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COMPUTER NETWORKS

III- B.Tech –II SEMESTER-IT

Course Type: Under Graduate(B.Tech)

Text Book: 1. Computer Networks — Andrew S Tanenbaum, 3rd & 4th Edition. Pearson

Education/PHI

2. Data Communications and Networking – Behrouz A. Forouzan.3rd Edition

TMH.

References: 1. An Engineering Approach to Computer Networks-S. Keshav, 2nd Edition,

Pearson Education

2. Understanding communications and Networks,3rd Edition, W.A.Shay,

Thomson

3. Data Communications and Networking – Behrouz A. Forouzan.4th Edition

TMH

• Network layer protocols and network routing algorithms

• Transport layer protocols( TCP/IP, UDP)

• IPv4 & IPv6 addressing

• Application layer protocols

• Introduction to Network security

A. Sai Prasad-HoD IT



UNIT-1

Introduction : OSI, TCP/IP and other networks models, Examples of Networks: Novell Networks ,Arpanet, Internet, Network Topologies WAN, LAN, MAN.

Learning Objectives:

By the end of this unit the student able to:

No of Teaching hours required: 08hrs

Mode of Teaching: Black board & PPT




Introduction:

Communication: The process of sharing resources, exchanging data, providing back up for

each other and allowing employees and individuals to perform their work from any location.

Some of the common objectives of the computer communication network are:

• To provide sharing of resources such as information or processors.

• To provide inter-process communication among users and processors.

• To provide distribution of processing functions.

• To provide centralized control for a geographically distributed system.

• To provide centralized management and allocation of network resources: host processors,

transmission facilities.

• To provide compatibility of dissimilar equipment and software.

• To provide network users with maximum performance at minimum cost.

• To provide an efficient means of transport large volumes of data among remote locations.

Structure of the Communication System:

Figure 1.1 illustrates a simple data communications system.




• The application process (AP) is the end-user application. It usually consists of software

such as a computer program.

• Typical examples are an accounts receivable program, a payroll program, and airline

reservation system, an inventory control package or a personnel system.

• In figure 1.1, site A could execute an application process (APA1) in the form of software

program to access an application process at site B (which is, in this case, a program

(APB1) and a database).

• Figure 1.1 also shows a site B program (APB2) accessing a file at site A through an

application program (APA2).

• The application resides in the Data Terminal Equipment, or DTE . DTE is a generic term

used to describe the end-user machine, which is usually a computer or terminal.

• The DTE could be a large mainframe computer, such as a large IBM or ICL machine, or itcould be a smaller machine, such as a terminal or a personal computer.

• The DTE takes many forms in the industry.

Here are several examples:

• A workstation for an air traffic controller.

• An automated teller machine in a bank.

• A point-of-sale terminal in a department store.

• A sampling device to measure the quality of air.

• A computer used to automate the manufacturing process in a factory.

• An electronic mail computer or terminal.

• A personal computer in the home or office.

The function of communications network is to interconnect DTEs so they can share

resources, exchange data, provide back up for each other, and allow employees andindividuals to perform their work from any location.

Figure 1.1 shows that a network provides logical and physical communications for

the computers and terminals to be connected. The applications and files use the physical

channel to effect logical communications.

Logical, in this context, means the DTEs are not concerned with the physical

aspects of the communications process. Application A1 need only issue a logical Read

request with an identification of the data. In turn, the communications system is responsible

for sending the Read request across the physical channels to application B1.




Figure 1.1 also shows the Data Circuit-Terminating Equipment, or DCE (also

called data communications equipment). Its function is to connect the DTEs into the

communication line or channel.

The primary function of the DCE remains to provide an interface of the DTE into the

communications network. The familiar modem is an example of a DCE.

Computer Networks: The old model of a single computer serving all of the organisations

computational needs is rapidly being replaced by one in which a large number of separate,

but interconnected computers do the job. These systems are called computer networks.

