Master of Computer Application (MCA) Semester 3

NAME - KRUSHITHA.V.P

ROLL NO. - 520791371ASSIGNMENT SET 1SUBJECT - MC0075 COMPUTER NETWORKS

Master of Computer Application (MCA) Semester 3

MC0075 Computer Networks

Assignment Set 1

1. Describe the classification of Computer Networks giving suitable real time examples for each class.

ANS:

The term computer network is used to mean an interconnected collection of autonomous computers. Two computers are said to be interconnected, if they are able to exchange information. With a network, the user must explicitly log into a machine, explicitly submit jobs remotely, explicitly move files around and generally handle all network management personally.

The computer networks are classified depending on transmission technology and scale. These may be classified according to the network layer at which they operate according to some basis reference models that are considered to be standards in the industry such as the seven layer OSI reference model and the four layers TCP/IP model.The classifications are as follows:a) By transmission techniques:

Computer networks may be classified as broadcast or point to point networks.

b) By scale:

To the scale or extent or reach of the network, for example as a Personal area Network (PAN), Local area network (LAN), Campus area network (CAN), Metropolitan area network(MAN), or Wide area network (WAN).

c) By connection method:

Computer networks may be classified according to the technology that is used to connect the individual devices in the network such Home PNA Power line communication, Ethernet or Wireless LAN.

d) By functional relationship:

Computer networks may be classified according to the functional relationships which exist between the elements of the network, for ex., Active Networking, Client-Server and Peer to peer (workgroup) architectures. e) By network topology:

Computer networks may be classified according to the network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network, etc. f) By services provided;

Computer networks may be classified according to the services which they provide, such as Storage area networks, Server farms, Process control networks, Value-added network, Wireless community network, etc. g) By Protocol:

Computer networks may be classified according to the communications protocol that is being used on the network.

The most common way of classifications is by transmission techniques, by scale and by the way the computers are connected.

Based on transmission technology:

Broadcast networks have a single communication channel that is shared by all the users on the network. Short messages are commonly called as packets or frames . The user on the network sends packets. All other machines receive these packets. An address field within the packet or frame specifies the address of the destination machine. So upon receiving the packet, all machines check the address field. Only intended user uses or processes the pocket or frame and others neglect and discard it.

Ex:- When a teacher puts a question to a student in the a class of about 50 students, all the students will hear the question but will not answer as the question is intended to that particular student only. Hence only that student will analyze the question and others will not respond.

Broadcast system generally allows the possibility of addressing a packet to all the destinations by using a special code in the address field. When this code is transmitted, it is received and processed by every machine on the network. This mode of operation is referred as Broadcasting. Some broadcasting systems also support transmission to a subset of the users, which is a group of users. This mode is called as multicasting..

In contrast, the point-to-point network consists of many connections between individual pairs of machines. A packet to be sent from sources to destination may have to first visit one or more intermediate machines. Usually, different routes of different length are possible. So finding the best path or route is important in point to point networks. This type of transmission with one sender and one receiver is also referred to as unicasting.Based on their scale:

We classify multiple processor system based on physical size.

Interprocessor DistanceProcessors located in same Example

1 mSquare MeterPersonal Area Network

10 mRoomLAN

100 mBuildingLAN

1 kmCampusLAN

10 kmCityMetropolitan Area Network

100 KmCountryWAN

1000 KmContinentWAN

10000 KmPlanetThe Internet

Local Area Network

These are generally called LANs. They are privately owned networks with a single building or campus of up to few kilometers in size. Most of the LANs use Bus or ring topology for connection. They are used to connect personal computers and workstations in company offices and factories to share resources and exchange information. Traditional LANs run at speeds of 10Mbps to 100Mbps, have low delay (microseconds and nanoseconds) and make very few errors. Newew LANs operate 10Gbps. Various topologies are possible for broadcast LANs.

