computer network unit i rgpv

UNIT-I/Computer Networking Truba College of Science & Tech., Bhopal

Prepared By: Ms. Nandini Sharma(CSE Deptt.)

COMPUTER NETWORK

A computer network, or simply a network, is a collection of computers and other hardware

interconnected by communication channels that allow sharing of resources and information.

Where at least one process in one device is able to send/receive data to/from at least one process

residing in a remote device, then the two devices are said to be in a network.

A network is a group of devices connected to each other.

Computer networking is sometimes considered a sub-discipline of electrical engineering,

telecommunications, computer science, information technology or computer engineering.

NETWORKING HISTORY

Early networks

From a historical perspective, electronic communication has actually been around a long time,

beginning with Samuel Morse and the telegraph. He sent the first telegraph message May 24,

1844 from Washington DC to Baltimore MD, 37 miles away. The message ? “What hath God

wrought?”

Less than 25 years later, Alexander Graham Bell invented the telephone. This led to the

development of the ultimate analog network – the telephone system.

Telephone Network

The telephone network that has had the greatest impact on how businesses communicate and connect today.

Until 1985, the Bell Telephone Company, now known as AT&T, owned the telephone network from end to end. It represented a phenomenal network, the largest then and stills the largest

today.

Developments in Communication

In 1966, an individual named “Carter” invented a special device that attached to a telephone

receiver that would allow construction workers to talk over the telephone from a two-way radio. Bell telephone had a problem with this used – and eventually lost.

Eventually led to the breakup of American Telephone and Telegraph in 1984, thus creating nine regional Bell operating companies like Pacific Bell, Bell Atlantic, Bell South, Mountain Bell,

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etc. The breakup of AT&T in 1984 opened the door for other competitors in the telecommunications market.

1960's - 1970's Communication

In the 1960’s and 1970’s, traditional computer communications centered around the mainframe host. The mainframe contained all the applications needed by the users, as well as file

management, and even printing. This centralized computing environment used low-speed access lines that tied terminals to the host. These large mainframes used digital signals – pulses of

electricity or zeros and ones, what is called binary -- to pass information from the terminals to the host. The information processing in the host was also all digital.

Problems faced in communication

This brought about a problem. The telephone industry wanted to use computers to switch calls faster and the computer industry wanted to connect remote users to the mainframe using the telephone service. But the telephone networks speak analog and computers speak digital.

Digital signals are seen as one’s and zero’s. The signal is either on or off. Whereas analog

signals are like audio tones – for example, the high-pitched squeal you hear when you accidentally call a fax machine.

So, in order for the computer world to use the services of the telephone system, a conversion of

the signal had to occur. The solution – a modulator/demodulator or “modem.”

The modem takes the digital signals from the computer and modulates the signal into analog

format. In sending information from a desktop computer to a host using POTS or plain old

telephone service, the modem takes the digital signals from the computer and modulates the

signal into analog format to go through the telephone system. From the telephone system, the

analog signal goes through another modem which converts the signal to digital format to be

processed by the host computer. This helped solve some of the distance problems, at least to a

certain extent.

Multiplexing

Another problem is how to connect multiple terminals to a single cable. The technology solution is multiplexing. What we can do with multiplexing is we can take multiple remote terminals, connect them back

to our single central site, our single mainframe at the central site, but we can do it all over a single communications channel, a single line.

How networks are growing

With all the technologies available, companies were able to team up with the phone company and tie branch offices to the headquarters. The speeds of data transfer were often slow and were

still dependent on the speed and capacity of the host computers at the headquarters site. The



phone company was also able to offer leased line and dial-up options. With leased-lines, companies paid for a continuous connection to the host computer. Companies using dial-up

connections paid only for time used. Dial-up connections were perfect for the small office or branch.

Birth of the personal computer

The birth of the personal computer in 1981 really fueled the explosion of the networking marketplace. No longer were people dependent on a mainframe for applications, file storage,

processing, or printing. The PC gave users incredible freedom and power. The Internet 1970's - 1980's

The 70’s and 80’s saw the beginnings of the Internet. The Internet as we know it today began as

the ARPANET — The Advanced Research Projects Agency Network – built by a division of the Department of Defense essentially in the mid ‘60's through grant-funded research by universities and companies.

The first actual packet-switched network was built by BBN. It was used by universities and the

federal government to exchange information and research. Many local area networks connected to the ARPANET with TCP/IP.

TCP/IP was developed in 1974 and stands for Transmission Control Protocol / Internet Protocol.

The ARPANET was shut down in 1990 due to newer network technology and the need for greater bandwidth on the backbone.

NETWORK GOALS:

The main goal of networking is "Resource sharing", and it is to make all programs, data and equipment available to anyone on the network without the regard to the physical

location of the resource and the user. A second goal is to provide high reliability by having alternative sources of supply. For

example, all files could be replicated on two or three machines, so if one of them is unavailable, the other copies could be available.