Therefore computer network means an interconnected collection of autonomous

computer, “If one computer can forcibly start, stop or control another one, the computers

are not autonomous”.

NEED OF COMPUTER NETWORKS

Computer Network satisfies a broad range of purposes and meets various

requirements.

Need of computer network arises for various purposes, and these are:

1. To provide sharing of resources such as information or processors.

2. To provide inter-process communication among users and processors.

3. To provide distribution of processing functions.

4. To provide centralized control for a geographically distributed system.

5. To provide centralized management and allocation of network resources.

6. To provide compatibility of dissimilar equipment and software.

7. To provide network users with maximum performance at minimum cost.

8. To provide an efficient means of transport large volumes of data among remote locations.

ADVANTAGES OF COMPUTER NETWORKS:These purposes must be fulfilled by various advantages of networks.

1. Resource Sharing: Resource sharing means the goal is to make all programs, data and

equipment available to anyone on the network without regard to the physical location of the

resource and the user.

Example: Suppose a user happens to be 1000 km away from his data should not prevent him

from using the data as though they were local. Also load sharing is another aspect of resource

sharing.

2. High Reliability: Network provides high reliability by having alternative sources of

supply.




Example: Suppose all files could be replicated on two or three machines, so if one of them is

unavailable (due to a hardware failure), the other copies could be used. For military, banking,

air traffic control, and many other applications, the ability to continue operating the face of

hardware problems is of great importance.

3. Low Cost/Saving Money: Small computers have a much better price/performance ratiothan large one. Mainframes are roughly a factor of fourty faster than the fastest single chip

microprocessors, but they cost a thousand times more. This imbalance has caused many

system designers to build systems consisting of powerful personal computers, as per user,

with data kept on one or more shared file server machines.

4. Communications: Another goal of setting up a computer network has little to do with

technology at all. A computer network can provide a powerful communication medium

among widely separated people. Using a network, it is easy for two or more people who live

far apart to write a report together. i.e. when one author makes a change to the document,

which is kept online, the others can see the change immediately, instead of waiting severaldays for a letter.

USES OF COMPUTER NETWORKS

1. Access to remote programs: A company that has produced a model simulating the world

economy may allow its clients to log in over the network and run the program to see how

various projected inflation rates, interest rates, and currency fluctuations might affect their

business. This approach is often preferable to selling the program outright, especially if the

model is constantly being adjusted or requires an extremely large mainframe computer to run.

2. Access to remote data bases: It may soon be easy for the average person sitting at home to

make reservations for aeroplanes, trains, buses, boats, hotels, restaurants, theatres and so on,

anywhere in the world with instant confirmation. Home banking and the automated

newspaper also fall in this category.

3. Value-added communication facilities: High-quality communication facilities tend to

reduce the need for physical proximity. Everyone in the world, have ability to send and

receive electronic mail. These mails are also be able to contain digitized voice, still pictures

and possibly even moving television and video images.

4. using for entertainment purpose.

5. Accessing the information systems like World Wide Web, contains almost any

information.

Basic Concepts

CHANNELS AND TRANSMISSION MODE

The generic interconnection between a message source and its destination, or message sink, is

called a channel.

Two types of transmission technologies

1. Broadcast links 2. Point – to – point link




2. Broad-Casting Channels/ Multicasting Channels

• Broadcast networks have a single communication channel that is shared by all the

machines on the network. Short messages, called packets sent by any machine are

received by all the others.

• Upon receiving the packet, a machine checks the address field. If the packet is intended for

the receiving machine that is processed by the machine otherwise it is ignored.

• For example in an airport announcement asking all flight 644 passengers to report to gate

12 for immediate boarding.

• Some broadcast networks also supports to a subset of machines, something called

multicasting.

•Each machine can subscribe to any or all of the groups. When the packet is sent to a certaingroup, it is delivered to all machines subscribing to that group.