Metropolitan Area networks

These generally referred as MANs cover a city. The best known example is cable television network available in many cities. Earlier these were used for TV reception only but with changes a two way internet service could be provided. In this system both television signals and internet being fed into centralized head end for distributions to peoples home.Wireless Networks

In 1901, the Italian Physicist, Guglielmo Marconi demonstrated a ship-to-shore telegraph, using Morse code. Morse code is a collection of binary digits called dots and dashes. Modern digital wireless systems have better performance, but the basic idea is the same. Wireless networks are divided into 3 main categories- System Interconnection

It is all about interconnecting the components of a computer using short range radio. Every computer has a monitor, keyboard, mouse, printer connected to the main unit by cables. Some companies got together to design a short range wireless network called Bluetooth to connect these components without wires. Bluetooth allows digital cameras, headsets, scanners and other devices live even computers to connect to a computer merely being brought within range. No cables, no drier installation, just put them on and turn them on and they work. Wireless LANs

These are systems in which every computer has a radio modem and a antenna with which it can communicate with other systems. Often there is an antenna on the ceiling that the machines talk to. Wireless LANs are becoming common in small offices and homes, where installing Ethernet is considered too much trouble. Also used in older building, company cafeterias, conference rooms etc. IEEE 802.11 is the standard for wireless LANs.

Wireless WANs

This is also wireless network, but is a wide area system.

Wide area Network

This is referred as WAN. WAN spans a large geographical area often a continent or country. WAN contains a collection of machines, traditionally called as hosts. These hosts can be on LANs and we are connected by a subnet.or also called communication subnet. The hosts are owned be customers or are personal computers. The communication subnets are owned by a telephone company or internet service provider. The subnet carries the messages from hosts to hosts, just as telephone system carries words from speaker to listener. Each host is connected to a LAN on which a router is present. Sometimes a host may be connected directly to a router. The collection of communication lines and routers is called a communication subnet.

In most WANs the network contains many transmission lines each connecting a pair of routers. A packet is sent from one router to another via one or more intermediate routers. The packet is received at each intermediate router in its entirely. That is store the packet in full until the required output line is free, and then forwards it. A subnet that works according to this principle is called store and forward or packet switched subnet. Not all WANs are packet switched. A second possibility for a WAN is a satellite system. Satellite networks are inherently broadcast networks.4. Explain the following concepts of Internetworking:

A) Internet architecture B) Protocols and Significance for internetworking

C) Internet layering model

A) Internet Architecture

The internet is a worldwide, publicly accessible network of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol (IP). It is a network of networks, that consists of millions of smaller domestic, academic, business, and government networks, which together carry various information and services, such as electronic mail, inline chat, file transfer, and the interlinked web pages and other documents of the World Wide Web.

Physically, two networks can only be connected by a computer that attaches both of them. But just a physical connection cannot provide interconnection where information can be exchanged as there is no guarantee that the computer will cooperate with other machines that wish to communicate.

Internet is not restricted in size. Internets exist that contain a few networks and internets also exists that contain thousands of networks. Similarly, the number of computers attached to each network in an internet can vary. Some networks have no computers attached, while others have hundreds. To have a viable internet, we need a special computer that is willing to transfer packets from one network to another. Computers that interconnect two networks and pass packets from one to the other are called internet gateways or internet routers.

An organization uses single router to connect its entire network. The 2 reasons for this is

Because the CPU and memory in a router are used to process each packet, the processor in one router is insufficient. Redundancy improves internet reliability. Protocol software continuously monitors Internet connections and instructs the routers to send traffic along alternative paths when a network or router fails.Hence when planning an internet, organization must choose a design that meets its need for reliability, capacity, and cost. In particular, the exact details of the expected traffic, the organizations reliability requirements, internet topology, that often depends on the bandwidth of the physical networks and finally the cost of available router hardware.

For example, a client calls his/her Internet Service Provider (ISP) over a a dial up telephone line. The modem is a card with a PC that converts the digital signals the computer produces to analog signals that can pass over telephone system. These signals are transferred to the ISPs point of presence (POP), where they are removed from the telephone system and injected into the ISPs regional network. From this point onwards, the system is fully digital and packet switched.

The ISPs regional network consists of interconnected routers in the various cities the ISP serves. If the packet is destined for a host served directly the ISP, then the packet is delivered to the host. Otherwise, it is handed over to the ISPs backbone operator.