Another goal is saving money. Small computers have a much better price/performance

ratio than larger ones. Mainframes are roughly a factor of ten times faster than the fastest single chip microprocessors, but they cost thousand times more. This imbalance has

caused many system designers to build systems consisting of powerful personal computers, one per user, with data kept on one or more shared file server machines. This goal leads to networks with many computers located in the same building. Such a

network is called a LAN (local area network). Another closely related goal is to increase the systems performance as the work load

increases by just adding more processors. With central mainframes, when the system is full, it must be replaced by a larger one, usually at great expense and with even greater disruption to the users.



Computer networks provide a powerful communication medium. A file that was updated/modified on a network can be seen by the other users on the network

immediately.

List the Basic essential components of a computer network

Servers: servers are faster computers that run various software’s, store and process

information and also provide a human interface for the users to be able to use the networked computers.

Nodes: nodes are the computers on the network, which are provided to the users to carry

out their tasks using the network.

Workstation: A node, which is more powerful, and can handle local information

processing or graphics processing is calls a workstation. The workstation works only for the person sitting in front of it, where as a server serves all the people on the network to

share its resources. A workstation usually has an inexpensive, small hard disk to carry out local tasks. Some workstations, called diskless workstations, have no disk drive of their own. Such workstations also called dumb terminals and they rely completely on the LAN

for their access.

Network Operating System (NOS): The network requires some software to control all

the information transfer activity on the network, like the traffic police to control the traffic. The software called NOS handles these tasks. Networks, which are more complex,

require network devices like hubs, switches & routers to carry out different network function.

LAN Software: On the network, each computer is called a node or a workstation unless

there are certain computers designed as servers. LAN cables connect all the nodes and servers together to form the network. In addition to its local disk operating system, each

node requires networking software that enables the nodes to communicate with the servers. In file servers run network software that communicates with the nodes.

LAN Cable: This is the medium or channel over which the information travels from computer to computer. The information travels from one computer onto the medium and then from the medium to another computer in the form that it can be read.

Network Interface Card: Each computer contain a network interface card. This card is used to connect the cables to the computers. These cards help the computer to transfer the

data at a faster rate and in the form of packets. These cards are plugged into the computer motherboard. These cards are generally called as Ethernet cards.

Network Topologies

A network topology is the basic design of a computer network. It is very much like a map of a

road. It details how key network components such as nodes and links are interconnected. A

network's topology is comparable to the blueprints of a new home in which components such as

the electrical system, heating and air conditioning system, and plumbing are integrated into the

overall design.

In the Greek word "Topos" meaning "Place," Topology, in relation to networking, describes the

configuration of the network; including the location of the workstations and wiring connections.

Basically it provides a definition of the components of a Local Area Network (LAN).



Definition: The term topology refers to the way a network is laid out, either physically or

logically. Two or more devices connect to a link; two or more links form a topology.

1. Star Topology: All devices connected with a Star setup communicate through a central Hub by cable segments. Signals are transmitted and received through the Hub. It is the

simplest and the oldest and all the telephone switches are based on this.

Advantages

o Network administration and error detection is easier because problem is isolated to central node

o Networks runs even if one host fails o Expansion becomes easier and scalability of the network increases

o More suited for larger networks o Less expensive & less cable required than a mesh topology

Disadvantages

o Broadcasting and multicasting is not easy because some extra functionality needs

to be provided to the central hub o If the central node fails, the whole network goes down; thus making the switch

some kind of a bottleneck

o Installation costs are high because each node needs to be connected to the central switch

2. Bus Topology: The simplest and one of the most common of all topologies, Bus consists of a single cable, called a Backbone that connects all workstations on the network using a single line. All transmissions must pass through each of the connected devices to

complete the desired request. Each workstation has its own individual signal that identifies it and allows for the requested data to be returned to the correct originator. In

the Bus Network, messages are sent in both directions from a single point and are read by the node (computer or peripheral on the network) identified by the code with the message. Most Local Area Networks (LANs) are Bus Networks because the network will

continue to function even if one computer is down. This topology works equally well for either peer to peer or client server.



The purpose of the terminators at either end of the network is to stop the signal being

reflected back.

Advantages

o Broadcasting and multicasting is much simpler

o Network is redundant in the sense that failure of one node doesn't affect the network. The other part may still function properly

o Least expensive since less amount of cabling is required and no network switches are required

o Good for smaller networks not requiring higher speeds

Disadvantages

o Trouble shooting and error detection becomes a problem because, logically, all

nodes are equal o Less secure because sniffing is easier

o Limited in size and speed 3. Ring Topology: All the nodes in a Ring Network are connected in a closed circle of

cable. Messages that are transmitted travel around the ring until they reach the computer

that they are addressed to, the signal being refreshed by each node.