1. Point-to-Point Channel

In this network two machines are connected by network. Here the message is send from one

machine to another machine, it is received by all intermediate machines, and stored there,

until the request output line is free.

In general all larger networks are point – to –point networks.

Point- to- point transmission with one sender and one receiver is called unicasting




Transmissions modes

There are three modes of Transmission modes used in communications.

1. Simplex

• A simplex arrangement allows communication in one direction only. Here the role of

source and destination are permanently assigned.

• Only of the two stations on a link transmit, the other can only receive It is common in

television and commercial radio.

• Simplex systems are found in some applications such as telemetry, burglar alarm.

2. Half duplex

• Half duplex arrangement allows communication in both directions but only one direction at

a time.

• Here the roles of source and destination are allowed to change. This is also called a Two-

Way-Alternate i.e., (TWA). Half-duplex systems found in many systems, such as

inquiry/response applications, where in a workstation sends a query to another workstation

and waits for the applications process to access and/or compute the answer and transmit the

response back.

• Terminal-based systems (keyboard terminals and terminals with CRT screens) often use

half-duplex techniques.


Transmission

Simplex Half-Duplex Full



3. Full duplex (or Duplex)

• Full duplex arrangement allows communication in both directions simultaneously. This

is also called Two-Way-Simultaneous (TWS).

• Full duplex or (duplex) provides for simultaneous two-way transmission, without

the intervening stop-and-wait aspect of half-duplex.

• Full duplex is widely used in applications requiring continuous channel usage, highthroughput, and fast response time.

• One example of full-duplex communication is the telephone network.

Network Topology:

The term “TOPOLOGY” refers to the way in which the end points or

stations/computer systems, attached to the networks, are interconnected

Depending on the requirement there are different Topologies to construct a network.

(1) Mesh topology

(2) Star topology

(3) Tree topology

(4) Bus topology

(5) Ring topology

• Ring and mesh topologies are felt convenient for peer to peer transmission.

• Star and tree are more convenient for client server.

• Bus topology is equally convenient for either of them.




Mesh Topology

In mesh topology each and every computer is connected to each other with direct point to

point link. A fully connected mesh network therefore has n (n –1/2) physical channels to link

n devices.

To accommodate these, every device on the network must have n –1 input/output parts.

Advantages

• Use of dedicated links eliminates the traffic problems.

• It is robust, i.e. if one link becomes unusable it does not incapacitate the entire system.

• Privacy is maintained since the message travels along the dedicated lines.

• Point-to-point link makes fault identification and fault isolation easy.

Disadvantages

• The amount of cabling required is high.

• The number if I/O ports required is high.

Star Topology

In a star topology, cables run from every computer to a centrally located device called a

HUB.

Star topology networks require a central point of connection between media segment.

These central points are referred to as Hubs.

Hubs are special repeaters that overcome the electromechanical limitations of a media.

Each computer on a star network communicates with a central hub that resends the

message either to all the computers. (In a broadcast network) or only the destination

computer. (In a switched network).

Ethernet 10 base T is a popular network based on the star topology.

Advantages

• Easy to modify and to add new computers without disturbing the rest of the network.

• Less expensive than mesh topology.

• Each device needs only one link and one port.

• Easy to install and configure.

• Easy to diagnose network faults.

• Single computer failure does not affect the network.

• Ordinary telephone cables can be used.

Disadvantages

• More cabling is required as compare to others.




• Failure of the central hub brings the entire network down.

Bus Topology

• A bus topology is multipoint.

• One long cable acts as a backbone to link all the devices in the network.

• Nodes are connected to the bus cable by drop lines and taps.

• A drop line is a line running between the device and main cable.

• A tap is connector that splices into the main cable. There is a limit on the number of taps

used and distance between the taps.

Advantages

• Simple, reliable and easy to use.

• Easy to installation and cheaper than when compared with others.

• Less cabling.

Disadvantages

• Can be used in relatively small networks.

• All computers share the same bus.

• Reconfiguration is difficult.