AT&T, Sprint are some of the major backbone operators. They operate large international backbone networks with thousands of routers connected by high bandwidth optical fibers. Large companies run server farms often connect directly to the backbone. Backbone operators encourage this direct connection by renting space called Carrier hotels. If a packet given to the backbone is destined for an ISP or company served by the backbone, it is sent to the closet router and handed off there. Many backbones of varying sizes exist in the world. To allow packets to hop between backbones, all major backbones connect at the network access point (NAP). A NAP is a room full of routers, at least one packet per backbone. A LAN in a room connects all these routers, so packets can be forwarded from any backbone to any other backbone. In addition to being interconnected at NAPs, the larger backbone have numerous direct connections between their routers, a technique known as Private peering. B) Protocols and Significance for Internetworking:

Protocols for internetworking

Many protocols have been used for use in an internet. One suite known as the TCP/IP internet protocol stands out most widely used for internets. Most networking professional simply refer this protocol as TCP/IP. Work on the transmission control protocol (TCP) began in the 1970s. the U.S military funded the research in TCP/IP and internetworking through the Advanced Research Projects Agency in short known as ARPA.Significance of internetworking and TCP/IP

Internetworking has become one of the important technique in the modern networking. Internet technology has revolutionized the computer communication. The TCP?IP technology has made possible a global Internet, which reaches millions of schools, commercial organizations, government and military etc., around the world.

The worldwide demand for Internetworking products has affected most companies sell networking technologies. Competition has increased among the companies that sell the hardware and software needed for internetworking. Companies have extended the designs in two ways.

1. The protocols have adapted to work with many network technologies.

2. And new features have been adapted that allow the protocols to transfer data across the internets.

C) Internet Layering Model

Internet uses the TCP/IP reference model. This model is also called as Internet Layering Model or Internet reference Model. This model consists of 5 layers.

Layer 5 - APPLICATION

Layer 4 - TRANSPORT

Layer 3 - INTERNET

Layer 2 - NETWORK INTERFACE

Layer 1 - PHYSICAL

A goal was of continuing the conversation between source and destination even if transmission went out of operation. The reference model was named after two of its main protocols. TCP (Transmission Control Protocol) and IP (Internet Protocol).

Layer 1 : Physical layer

This layer corresponds to basic network hardware.Layer 2 : Network Interface

This layer specifies how to organize data into frames and how a computer transfers frames over a network. It interfaces the TCP?IP protocol stack to the physical network.Layer 3 : Internet

This layer specifies the format of packets sent across an internet. It also specifies the mechanism used to forward packets from a computer through one or more routers to the final destination.

Layer 4 : Transport

This layer deals with opening and maintaining connection, ensuring that packets are in fact received. The transport layer is the interface between the application layer and the complex hardware of the network. It is designed to allow peer entities on the source and destination hosts to carry on conversations.Layer 5 : Network Interface

Each protocol of this layer specifies how one application uses an internet.

5. Explain the following different classes of IP addresses:

A) Primary classful addresses B) Class A

C) Class B D) Class C

ANS :

In order to provide the flexibility required to support different size networks, the designers decided that the IP address space should be divided into five different address classes. They are Class A, Class B, Class C, Class D and Class E.

A) Primary classful addresses

Out of the five above classes, the three classes are called Class A, Class B, and Class C. These three classes together are often referred to as classful addressing or Primary Address class.

Each class fixes the boundary between the network-prefix and the host-number at a different point within the 32-bit address. One of the fundamental features of classful IP addressing is that each address contains a self-encoding key that identifies the dividing point between the network-prefix and the host-number.

B) Class A Networks (/8 Prefixes)

Each Class A network address has an 8-bit network-prefix with the highest order bit set to 0 and a 7-bit network number, followed by a 24-bit host number. Today, it is no longer considered modern to refer to a Class A network. Class A network are now referred to as /8s (pronounced slash eight or just eights) since they have an 8-bit network-prefix.

A maximum of 126 ( 27 -2)/8 networks can be defined. The calculation requires that the 2 is subtracted because the /8 network 0.0.0.0 is reserved for use as the default route and the /8 network 127.0.0. (also written as 127/8 or 127.0.0.0/8) has been reserved for the loop back function. Each /8 supports a maximum of 16,777,214 (224 -2)hosts per function. The host calculation requires that 2 is subtracted because the all-0s (this network) and all-1s (broadcast) host-numbers may no be assigned to individual hosts.