In a ring topology, the network signal is passed through each network card of each device and passed on to the next device. Each device processes and retransmits the signal,

so it is capable of supporting many devices in a somewhat slow but very orderly fashion. There is a very nice feature that everybody gets a chance to send a packet and it is

guaranteed that every node gets to send a packet in a finite amount of time.



Advantages

o Broadcasting and multicasting is simple since you just need to send out one

message o Less expensive since less cable footage is required o It is guaranteed that each host will be able to transmit within a finite time interval

o Very orderly network where every device has access to the token and the opportunity to transmit

o Performs better than a star network under heavy network load

Disadvantages

o Failure of one node brings the whole network down o Error detection and network administration becomes difficult

o Moves, adds and changes of devices can affect the network o It is slower than star topology under normal load

4. Tree Topology: The type of network topology in which a central 'root' node (the top

level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a point-to-point link between each of the second

level nodes and the top level central 'root' node, while each of the second level nodes that are connected to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy (i.e., the third level) connected to it, also with a

point-to-point link, the top level central 'root' node being the only node that has no other node above it in the hierarchy (The hierarchy of the tree is symmetrical.)

Each node in the network having a specific fixed number, of nodes connected to it at the next lower level in the hierarchy, the number, being referred to as the 'branching factor' of the hierarchical tree. This tree has individual peripheral nodes.

Advantages

It is scalable. Secondary nodes allow more devices to be connected to a central node.

Point to point connection of devices.

Having different levels of the network makes it more manageable hence easier fault identification and isolation.

Disadvantages

Maintenance of the network may be an issue when the network spans a great area.

Since it is a variation of bus topology, if the backbone fails, the entire network is

crippled.

An example of this network could be cable TV technology.



5. Mesh Topology: every device has a dedicated point-to-point link to every other device.

The term dedicated means that the link carries traffic only between the two devices it

connects. A fully connected mesh network therefore has n(n-1)/2 physical channels to

link n devices. To accommodate that many links, every device on the network must have

n-1 I/O ports.

Advantages-

o The use of dedicated links guarantees that each connection can carry its own data load,

thus eliminating the traffic problem that can occur when links must be shared by multiple

devices.

o Its Robust, If one link becomes unusable, it does not in capacitate the entire system.

o Privacy & Security.

o Ease to fault isolation & fault identification.

Disadvantages-

o Mesh are related to amount of cabling and the number of I/O ports required.

o Installation and Reconfiguration are difficult.

o Sheer bulk of the wiring can be greater than the available the space ( In walls, ceiling, floors etc) can accommodate.

6. Hybrid

Hybrid networks use a combination of any two or more topologies in such a way that the

resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.).



A hybrid topology is always produced when two different basic network topologies are

connected. Two common examples for Hybrid network are: star ring network and star bus

network

A Star Ring network consists of two or more star topologies connected using a multi-station access unit (MAU) as a centralized hub.

A Star Bus network consists of two or more star topologies connected using a bus trunk (the bus trunk serves as the network's backbone).

Categories of Network

Computer Networks may be classified on the basis of geographical area in two broad categories.

I. Local Area Network:

Networks used to interconnect computers in a single room, rooms within a building or buildings on one site are called Local Area Network (LAN). LAN transmits data with a speed of several megabits per second (106 bits per second). The transmission medium is normally coaxial cables.

LAN links computers, i.e., software and hardware, in the same area for the purpose of sharing

information. Usually LAN links computers within a limited geographical area because they must be connected by a cable, which is quite expensive. People working in LAN get more capabilities

in data processing, work processing and other information exchange compared to stand-alone computers. Because of this information exchange most of the business and government organizations are using LAN.

Major Characteristics of LAN are as follows:

Every computer has the potential to communicate with any other computers of the network

High degree of interconnection between computers

Easy physical connection of computers in a network Inexpensive medium of data transmission

High data transmission rate

Advantages:

The reliability of network is high because the failure of one computer in the network does not affect the functioning for other computers.

Addition of new computer to network is easy.

High rate of data transmission is possible.

Peripheral devices like magnetic disk and printer can be shared by other computers.

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Disadvantages:

If the communication line fails, the entire network system breaks down.

Followings are the major areas where LAN is normally used

File transfers and Access

Word and text processing

Electronic message handling

Remote database access

Personal computing

Digital voice transmission and storage II.

II. Wide Area Network:

The term Wide Area Network (WAN) is used to describe a computer network spanning a

regional, national or global area. For example, for a large company the head quarters might be at Delhi and regional branches at Bombay, Madras, Bangalore and Calcutta. Here regional centers

are connected to head quarters through WAN. The distance between computers connected to WAN is larger. Therefore the transmission medium used is normally telephone lines, microwaves and satellite links.

