big advantage-it is less susceptible to noise. In ASK, the criterion for bit detection is the amplitude of the signal;in PSK, it is the phase. Noise can change the amplitude easier than it can change the phase. In other words, PSK is less susceptible to noise than ASK. PSK is superior to FSK because we do not need two carrier signals.

Bandwidth Figure also shows the bandwidth for BPSK. The bandwidth is the same as that for binary ASK, but less than that for BFSK. No bandwidth is wasted for separating two carrier signals.Implementation The implementation of BPSK is as simple as that for ASK. The reason is that the signal element with phase 180° can be seen as the complement of the signal element with phase 0°. This gives us a clue on how to implement BPSK.

5.a.Q Explain with a suitable diagram,the working of OS1reference model. 9

Ans: Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection model. It was first introduced in the late 1970s. An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture.The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable.ISO is the organization. OSI is the model.

Seven layers of the OSI model:

Physical Layer:The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to Occur.The physical layer is also concerned with the following:

oThe physical layer defines the characteristics of the interface between the devices and the transmissionmedium. It also defines the type of transmission edium.

o Representation of bits. The physical layer data consists of a stream of bits (sequence of Os or 1s) with no interpretation. To be transmitted, bits must be encoded into signals--electrical or optical. The physical layer defines the type of encoding (how Os and I s are changed to signals).

o Data rate. The transmission rate-the number of bits sent each second-is also defined by the physical layer. In

other words, the physical layer defines the duration of a bit, which is how long it lasts.

o Synchronization of bits. The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.

o Line configuration. The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices.

o Physical topology. The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology (every device is connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device is on a common link), or a hybrid topology (this is a combination of two or more topologies).

o Transmission mode. The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex. In simplex mode, only one device can send; the other can only receive. The simplex mode is a one-way communication. In the half-duplex mode, two devices can send and receive, butnot at the same time. In a full-duplex (or simply duplex) mode, two devices can send and receive at the same time.___________________________________________________________________________________The physical layer is responsible for movements of individual bits from one hop (node) to the next.

Data Link LayerThe data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). Figure 2.6 shows the relationship of the data link layer to the network and physicallayers.Other responsibilities of the data link layer include the following:

Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.

Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

_______________________________________________________________________________________The data link layer is responsible for moving frames from one hop (node) to the next.

Network LayerThe network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination.If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source-to-destination delivery.Other responsibilities of the network layer include the following:

Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver. We discuss logical addresses later in this chapter.

Routing. When independent networks or links are connected to create intemetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

__________________________________________________________________________________________The network layer is responsible for the delivery of individual packets from the source host to the destination host.Transport LayerThe transport layer is responsible for process-to-process delivery of the entire message.A process is an application program running on a host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belonged to a separate message, whether or not it does. The transport layer,on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. Other responsibilities of the transport layer include the following:

o Service-point addressing. Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

o Segmentation and reassembly. A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

o Connection control. The transport layer can be either connectionless or connecti onoriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connecti onoriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred,the connection is terminated.

o Flow control. Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.

o Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission.__________________________________________________________________________________________The transport layer is responsible for the delivery of a message from one process to another.

Session LayerThe services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.Specific responsibilities of the session layer include the following:

o Dialog control. The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either halfduplex (one way at a time) or full-duplex (two ways at a time) mode.

o Synchronization. The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent.__________________________________________________________________________________________The session layer is responsible for dialog control and synchronization.

Presentation LayerThe presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. Specific responsibilities of the presentation layer include the following:

o Translation. The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The infonnation must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes thecommon format into its receiver-dependent format.

o Encryption. To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.

o Compression. Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.__________________________________________________________________________________________The presentation layer is responsible for translation, compression, and encryption.Application LayerThe application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services. Specific services provided by the application layer include the following:

o Network virtual terminal. A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user's computer talks to the software terminal which, in turn, talks to the host, and vice versa. The remote host believes it is communicating with one of its own terminals and allows the user to log on.

o File transfer, access, and management. This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.

o Mail services. This application provides the basis for e-mail forwarding and storage.

o Directory services. This application provides distributed database sources and access for global information about various objects and services.__________________________________________________________________________________________The application layer is responsible for providing services to the user.

5.b.Q What do you understand by the terms –i) LANii) MANiii) WAN?Give examples for each.Ans:Local Area NetworkA local area network (LAN) is usually privately owned and links the devices in a single office, building, or campus (see Figure 1.10). Depending on the needs of an organization and the type of technology used, a LAN can be as simple as two PCs and a printer in someone's home office; or it can extend throughout a company and include audio and video peripherals. Currently, LAN size is limited to a few kilometers. LANs are designed to allow resources to be shared between personal computers or workstations. The resources to be shared can include hardware (e.g., a printer), software (e.g., an application program), or data. A common example of a LAN, found in many business environments, links a workgroup of task-related computers, for example, engineering workstations or accounting PCs. Metropolitan Area NetworksA metropolitan area network (MAN) is a network with a size between a LAN and a WAN. It normally covers the area inside a town or a city. It is designed for customers who need a high-speed connectivity, normally to the Internet, and have endpoints spread over a city or part of city. A good example of a MAN is the part of the telephonecompany network that can provide a high-speed DSL line to the customer. Another example is the cable TV network that originally was designed for cable TV, but today can also be used for high-speed data connection to the Internet.Wide Area NetworkA wide area network (WAN) provides long-distance transmission of data, image, audio,and video information over large geographic areas that may comprise a country, a continent,or even the whole world. In Chapters 17 and 18 we discuss wide-area networks in greater detail. A WAN can be as complex as the backbones that connect the Internet or as simple as a dial-up line that connects a home computer to the Internet. We normally refer to the first as a

switched WAN and to the second as a point-to-point WAN. The switched WAN connects the end systems, which usually comprise a router (internetworking connecting device) that connects to another LAN or WAN. The point-to-point WAN is normally a line leased from a telephone or cable TV provider that connects a home computer or a small LAN to an Internet service provider (lSP). This type of WAN is often used to provide Internet access. An early example of a switched WAN is X.25, a network designed to provide connectivity between end users.