• Fault identifications is difficult.

• Adding new nodes is difficult.

• A fault on the cable stops all transmission.




Ring Topology

• In ring topology, each device has a dedicated point-to-point line configuration only with

two devices on either side of it.

•

A signal is passed along the ring in one direction, from device to device until it reaches itsdestination.

• Each device in the ring has a repeater. When the devices receive the signal intended for the

other node, it just regenerates the bits and passes them along.

• Ring network passes a token. A token is a short message with the electronic address of the

receiver.

• Each network interface card is given a unique electronic address, which is used to identify

the computer on the network.

Advantages

• Easy to install and reconfigure.

• Adding/deleting the new device is easy as only two connections have to be adjusted.

• Fault isolation is simplified.

• No terminators required.

Disadvantages

• A break in the ring can stop the transmission the entire network.

• Difficult to troubleshoot.

• Adding/removing computer disrupts the entire network.

• Expensive when compared with other topologies.

• When one computer fails overall network disrupts.

Tree Topology

• It is similar to the star network, but the nodes are connected to the secondary hub that in

turn is connected to the central hub.

• The central hub is the active hub. The active hub contains the repeater, which regenerates

the bits pattern it receives before sending them out.

• The secondary hub can be either active or passive.

• A passive hub provides a simple physical connection between the attached devices.

Advantages and Disadvantages of the tree are same as that of the star network. Also, the

addition of the secondary hub allows more devices to be attached to the central hub. It also

allows the network to isolate priorities communication from different computers.

NETWORK MODELS




1. Centralised network model: Here the terminals allow user has to enter data. But the

processing is done on the server. It gives the ability to the user to access the data from the

remote location.

2. Distributed network model: Here data storage and processing is done on the local

computer. Hence the computers used in the distributed network are capable of working as

stand alone.

4.5 CATEGORIES OF NETWORKS AND INTERNETWORKS

Today when we speak of networks, we are generally referring to three primary

categories based on its size, its ownership, the distance it covers and its physical architecture.

• Local-Area Networks.

• Metropolitan-Area Networks.

• Wide-Area Networks.

Local Area Network:

A Local-Area Network (LAN) is generally a privately owned network within a single office,

building or campus covering a distance of a few kilometers shown in given Fig. They are

widely used to connect personal computers and workstations in company offices and factories

to share resources and exchange information

• The main reason for designing a LAN is to share resources such as disks, printers, programs

and data.

• It also enables the exchange of information.

• LAN having data rate of 10 Mbps to hundreds of Mbps.

• LANs typically can use the star, bus or a ring topology.

• Example Ethernet LANs, Token Bus LANs, Token Ring LANs, FDDI.




Metropolitan–Area Network (MAN)

• A Metropolitan-Area Network (MAN) is designed to cover an entire city.

• It may be a single network such as a cable television network, or it may be a means of

connecting a number of LANs into a larger network so that resources may be shared

LAN- to-LAN as well as device-to-device.

• A Metropolitan-Area Network is shown in Fig.

• A MAN may be wholly owned and operated by a private company, or it may be a service

provided by a public company, such as local telephone company.

• Many telephone companies provide a popular MAN device called switched Multi-megait

Data Services.

• A MAN has a larger geographical scope compared to a LAN and can range from 10 km to a

few hundred kms in length.

• A typical LAN operates at a speed of 1.5 to 150 Mbps.




A metropolitan area network based on cable TV.

Wide-Area Network (WAN)

• A WAN is designed to interconnect computer systems over large geographic scope, such as

country, a continent, or even the whole world, as shown in fig.

• It contains a collection of machines intended for running user programs. These machines

are called Hosts.

• The hosts are connected by a communication subnet or subnet. The job of subnet is to

carry message from host to host.

• The subnet consists of Transmission lines and Switching elements. The transmissions

lines are used to move bits between machines.

• Switching elements are specialized computers that are called “routers”.