Characteristics of WAN are as follows:

Communication Facility: For a big company spanning over different parts of the country

the employees can save long distance phone calls and it overcomes the time lag in overseas communications. Computer conferencing is another use of WAN where users

communicate with each other through their computer system.

Remote Data Entry: Remote data entry is possible in WAN. It means sitting at any

location you can enter data, update data and query other information of any computer attached to the WAN but located in other cities. For example, suppose you are sitting at

Madras and want to see some data of a computer located at Delhi, you can do it through WAN.

Centralized Information: In modern computerized environment you will find that big

organizations go for centralized data storage. This means if the organization is spread over many cities, they keep their important business data in a single place. As the data are

generated at different sites, WAN permits collection of this data from different sites and save at a single site.

Examples of WAN are as follows:

Ethernet: Ethernet developed by Xerox Corporation is a famous example of WAN. This

network uses coaxial cables for data transmission. Special integrated circuit chips called controllers are used to connect equipment to the cable.



Arpanet: The Arpanet is another example of WAN. It was developed at Advanced

Research Projects Agency of U. S. Department. This Network connects more than 40 universities and institutions throughout USA and Europe.

Difference between LAN and WAN are as follows:

LAN is restricted to limited geographical area of few kilometers. But WAN covers great

distance and operate nationwide or even worldwide.

In LAN, the computer terminals and peripheral devices are connected with wires and

coaxial cables. In WAN there is no physical connection. Communication is done through telephone lines and satellite links.

Cost of data transmission in LAN is less because the transmission medium is owned by a

single organization. In case of WAN the cost of data transmission is very high because the transmission medium used is hired either telephone lines or satellite links.

The speed of data transmission is much higher in LAN than in WAN. The transmission speed in LAN varies from 0.1 to 100 megabits per second. In case of WAN the speed

ranges from 1800 to 9600 bits per second (bps).

Few data transmission errors occur in LAN compared to WAN. It is because in LAN the

distance covered is negligible.

III. Hybrid Networks:

Between the LAN and WAN structures, you will find hybrid networks such as campus area networks (CANs) and metropolitan area networks (MANs). In addition, a new form of network

type is emerging called home area networks (HANs). The need to access corporate Web sites has created two classifications known as intranets and extranets. The following sections introduce these networks.

a. Campus Area Networks (CANs): A campus area network (CAN) follows the same

principles as a local area network, only on a larger and more diversified scale. With a CAN, different campus offices and organizations can be linked together.

For example, in a typical university setting, accounts office might be linked to a registrar's

office. In this manner, once a student has paid his or her tuition fees in the accounts section, this information is transmitted to the registrar's system so the student can enroll for classes. Some university departments or organizations might be linked to the CAN even though they

already have their own separate LANs.

b. Metropolitan Area Networks (MANs): The metropolitan area network (MAN) is a large-scale network that connects multiple corporate LANs together. MANs usually are not owned by

a single organization; their communication devices and equipment arc usually maintained by a group or single network provider that sells its networking services to corporate customers.

MANs often take the role of a high-speed network that allows for the sharing of regional resources. MANs also can provide a shared connection to other networks using a WAN link.



c. Home Area Networks (HANs): A home area network (HAN) is a network contained within a user’s home connects a person’s digital devices, from multiple computers and their peripheral

devices, such as a printer, to telephones, VCRs, DVDs, televisions, video games, between LANs, MANs. Home security systems, “smart” appliances, fax machines, and other digital devices that

are wired into the network.

d. Intranets and Extranets: An "intranet" is the generic term for a collection of private computer networks within an organization. An "extranet" is a computer network that allows controlled access from the outside for specific business or educational purposes. Intranets and

extranets are communication tools designed to enable easy information sharing within workgroups.

NETWORK APPLICATION AREAS

There is a long list of application areas, which can be benefited by establishing Computer Networks.

Few of the potential applications of Computer Networks are:

1. Information retrieval systems which

search for books, technical reports, papers

and articles on particular topics

2. News access machines, which can search

past news, stories or abstracts with given search criteria.

3. Airline reservation, hotel booking, railway-reservation, car-rental, etc.

4. A writer's aid: a dictionary, thesaurus,

phrase generator, indexed dictionary of quotations, and encyclopedia.

5. Stock market information systems which

allow searches for stocks that meet certain

criteria, performance comparisons, moving

averages, and various forecasting techniques.

6. Electronic Financial Transactions (EFT)

between banks and via cheque clearing house.

7. Games of the type that grow or change

with various enthusiasts adding to the

complexity or diversity.

8. Electronic Mail Messages Systems

(EMMS).

9. Corporate information systems such as

marketing information system, customer

information system, product information system, personnel information system, etc.

10. Corporate systems of different systems

such as Order-Entry System, Centralized

Purchasing, Distributed Inventory Control,

etc.