5.b.Q Describe the features of the following devices :a) Switchb) Gatewayc) Repeater.

Ans: a ) Switch : A network is a set of connected devices. Whenever we have multiple devices, wehave the problem of how to connect them to make one-to-one communication possible.One solution is to make connection by meshtopology or startopology etc.These methods,however, are impractical and wasteful when applied to very large networks.The numberand length of the links require too much infrastructure to be cost-efficient.A bettersolutionis switching. 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 areconnected to the end systems (computers or telephones, for example).Others are usedonly for routing.Example:

We can divide today's networks into three broad categories: circuit-switched networks,packet-switched networks, and message-switched. Packet-switched networks can further be divided into two subcategories-virtual-circuit networks and datagram networks as shown below

b)Gateway:Although some textbooks use the terms gateway and router interchangeably, most of the literature distinguishes between the two. A gateway is normally a computer that operates in all five layers of the Internet or seven layers of OSI model. A gateway takes an application message, reads it, and interprets it. This means that it can be used as a connecting device between two internetworks that use different models. For example,a network designed to use the OSI model can be connected to another network using the Internet model. The gateway connecting the two systems can take a frame as it arrives from the first system, move it up to the OSI applic-

ation layer, and remove the message. Gateways can provide security. The gateway is used to filter unwanted application-layer messages.

A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point can be either a gateway node or a host (end-point) node. Both the computers of Internet users and the computers that serve pages to users are host nodes. The computers that control traffic within your company's network or at your local Internet service provider (ISP) are gateway nodes. In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet.

c) Repeater:A repeater is a device that operates only in the physical layer. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates the original bit pattern. The repeater then sends the refreshed signal. A repeater can extend the physical length of a LAN. A repeater does not actually connect two LANs; it connects two segments of the same LAN. The segments connected are still part of one single LAN. A repeater is not a device that can connect two LANs of different protocols. A repeater can overcome the 10Base5 Ethernet length restriction. In this standard,the length of the cable is limited to 500 m. To extend this length, we divide the cable into segments and install repeaters between segments. Note that the whole network is still considered one LAN, but the portions of the network separated by repeaters are called segments. The repeater acts as a two-port node, but operates only in the physical layer. When it receives a frame from any of the ports, it regenerates and forwards it to the other port. A repeater forwards every frame; it has no filtering capability.An amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it. A repeater does not amplify the signal; it regenerates the signal. When it receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength.A repeater is a regenerator, not an amplifier.

6.Q Write short notes on the following:a) X.25 protocolb)CSMA CDc)Circuit switched and packet switched networks.

Ans: a) X . 25 protocol: Frame Relay is a virtual-circuit wide-area network that was designed in response to demands for a new type ofWAN in the late 1980s and early 1990s. Prior to Frame Relay, some organizations were using a virtual-circuit switching network called X.25 that performed switching at the network layer. For example,the Internet, which needs wide-area networks to carry its packets from one place to another, used X.25. And X.25 is still being used by the Internet, but it is being replaced by other WANs. However, X.25 has several drawbacks:a. X.25 has a low 64-kbps data rate. By the 1990s, there was a need for higherdata-rate WANs.b. X.25 has extensive flow and error control at both the data link layer and the network layer. This was so because X.25 was designed in the 1970s, when the available transmission media were more prone to errors. Flow and error control at both layers create a large overhead and slow down transmissions. X.25 requires acknowledgments for both data link layer frames and network layer packets that are sent between nodes and between source and destination. c. Originally X.25 was designed for private lise, not for the Internet. X.25 has its own network layer. This means that the user's data are encapsulated in the network layer packets of X.25. The Internet, however, has its own network layer, which means if the Internet wants to use X.25, the Internet must deliver its network layer packet, called a datagram, to X.25 for encapsulation in the X.25 packet. This doubles the overhead.

b)CSMA CD:__________________________________

To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed. The chance of collision can be reduced if a station senses the medium before trying to use it. Carrier sense multiple access (CSMA) requires that each station first listen to the medium (or check the state of the medium) before sending. In other words, CSMA is based on the principle "sense before transmit" or "listen before talk."CSMA can reduce the possibility of collision, but it cannot eliminate it. The possibility of collision still exists because of propagation delay.

-----------------------------------------------------The CSMA method does not specify the procedure following a collision. Carrier sense multiple access with collision detection (CSMA/CD) augments the algorithm to handle the collision.

(i) For CSMAlCD to work, we need a restriction on the frame size. Before sending the last bit of the frame, the sending station must detect a collision, if any, and abort the transmission.This is so because the station, once the entire frame is sent, does not keep a copy of the frame and does not monitor the line for collision detection. Therefore, the frame transmission time must be at least two times the maximum propagation time.

(ii) Carrier Sense Multiple Access/Collision Detect (CSMA/CD) is the protocol for carrier transmission access in Ethernet networks. On Ethernet, any device can try to send a frame at any time. Each device senses whether the line is idle and therefore available to be used. If it is, the device begins to transmit its first frame. If another device has tried to send at the same time, a collision is said to occur and the frames are discarded. Each device then waits a random amount of time and retries until successful in getting its transmission sent. CSMA/CD is specified in the IEEE 802.3 standard.