• A subnet organized by the principal of store-and-forward or packet-switching subnet as

in Fig

• A WAN speed ranges from 1.5 Mbps to 100 Gbps.

• WANs may utilize public, leased or private communication devices, usually in

combinations, and can therefore span an unlimited number of miles.




• A good example of such a network is internet, which has a connection to similar networks

in other countries.

Wide-Area Network (WAN).

Wireless Networks

Wireless networks can be divided into three main categories.1. System interconnection.

2. Wireless LANs

3. Wireless WANs

System interconnection:

• It is about interconnecting the components of a computer using short-range radio.

• This short-range wireless network called Bluetooth to connect components like keyboard,

printers, monitors etc. without wires.

• In the fig 4.5 (a) the system interconnection networks use the master-slave paradigm. Thesystem is the master, talking to the mouse, keyboard, etc.; as slaves.




Fig 4.5 (a) Bluetooth configuration (b) Wireless LAN

Wireless LANs:

Wireless LANs are becoming increasingly common in small offices and homes where

installing of Ethernet is too much trouble. There is a standard for wireless LANs called IEEE

802.11. Fig 4.5 (b) shows the wireless LAN where the base station is used to communicate

by the machines.

Wireless WANs:

• The radio network used for cellular telephones is an example of a low-bandwidth wireless

system.

• The system has gone through to three generation.

• The first generation was analog and for voice only.

• The second generation was digital for voice only.

• The third generation is digital and is for both voice and data.

• These systems operates below 1 Mbps, but the distance between the base station and

computer is in Km.

Internetworks

• When two or more networks are connected, they become an internetwork, or internet as

shown in Fig.

• The boxes labeled R represent routers.

• Individual networks are joined into internetworks by the use of internetworking devices.

• These devices, which include routers and gateways.




• The term internet (lower case i ) should not be confused with the internet (upper

case I). The first is a generic term used to mean an interconnection of networks. The

second is the name of a specific worldwide network.

Protocol Hierarchies

• A protocol is an agreement between the communicating parties on how

communication is to proceed.

• To reduce their design complexity, most networks layers are organised as a stack of layers

or levels, each one built upon its predecessor.

• The number of layers, the name of each layer, the contents of each layer, and the function

of each layer differ from network to network.

• However, in all networks, the purpose of each layer is to offer certain services to the higher

layers, shielding those layers from the details of how the offered services are actuallyimplemented.

• A LAYER N on one machine carries on a conversation with layer N on another machine.

• The rules and conventions used in this conversation are collectively known as the layer N

protocol.

• A list of protocols used by a certain systems, one protocol per layer, is called protocol

stack.

• The entities comprising the corresponding layers on different machines are called peer.

• The peers may be processes, hardware devices, or human beings.

• Between each pair of adjacent layer is an interface. The interface defines which primitive

operations and services provided to the upper layer by the lower layer.

• In general clear cut interface is required because it is simpler while replacing the

implementation of one layer with the complete different implementation.

• The set of layers and protocols is called the network architecture.

• The specification of the architecture must contain enough information to allow an

implemented to write the program or build the hardware for each layer so that it will

correctly obey the appropriate protocol.




Example:

Imagine two philosophers, one in Kenya and one in Indonesia, who want to

communicate, since they have no common language, they each engage a translator, each of

whom in turn contacts an engineer. Philosopher 1 wishes to convey his affection for

oryctolagus cuniculus to his peer. To do so, he passes a message across the 2/3 interface to

his translator, who might render it, as “I like rabbits” or ‘‘Jaime des lapins or’’ “IK hou van

kon” “nen” depending on the layer 2 protocol.

The translator then gives the, message to his engineer for transmission by telegram,

telephone, computer network, or some other means depending on what the two engineershave agreed. On in advance (the layer 1 protocol). When the message arrives, it is translated

into Indonesian and passed across the 2/3 interface to philosopher 2. Note that each protocol

is completely independent of the other ones as long as the interfaces are not changed. The

translators can switch from French to Hindi at will, provided that they both agree, and neither

changes his interface with either layer 1 or layer 3.