11. On-line systems for Investment Advice

and Management, Tax Minimization, etc.

12. Resources of interest to a home user.

13. Sports results.

14. Theatre, movies, and community events information.



15. Shopping information, prices, and advertisements.

16. Restaurants; good food guide.

17. Household magazine, recipes, book reviews, film reviews.

18. Holidays, hotels, travel booking.

19. Radio and TV programmes.

20. Medical assistance service.

21. Insurance information.

22. Computer Assisted Instruction (CAI).

23. School homework, quizzes, tests.

24. Message sending service.

25. Directories.

26. Consumer reports.

27. Employment directories and Job

opportunities.

28. Tax information and Tax assistance.

29. Journey planning assistance viz. Train,

bus, plane etc.

30. Catalogue of Open University and

Virtual University courses.

KEY ISSUES TO COMPUTER NETWORK

The following are the major key issues to be trashed out very carefully before we go for a

computer network:

1. Nature of Nodes -Whether participating nodes are homogeneous or heterogeneous in nature?

2. Topology - Which of the computer topology has to be followed? Computer topology accounts

for the physical arrangement of participating computers in the network.

3. Interconnection Type - Whether interconnection type is point-to-point, multi-point, or

broadcast type.

4. Reliability - How reliable our network is? Reliability aspect includes error rate, redundancy and recovery procedures.

5. Channel Capacity Allocation - Whether allocation of channel capacity is time-division or frequency division?

6. Routing Techniques - Whether message between nodes are to be routed through: Deterministic, Stochastic, and Distributed routing techniques?

7. Models - Which of the models i.e. analytical models, queuing models, simulation models, measurement and validation models are applicable?

8. Channel Capacity - What are the channel capacities of the communication lines connecting nodes?



9. Access - Whether computer access in the network is direct-access or through a sub-network?

10. Protocols - What levels, standards and formats are to be followed while establishing communication between participating nodes?

11. Performance - How is higher performance of computer network achieved? Response time, time to connect, resource utilization, etc. contribute towards performance of computer network.

12. Control - Whether centralized control, distributed control or hierarchical control of participating nodes of computer network is suitable?

ISO-OSI 7-Layer Network Architecture

ISO-OSI layered architecture of Networks. It was first introduced in the late 1970s. An open system is a set of protocols that allow any two different systems to communicate regardless of

their underlying architecture. It is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable.

OSI model is a layered framework for the design of network systems that allows communication

between all types of computer system. According to the ISO standards, networks have been divided into 7 layers depending on the complexity of the functionality each of these layers provide.

1. Physical Layer

2. Data Link Layer 3. Network Layer

4. Transport Layer 5. Session Layer 6. Presentation Layer

7. Application Layer

Physical Layer

This layer is the lowest layer in the OSI model. It helps in the transmission of data between two machines that are communicating through a physical medium, which can be optical fibers,

copper wire or wireless etc. The following are the main functions of the physical layer:

1. Hardware Specification: The details of the physical cables, network interface cards, wireless radios, etc are a part of this layer.

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Coaxial Cable Hybrid Cable Wireless Card Network Card

2. Encoding and Signaling: How are the bits encoded in the medium is also decided by

this layer. For example, on the copper wire medium, we can use different voltage levels for a certain time interval to represent '0' and '1'. We may use +5mV for 1nsec to represent '1' and -5mV for 1nsec to represent '0'.

3. Data Transmission and Reception: The transfer of each bit of data is the responsibility of this layer. This layer assures the transmission of each bit with a high probability. The transmission of the bits is not completely reliable as there is no error correction in this

layer. 4. Topology and Network Design: The network design is the integral part of the physical

layer. Which part of the network is the router going to be placed, where the switches will be used, where we will put the hubs, how many machines is each switch going to handle, what server is going to be placed where, and many such concerns are to be taken care of

by the physical layer. The various kinds of topologies that we decide to use may be ring, bus, star or a hybrid of these topologies depending on our requirements.



Data Link Layer

This layer provides reliable transmission of a packet by using the services of the physical layer which transmits bits over the medium in an unreliable fashion. This layer is concerned with :

1. Framing : Breaking input data into frames (typically a few hundred bytes) and caring about the frame boundaries and the size of each frame.

2. Acknowledgment : Sent by the receiving end to inform the source that the frame was

received without any error. 3. Sequence Numbering : To acknowledge which frame was received.

4. Error Detection : The frames may be damaged, lost or duplicated leading to errors. The error control is on link to link basis.

5. Retransmission : The packet is retransmitted if the source fails to receive

acknowledgment. 6. Flow Control : Necessary for a fast transmitter to keep pace with a slow receiver.

Network Layer

Its basic functions are routing and congestion control.

Routing: This deals with determining how packets will be routed (transferred) from source to

destination. It can be of three types :

Static : Routes are based on static tables that are "wired into" the network and are rarely changed.