Since the/8 address block contains 231 ( 2,147,483,648) individual addresses and the IPv4 address space contains a maximum of 232 (4,294,967,296) addresses, the /8 address space is 50% of the total IPv4 unicast address space. C) Class B Networks (/16 prefixes)

Each Class B network address has a 16-bit network-prefix with the two highest order bits set to 1-0 and a 14-bit network number, followed by a 16-bit host-number. Class B networks are now referred to as /16s since they have a 16-bit network-prefix.

A maximum of 16,384 (214) /16 networks can be defined with up to 65,534 (216 -2) hosts per network. Since the entire /16 address block contains 230 (1,073,741,824) addresses, it represents 25% of the total IPv4 unicast address space.

D) Class C Networks (/24 prefixes)

Each class C network address has a 24-bit network-prefix with the three highest order bits set to 1-1-0 and a 21-bit network number, followed by an 8-bit host-number. Class C networks are now referred to as /24s since they have a 24-bit network-prefix. A maximum of 2,097,152 (221) /24 networks can be defined with up to 254 (28 -2) hosts per network. Since the entire /24 address block contains 229 (536,870,912) addresses, it represents12.5% (1/8th) of the total IPv4) unicast address space.6. Discuss the following with suitable examples:

A) Variable length subnets

B) Subnet Masks

ANS: A) Variable Length Subnets:

Most Sites that implement subnetting use a fixed length assignment. The TCP?IP subnet standard provides even more flexibility than the fixed length subnetting seen above. An organization may select a subnet partition independently for each physical network. Although the technique is known as variable length subnetting, the name is slightly misleading as the value does not vary over time. Once a partition is been selected for a particular network the partition never changes. All hosts and routers attached to that network must follow the decision.

Advantages:

The chief advantage is flexibility in size of physical networks in an organization. An organization can have a mixture of large and small networks and also can achieve higher utilization of address space.

Disadvantages:

Values of subnets must be assigned carefully to avoid address ambiguity. A situation may arise in which an address is interpreted differently depending on the physical network. An address may match two different subnets. Routers cannot resolve such ambiguity, which means an invalid assignment can only be repaired by re-numbering. So network managers are discouraged from using variable length subnetting.B) Subnet Masks

The subnet technology makes configuration of either fixed or variable length with the use of subnet masks. Usually sub netting is used by class B networks. The standard specifies a 32-bit subnet mask to identify the division. Thus a site using subnet addressing must choose a 32-bit subnet mask for each network.Implementation of subnet using subnet mask;

To implement subnetting, the router needs a subnet mask that indicates the division of the hierarchy for routing purpose. Bits in the subnet mask are set to 1 if machines on the network treats the corresponding bit in the IP address as part of subnet prefix, and 0 if they treat bit as part of the host identifier. A subnet mask should have 1s for all bits that corresponds to the network portion of the address.

It is recommended that sites uses contiguous subnet masks and that they use the same mask throughout an entire set of physical networks that share an IP address. Contiguous subnet masks means the 32-bit subnet mask consists of two groups of bits. The first group contains all 1s, the second group contains all 0s.

The number of bits that corresponds to 1s represents the network number, and the number of bits corresponding to 0s represents the host number.Representation of subnet masks:

Subnet mask specified in binary is very awkward and prone to errors. Hence most softwares allow alternative representation. Sometimes the representation follows whatever conventions the local operating system uses for representation of binary quantities. Ex., hexadecimal notation etc.

Most IP software uses dotted decimal representation for subnet masks. It works best when sites choose to align subnetting on octet boundaries. Many networks that use subnetting are Class B networks.

11111111 11111111 11111111

00000000

Network profile

Host on that network

Using the standard notation we write the subnet mask of the above as 255.255.255.0, which makes easy to write and understand.

Another way of representation is using3-tuple:

{< network number >, < subnet number > < host number > }

The disadvantage of this representation is that it does not accurately specify how many bits are used for each part of the address. The advantage is that it abstracts away from the details of bit fields and emphasizes the values of the three parts of the address.