Information flow supporting virtual communication in layer 5

Consider the message M is produced by an application process running in layer 5 and

given to layer 4 for transmission.

Layer 4 puts a header H4 to the message and passes to layer 5.

The header includes control information, such as sequence number, to deliver the

message in the right order.

The long message transmitted by layer 4 to layer 3 is break up in to smaller units,

called packets, and adding layer 3 headers H3 to the message.

Layer 3 decides which of the outgoing lines to use and passes the packets to layer 2.

Layer 2 adds a header H2 and also trailer, and transmits to layer1 for physical

transmission.

At the receiving machine the message moves upward, from layer to layer, with header

being stripped off as it progresses.




Design Issues for the Layers: Some of the key design issues that occur in computer

networking are present in several layers. Below, we will briefly mention some of the more

important ones.

Every layer must have a mechanism for connection establishment.

1. Addressing: Addressing is needed in order to specify destination. Since networks

normally have many computers, a means is needed to specify the machine to

whom the processing machine wants to talk

2. Error control: It is an important issue because physical communication networks are not

perfect. Many error-detecting and error-correction are required, but bothends of the connection must agree on which is being used

3. Flow control: An issue that occurs at every level is how to keep a fast sender from

swamping a slow receiver with data.

4. Assembling: Another problem that must be solved at several levels is the inability of all

processes to accept arbitrarily long messages. This property leads to

mechanisms for disassembling, transmitting, and then reassembling

messages.

5. Routing: When there are multiple paths between source and destination, a route must be

chosen. Sometimes this decision must be split over two or more layers. This iscalled routing.

Some terminology

The active elements in each layer are called entities. An entity can be a software entity (such

as a process), or a hardware entity (such as an intelligent I/O chip). Entities in the same layer

on different machines are called peer entities.

The entities in layer n implement a service used by layer n+1. Layer n is the service provider

for the layer n+1 being the service user. Layer n may use the services of layer n - 1 in order to

provide its service.




Services are available at SAPs ( Service Access Points). The layer n SAPs are the places,

where layer n+1 can access the services offered. Each SAP has an address that uniquely

identifies it.

At a typical interface, the layer n+1 entity passes an IDU (Interface Data Unit) to the layer n

entity through the SAP. The IDU consists of an SDU (Service Data Unit) and some controlinformation. The SDU is the information passed across the network to the peer entity and

then up to layer n+1. The control information is needed to help the lower layer do its job (e.g.

the number of bytes in the SDU) but is not part of the data itself.

In order to transfer SDU, the layer n entity may have to fragment it into several pieces, each

of which is given a header and sent as a separate PDU (Protocol Data Unit) such as a packet.

The PDU headers are used by the peer entities to carry out their peer protocol. They identify

which PDU contain data and which contain control information, provide sequence numbers

and counts, and so on.

Connection-oriented and Connectionless Services

Layers can offer two different types of service to the layers above

them.

Connection-oriented

Connectionless.

Connection-oriented service (modeled after the telephone system):

• The service user first establishes a connection, uses the connection, and then releases

the connection.

• The essential aspect of a connection is that it acts like a tube, the sender pushes

objects (bits) in at one end, and the receiver takes them out in the same order at the

other end.

Connectionless service (modeled after the postal system):

• Each message carries the full destination address, and each one is routed through the

system independent of all the others.

Both the service is characterized by Quality of Service.

Some services are reliable in the sense that they never lose data.

Reliability is usually implemented by having the receiver acknowledge the receipt

of each message.

The acknowledgment process is often worth but introduces sometimes undesirable

overheads and delays.

Reliable connection-oriented service has two minor variations:




• Message sequences - the message boundaries are preserved. For example when two

1024-byte messages are sent, they arrive as two distinct 1024 messages but not as one

2048 message. Used to send pages of the book.