Dynamic : All packets of one application can follow different routes depending upon the

topology of the network, the shortest path and the current network load. Semi-Dynamic : A route is chosen at the start of each conversation and then all the

packets of the application follow the same route.



The services provided by the network can be of two types :

Connection less service: Each packet of an application is treated as an independent entity. On each packet of the application the destination address is provided and the packet is routed.

Connection oriented service: Here, first a connection is established and then all packets

of the application follow the same route. To understand the above concept, we can also draw an analogy from the real life. Connection oriented service is modeled after the

telephone system. All voice packets go on the same path after the connection is established till the connection is hung up. It acts like a tube ; the sender pushes the objects in at one end and the receiver takes them out in the same order at the other end.

Connection less service is modeled after the postal system. Each letter carries the destination address and is routed independent of all the others. Here, it is possible that the

letter sent first is delayed so that the second letter reaches the destination before the first letter.

Congestion Control: A router can be connected to 4-5 networks. If all the networks send packet

at the same time with maximum rate possible then the router may not be able to handle all the packets and may drop some/all packets. In this context the dropping of the packets should be minimized and the source whose packet was dropped should be informed. The control of such

congestion is also a function of the network layer. Other issues related with this layer are transmitting time, delays, jittering.

Internetworking: Internetworks are multiple networks that are connected in such a way that they act as one large network, connecting multiple office or department networks. Internetworks

are connected by networking hardware such as routers, switches, and bridges. Internetworking is a solution born of three networking problems: isolated LANs, duplication of resources, and the lack of a centralized network management system. With connected LANs, companies no longer



have to duplicate programs or resources on each network. This in turn gives way to managing the network from one central location instead of trying to manage each separate LAN. We should be

able to transmit any packet from one network to any other network even if they follow different protocols or use different addressing modes.

Network Layer does not guarantee that the packet will reach its intended destination. There are

no reliability guarantees.

Transport Layer

Its functions are :

Multiplexing / Demultiplexing : Normally the transport layer will create distinct network connection for each transport connection required by the session layer. The

transport layer may either create multiple network connections (to improve throughput) or it may multiplex several transport connections onto the same network connection

(because creating and maintaining networks may be expensive). In the latter case, demultiplexing will be required at the receiving end. A point to note here is that communication is always carried out between two processes and not between two

machines. This is also known as process-to-process communication. Fragmentation and Re-assembly : The data accepted by the transport layer from the

session layer is split up into smaller units (fragmentation) if needed and then passed to the network layer. Correspondingly, the data provided by the network layer to the transport layer on the receiving side is re-assembled.

Fragmentation Reassembly



Types of service : The transport layer also decides the type of service that should be provided to the session layer. The service may be perfectly reliable, or may be reliable

within certain tolerances or may not be reliable at all. The message may or may not be received in the order in which it was sent. The decision regarding the type of service to be provided is taken at the time when the connection is established.

Error Control : If reliable service is provided then error detection and error recovery operations are also performed. It provides error control mechanism on end to end basis.

Flow Control : A fast host cannot keep pace with a slow one. Hence, this is a mechanism to regulate the flow of information.

Connection Establishment / Release: The transport layer also establishes and releases

the connection across the network. This requires some sort of naming mechanism so that a process on one machine can indicate with whom it wants to communicate.

Session Layer

It deals with the concept of Sessions i.e. when a user logins to a remote server he should be

authenticated before getting access to the files and application programs. Another job of session layer is to establish and maintain sessions. If during the transfer of data between two machines the session breaks down, it is the session layer which re-establishes the connection. It also

ensures that the data transfer starts from where it breaks keeping it transparent to the end user. e.g. In case of a session with a database server, this layer introduces check points at various

places so that in case the connection is broken and reestablished, the transition running on the database is not lost even if the user has not committed. This activity is called Synchronization. Another function of this layer is Dialogue Control which determines whose turn is it to speak in

a session. It is useful in video conferencing.

Presentation Layer

This layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible for computers with different data representations to communicate data structures

to be exchanged can be defined in abstract way along with standard encoding. It also manages these abstract data structures and allows higher level of data structures to be defined an



exchange. It encodes the data in standard agreed way (network format). Suppose there are two machines A and B one follows 'Big Indian' and other 'Little Indian' for data representation. This

layer ensures that the data transmitted by one gets converted in the form compatible to other machine. This layer is concerned with the syntax and semantics of the information transmitted.

In order to make it possible for computers with different data representations to communicate data structures to be exchanged can be defined in abstract way along with standard encoding. It also manages these abstract data structures and allows higher level of data structures to be

defined an exchange. Other functions include compression, encryption etc.