Yet another notation can be used at power with dotted decimal notation is using a slash followed by the number of bits in the network and subnet part. The subnet mask can be represented as /24 to indicate that 24 bits of the IP address is used as network address and subnet address.Maintenance of subnet masks:

Each site is free to choose subnet masks for its networks. When making assignments, network managers make an attempt to balance sizes of networks, number of physical networks, expected growth and ease of maintenance. Difficulty arises in case of non-uniform masks, as it may be possible that such an assignments that lead to ambiguous routes.

Typically a site selects contiguous bits from local portion of the address to identify a subnet or local network and uses the same partition for all local physical networks at the site. For easy maintenance many sitesise simply a single subnet octet when subnetting a class B address.

2. Describe the OSI reference model and compare it with TCP / IP model.

ANS:The O.S.I. model (O.S.I. - Open System Interconnection) is a way of sub-dividing a System into smaller parts (called layers) from the point of view of communications. A layer is a collection of conceptually similar functions that provide services to the layer above it and receives services from the layer below it. On each layer an instance provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free communications, across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.Description of OSI layersOSI Model

Data unitLayerFunction

HostlayersData7. ApplicationNetwork process to application

6. PresentationData representation,encryption and decryption

5. SessionInterhost communication

Segments4. TransportEnd-to-end connections and reliability,Flow control

MedialayersPacket3. NetworkPath determination and logical addressing

Frame2. Data LinkPhysical addressing

Bit1. PhysicalMedia, signal and binary transmission

Layer 1: Physical LayerThe Physical Layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBAs used in storage area networks) and more.

To understand the function of the Physical Layer, contrast it with the functions of the Data Link Layer. Think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, whereas the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. Standards such as RS-232 do use physical wires to control access to the medium.

The major functions and services performed by the Physical Layer are:

Establishment and termination of a connection to a communications medium.

Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.

Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as token ring, FDDI, ITU-T G.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

Layer 2: Data Link LayerThe Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work in IEEE Project 802. IEEE work assumed sub layering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages.

The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete Data Link Layer which provides both error correction and flow control by means of a selective repeat Sliding Window Protocol.

Both WAN and LAN service arrange bits, from the Physical Layer, into logical sequences called frames. Not all Physical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layer functions. For example, every fifth bit of the FDDI bit stream is not used by the Layer.

WAN Protocol architectureConnection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They are also capable of controlling the rate of transmission. A WAN Data Link Layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD.

Practical, connectionless LANs began with the pre-IEEE Ethernet specification, which is the ancestor of IEEE 802.3. This layer manages the interaction of devices with a shared medium, which is the function of a Media Access Control sub layer. Above this MAC sub layer is the media-independent IEEE 802.2 Logical Link Control (LLC) sub layer, which deals with addressing and multiplexing on multiaccess media.

While IEEE 802.3 is the dominant wired LAN protocol and IEEE 802.11 the wireless LAN protocol, obsolescent MAC layers include Token Ring and FDDI. The MAC sub layer detects but does not correct errors.

Layer 3: Network LayerThe Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layersending data throughout the extended network and making the Internet possible. This is a logical addressing scheme values are chosen by the network engineer. The addressing scheme is hierarchical.

Careful analysis of the Network Layer indicated that the Network Layer could have at least 3 sub layers: 1.Subnetwork Access - that considers protocols that deal with the interface to networks, such as X.25; 2.Subnetwork Dependent Convergence - when it is necessary to bring the level of a transit network up to the level of networks on either side; 3.Subnetwork Independent Convergence - which handles transfer across multiple networks. The best example of this latter case is CLNP, or IPv7 ISO 8473. It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of error packets so they may be discarded. In this scheme, IPv4 and IPv6 would have to be classed with X.25 as Subnet Access protocols because they carry interface addresses rather than node addresses.

A number of layer management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the Network Layer. These include routing protocols, multicast group management, Network Layer information and error, and Network Layer address assignment. It is the function of the payload that makes these belong to the Network Layer, not the protocol that carries them.

Layer 4: Transport LayerThe Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail.

Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

Of the actual OSI protocols, there are five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries, both of which TCP is incapable. Detailed characteristics of TP0-4 classes are shown in the following table: Feature NameTP0TP1TP2TP3TP4

Connection oriented networkYesYesYesYesYes

Connectionless networkNoNoNoNoYes

Concatenation and separationNoYesYesYesYes

Segmentation and reassemblyYesYesYesYesYes

Error RecoveryNoYesNoYesYes

Reinitiate connection (if an excessive number of PDUs are unacknowledged)NoYesNoYesNo

multiplexing and demultiplexing over a single virtual circuitNoNoYesYesYes

Explicit flow controlNoNoYesYesYes

Retransmission on timeoutNoNoNoNoYes

Reliable Transport ServiceNoYesNoYesYes

Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the Transport Layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a Network Layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet.Layer 5: Session LayerThe Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session check pointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls.

Layer 6: Presentation LayerThe Presentation Layer establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the presentation service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol data units, and moved down the stack.

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.

Comparison with TCP/IPIn the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.[5] RFC 3439 contains a section entitled "Layering considered harmful." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network.

Even though the concept is different from the OSI model, these layers are nevertheless often compared with the OSI layering scheme in the following way: The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its end-to-end Transport Layer includes the graceful close function of the OSI Session Layer as well as the OSI Transport Layer. The internetworking layer (Internet Layer) is a subset of the OSI Network Layer (see above), while the Link Layer includes the OSI Data Link and Physical Layers, as well as parts of OSI's Network Layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the Network Layer document.

The presumably strict peer layering of the OSI model as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application Layer protocol in its own right.

3. Explain the following with respect to Data Communications:

A) Fourier analysis B) Band limited signals

C) Maximum data rate of a channel

A) Fourier AnalysisIn 19th century, the French mathematician Fourier proved that any periodic function of time g (t) with period can be constructed by summing a number of cosines and sines.

Where f= 1/T is the fundamental frequency, and are the sine and cosine amplitudes of thenth harmonics. Such decomposition is called a Fourier seriesC) Maximum data rate of a channel

In 1924, H. Nyquist realized the existence of the fundamental limit and derived the equation expressing the maximum data for a finite bandwidth noiseless channel. In 1948, Claude Shannon carried Nyquist work further and extended it to the case of a channel subject to random noise.

In communications, it is not really the amount of noise that concerns us, but rather the amount of noise compared to the level of the desired signal. That is, it is the ratio of signal to noise power that is important, rather than the noise power alone. This Signal-to-Noise Ratio (SNR), usually expressed in decibel (dB), is one of the most important specifications of any communication system. The decibel is a logarithmic unit used for comparisons of power levels or voltage levels. In order to understand the implication of dB, it is important to know that a sound level of zero dB corresponds to the threshold of hearing, which is the smallest sound that can be heard. A normal speech conversation would measure about 60 dB.

If an arbitrary signal is passed through the Low pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H samples per second. Sampling the line faster than 2H per second is pointless. If the signal consists of V discrete levels, then Nyquist theorem states that, for a noiseless channel.

Maximum data rate =2H.log2 ( V) bits per second.

For a noisy channel with bandwidth is again H, knowing signal to noise ratio

S/N, the maximum data rate according to Shannon is given as

Maximum data rate =H.log2 (1+ S/N) bits per second.

B) Band limited signalsA signal is said to be a band limited signal if all of it's frequency components are zero above a certain finite frequency. i.e it's power spectral density should be zero above the finite frequency.A band limited signal is a deterministic or stochastic signal whose Fourier transform or power spectral density is zero above a certain finite frequency. In other words, if the Fourier transforms or power spectral density has finite support then the signal is said to be band limited.A more practically useful correspondence between physical and digital waveguide networks is obtained by assuming the inputs to the physical networks are band limited continuous-time Kirchoff variables. The signals propagating throughout the physical network are assumed to consist of frequencies less than Hz, where denotes the sampling rate. Therefore, by the Shannon sampling theorem, if we record a sample of the pressure wave, say, at each unit-delay element every seconds, the band limited continuous pressure fluctuation can be uniquely reconstructed throughout the waveguide network. Saying the pressure variation is frequency band limited to less than is equivalent to saying the pressure distribution is spatially band limited to less than , or, a one-sample section of waveguide is less than half a cycle of the shortest wavelength contained in a traveling wave. In summary, a DWN is equivalent to a physical waveguide network in which the input signals are band limited to Hz. This equivalence does not remain true for time-varying or non-linear DWNs.

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