• Byte streams - the connection is simply a stream of bytes, with no message

boundaries. Applicable when a user logs into a remote server.

Connectionless services:

• Unreliable Service: In this type of service no acknowledge is received for the send

massage. Such connectionless services are often called datagram services.

Ex: Electronic junk mail

• Acknowledged datagram services - connectionless datagram services with

acknowledgment.

Ex: Sending the short messages with reliability.

• Request-reply service - the sender transmits a single datagram containing a request.

The reply contains the answer. Request-reply is commonly used to implement

communication in the client-server model

.

Service Primitives

• A service is formally specified by a set of primitives (operations) available to a user

process to access the service.

• These primitives tell the service to perform some actions or report on an action taken

by a peer entity.

• If the protocol stack is located in the operating system, the primitives are normally

system call. These calls cause a trap to kernel mode, which in turn control of the

machine over to the operating system to send the necessary packets.

Service primitives for implementing a simple connection-oriented service:




Reference models

The two important network architectures are:

• OSI reference model and

• The TCP/IP reference model.

Although the protocols associated with the OSI model are rarely used any more, the model

itself is actually quite general and still valid, and the features discussed at each layer are still

very important.

The TCP/IP model has the opposite properties: the model itself is not of much use but the

protocols are widely used.

1.4.1. The OSI Reference Model

The OSI model is based on a proposal develop by ISO as a first step toward international

standardization of the protocols used in the various layers. The model is called ISO OSI

(Open Systems Interconnection) Reference Model.

Open system is a system open for communication with other systems.

The OSI model has 7 layers (Fig. 1-16). The principles that were applied to arrive at the

seven layers are as follows:

1. A layer should be created where a different level of abstraction is needed.

2. Each layer should perform a well defined function.

3. The function of each layer should be chosen with an eye toward defining

internationally standardized protocols.

4. The layer boundaries should be chosen to minimize the information flow across the

interfaces.5. The number of layers should be large enough that distinct functions need not be

thrown together in the same layer out of necessity, and small enough that the

architecture does not become unwieldy.

The OSI model is not network architecture - it does not specify the exact services and

protocols. It just tells what each layer should do. However, ISO has also produced

standards for all the layers as separate international standards.




Bit 1. Physical

Lay

er

Media, signal and binary

transmission

This is the hardware level.

The physical layer is concerned with transmitting raw

bits over a communication channel.

It defines how the network devices work physically,

such as cable connections, voltage levels and timing.

It conveys the bit stream through the network at the

electrical

and mechanical level.

It provides the hardware means of sending and

receiving data on a carrier.

This includes the layout of pins, cable specifications,

hubs, repeaters, network adapters etc.

Frame2. Datalink

Physical addressing

This layer provides the reliable transmit of data

across the physical layer link. Different data link

layer specification define different network and

protocol characteristics:

Including physical addressing

Network topologies, Error notification, Sequencing

of frames

Flow control,

Broadcast networks have an additional issue in thedata link layer: how to control access to the shared

channel.

A special sublayer of the data link layer, the

medium access control sublayer, deals with this

problem.

Packet

3. Network

Path determination and

logical addressing

The main task of the network layer is to

determine how data can be delivered from

source to destination. That is, the network layer

is concerned with controlling the operation of

the subnet.

The issues that the layer has to solve:

• to implement the routing mechanism,

• to control congestions,

• to do accounting,

• to allow interconnection of heterogeneous networks.




In broadcast networks, the routing problem is

simple, so the network layer is often thin or

even nonexistent.

The user of the network layer may be sure that

his packet was delivered to the given

destination. However, the delivery of the

packets needs not to be in the order in which

they were transmitted.

Data Segments4. Transport

End-to-end

connections and

reliability,

Flow control

Manages the flow of data, providing

for error checking and recovery of data between the sending and

receiving devices. This layer takes

streams of data from multiple

applications and merges them

together into a single data stream

for the physical network. It's like

the railway controller managing

many trains coming on and off spur

lines into a single main rail line

heading for the city. This layer

manages the end-to-end control(for example, determining whether

all packets have arrived) and error-

checking. It ensures complete data

transfer.