Application Layer

The seventh layer contains the application protocols with which the user gains access to the network. The choice of which specific protocols and their associated functions are to be used at

the application level is up to the individual user. Thus the boundary between the presentation layer and the application layer represents a separation of the protocols imposed by the network

designers from those being selected and implemented by the network users. For example commonly used protocols are HTTP (for web browsing), FTP (for file transfer) etc.

Network Layers as in Practice

In most of the networks today, we do not follow the OSI model of seven layers. What is actually implemented is as follows. The functionality of Application layer and Presentation layer is

merged into one and is called as the Application Layer. Functionalities of Session Layer is not implemented in most networks today. Also, the Data Link layer is split theoretically into MAC

(Medium Access Control) Layer and LLC (Link Layer Control). But again in practice, the LLC layer is not implemented by most networks. So as of today, the network architecture is of 5 layers only.



Types of Medium

Medium can be classified into 2 categories.

1. Guided Media : Guided media means that signals is guided by the presence of physical

media i.e. signals are under control and remains in the physical wire. For e.g. copper wire.

2. Unguided Media : Unguided Media means that there is no physical path for the signal to

propagate. Unguided media are essentially electro-magnetic waves. There is no control on flow of signal. For e.g. radio waves.

Communication Links

In a network nodes are connected through links. The communication through links can be

classified as

1. Simplex : Communication can take place only in one direction. e.g. T.V broadcasting. 2. Half-duplex : Communication can take place in one direction at a time. Suppose node A

and B are connected then half-duplex communication means that at a time data can flow from A to B or from B to A but not simultaneously. e.g. two persons talking to each other such that when speaks the other listens and vice versa.

3. Full-duplex : Communication can take place simultaneously in both directions. eg. A discussion in a group without discipline.

Links can be further classified as

1. Point to Point : In this communication only two nodes are connected to each other.

When a node sends a packet then it can be received only by the node on the other side and none else.

2. Multipoint : It is a kind of sharing communication, in which signal can be received by all

nodes. This is also called broadcast.

Generally two kind of problems are associated in transmission of signals.

1. Attenuation : When a signal transmits in a network then the quality of signal degrades as the signal travels longer distances in the wire. This is called attenuation. To improve

quality of signal amplifiers are used at regular distances. 2. Noise : In a communication channel many signals transmits simultaneously, certain

random signals are also present in the medium. Due to interference of these signals our signal gets disrupted a bit.

Bandwidth

Bandwidth simply means how many bits can be transmitted per second in the communication channel. In technical terms it indicates the width of frequency spectrum.



Guided Transmission Media In Guided transmission media generally two kind of materials are used.

1. Copper o Coaxial Cable o Twisted Pair

2. Optical Fiber

1. Coaxial Cable: Coaxial cable consists of an inner conductor and an outer conductor which are separated by an insulator. The inner conductor is usually copper. The outer

conductor is covered by a plastic jacket. It is named coaxial because the two conductors are coaxial. Typical diameter of coaxial cable lies between 0.4 inch to 1 inch. The most application of coaxial cable is cable T.V. The coaxial cable has high bandwidth,

attenuation is less.

2. Twisted Pair: A Twisted pair consists of two insulated copper wires, typically 1mm

thick. The wires are twisted together in a helical form the purpose of twisting is to reduce cross talk interference between several pairs. Twisted Pair is much cheaper then coaxial

cable but it is susceptible to noise and electromagnetic interference and attenuation is large.

Twisted Pair can be further classified in two categories:

Unshielded twisted pair: In this no insulation is provided, hence they are susceptible to interference. Shielded twisted pair: In this a protective thick insulation is provided but shielded

twisted pair is expensive and not commonly used.



The most common application of twisted pair is the telephone system. Nearly all telephones are connected to the telephone company office by a twisted pair. Twisted pair

can run several kilometers without amplification, but for longer distances repeaters are needed. Twisted pairs can be used for both analog and digital transmission. The

bandwidth depends on the thickness of wire and the distance travelled. Twisted pairs are generally limited in distance, bandwidth and data rate.

3. Optical Fiber: In optical fiber light is used to send data. In general terms presence of light is taken as bit 1 and its absence as bit 0. Optical fiber consists of inner core of either

glass or plastic. Core is surrounded by cladding of the same material but of different refractive index. This cladding is surrounded by a plastic jacket which prevents optical

fiber from electromagnetic interference and harshly environments. It uses the principle of total internal reflection to transfer data over optical fibers. Optical fiber is much better in bandwidth as compared to copper wire, since there is hardly any attenuation or

electromagnetic interference in optical wires. Hence there is less requirement to improve quality of signal, in long distance transmission. Disadvantage of optical fiber is that end

points are fairly expensive. (eg. switches)

Differences between different kinds of optical fibers:

1. Depending on material Made of glass

Made of plastic. 2. Depending on radius

Thin optical fiber

Thick optical fiber 3. Depending on light source

LED (for low bandwidth) Injection laser diode (for high bandwidth)

Wireless Transmission

1. Radio: Radio is a general term that is used for any kind of frequency. But higher frequencies are usually termed as microwave and the lower frequency band comes under

radio frequency. There are many application of radio. For eg. cordless keyboard, wireless LAN, wireless Ethernet but it is limited in range to only a few hundred meters.