The Transport Layer provides

transparent transfer of data

between end users. Typical

examples of Layer 4 are the

Transmission Control Protocol (TCP)

and User Datagram Protocol (UDP).




Data 5. SessionInterhost communication

This layer sets up, coordinates, and

terminates conversations, exchanges, and

dialogs between the applications at each

end. It deals with session and connection

coordination.

Sessions offer various services, including

dialog control (keeping track of whose turn it

is to transmit), token management

(preventing two parties from attempting the

same critical operation at the same time), and

synchronization (checkpointing long

transmissions to allow them to continue from

where they were after a crash).

Data 6.presentation

Data representation,

encryption and decryption

The presentation layer works to transform data

into the form that the application layer can

accept.

It translates the data to/from the the

Application layer into a standard format that

the other layers can understand.

Usually part of an operating system, protocols

at this layer convert incoming and outgoing

data from one presentation format to another

(for example, from a text stream into a popup

window with the newly arrived text).

This layer formats and encrypts/decrypts data

to be sent across a network, providing freedom

from compatibility problems.

It is sometimes called the syntax layer.




Data 7. Application

Network process to

application

The application layer is the OSI layer closest to

the end user, which means that both the OSI

application layer and the user interact directly

with the software application.

The application layer determines the identity andavailability of communication partners for an

application with data to transmit.

The application layer contains a variety of

protocols that are commonly needed by users.

One widely-used application protocol is HTTP

(HyperText Transfer Protocol), which is the

basis for the World Wide Web.

When a browser wants a Web page, it sends thename of the page it wants to the server using

HTTP. The server then sends the page back.

Other application protocols File Transfer

Protocol (FTP), Simple Mail Transfer Protocol

(SMTP).

Novell NetWare

• It is the most popular network system in the PC world.

• It was designed to be used by companies downsizing from a mainframe to a network

of PCs.

• In these systems desktop PC’s functions like clients and some other PC’s acts like

server. Hence is based on the client-server model.

• NetWare uses protocol stack. It looks more like TCP/IP than like OSI.




• The physical and data layers can be chosen from among various industry standards

(Ethernet, IBM token ring, ARCnet).

• The network layer runs an unreliable internetwork connectionless protocol called

IPX, functionally similar to IP. IPX uses 10-byte addressing.

•

Above IPX is a connection-oriented transport protocol called NCP (Network CoreProtocol) it provides various other services besides user data transport.

• A second protocol, SPX, is also available, but provides only transport.

• The session and presentation layers do not exist. Various application protocols are

present in the application layer.

The IPX packet consists of the following fields:

• Checksum (2 bytes) - rarely used, since the underlying data link layer also provides a

checksum.

• Packet length (2 bytes) - determines how long the entire packet is.

•

Transport control (1 byte) -which counts, how many networks the packet hastraversed. When it exceeds a maximum, the packet is discarded.

• Packet type (1 byte) - used to mark various control packets.

• Two address fields (12 bytes each) - each contains a 32-bit network number, a 48 bit

machine number (the 802 LAN address), and 16 bit local address (socket) on that

machine.

• Finally the data field – which occupies the rest of the packet. It size is determined by

the network.

About once a minute, each server broadcasts a packet giving its address and telling what

services it offers by using SAP - Service Advertising Protocol.

The packets are collected by special agent processes running on the router machines. The

agents use the information contained in them to construct databases of which servers are

running where.

When a client machine is booted, the following procedures take place:

1. The client machine broadcasts a request asking where the nearest server is.

2. The agent on the local router machine looks in its database of servers and the best

choice of server send back to the client.

3. The client establishes an NCP connection with the server. From this point on, the

client can access the file system and other services using this connection.

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