Depending on frequency radio offers different bandwidths. 2. Terrestrial microwave: In terrestrial microwave two antennas are used for

communication. A focused beam emerges from an antenna and is received by the other

antenna, provided that antennas should be facing each other with no obstacle in between. For this reason antennas are situated on high towers. Due to curvature of earth terrestrial

microwave can be used for long distance communication with high bandwidth. Telecom department is also using this for long distance communication. An advantage of wireless communication is that it is not required to lay down wires in the city hence no

permissions are required.



3. Satellite communication: Satellite acts as a switch in sky. On earth VSAT(Very Small Aperture Terminal) are used to transmit and receive data from satellite. Generally one

station on earth transmits signal to satellite and it is received by many stations on earth. Satellite communication is generally used in those places where it is very difficult to

obtain line of sight i.e. in highly irregular terrestrial regions. In terms of noise wireless media is not as good as the wired media. There are frequency band in wireless communication and two stations should not be allowed to transmit simultaneously in a

frequency band. The most promising advantage of satellite is broadcasting. If satellites are used for point to point communication then they are expensive as compared to wired

media.

Digital Data Communication Techniques:

For two devices linked by a transmission medium to exchange data, a high degree of co-

operation is required. Typically data is transmitted one bit at a time. The timing (rate, duration, pacing) of these bits must be same for transmitter and receiver. There are two options for

transmission of bits.

1. Parallel All bits of a byte are transferred simultaneously on separate parallel wires. Synchronization between multiple bits is required which becomes difficult over large distance. Gives large band width but expensive. Practical only for devices close to each

other. 2. Serial Bits transferred serially one after other. Gives less bandwidth but cheaper. Suitable

for transmission over long distances.



Transmission Techniques:

1. Asynchronous: Small blocks of bits (generally bytes) are sent at a time without any time relation between consecutive bytes. When no transmission occurs a default state is

maintained corresponding to bit 1. Due to arbitrary delay between consecutive bytes, the time occurrences of the clock pulses at the receiving end need to be synchronized for

each byte. This is achieved by providing 2 extra bits start and stop.

Note: Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their duration are the same.

Start bit: It is prefixed to each byte and equals 0. Thus it ensures a transition from 1 to 0

at onset of transmission of byte. The leading edge of start bit is used as a reference for generating clock pulses at required sampling instants. Thus each onset of a byte results in resynchronization of receiver clock.

Stop bit: To ensure that transition from 1 to 0 is always present at beginning of a byte it

is necessary that default state be 1. But there may be two bytes one immediately following the other and if last bit of first byte is 0, transition from 1 to 0 will not occur.

Therefore a stop bit is suffixed to each byte equaling 1.

Advantages Disadvantages

Asynchronous

transmission

Simple, doesn't require synchronization of both

communication sides

Cheap, timing is not as critical as for synchronous

transmission, therefore hardware can be made cheaper

Set-up is faster than other transmissions, so well suited for

applications where messages are generated at irregular

intervals, for example data entry from the keyboard

Large relative overhead, a

high proportion of the

transmitted bits are uniquely

for control purposes and

thus carry no useful

information

Synchronous

transmission Lower overhead and thus, greater throughput Slightly more complex

Hardware is more expensive



2. Synchronous - Larger blocks of bits are successfully transmitted. Blocks of data are either treated as sequence of bits or bytes. To prevent timing drift clocks at two ends need

to be synchronized. This can done in two ways: 1. Provide a separate clock line between receiver and transmitter. OR

2. Clocking information is embedded in data signal i.e. biphase coding for digital signals.

Diagram



Multiplexing

When two communicating nodes are connected through a media, it generally happens that

bandwidth of media is several times greater than that of the communicating nodes. Transfer of a

single signal at a time is both slow and expensive. The whole capacity of the link is not being

utilized in this case. This link can be further exploited by sending several signals combined into

one. This combining of signals into one is called multiplexing.

1. Frequency Division Multiplexing (FDM): This is possible in the case where transmission media has a bandwidth greater than the required bandwidth of signals to be

transmitted. It is an analog technique. A number of signals can be transmitted at the same time. Each source is allotted a frequency range in which it can transfer it's signals, and a

suitable frequency gap is given between two adjacent signals to avoid overlapping. This is type of multiplexing is commonly seen in the cable TV networks.

2. Time Division Multiplexing (TDM): This is possible when data transmission rate of the media is much higher than that of the data rate of the source. It is digital process.

Multiple signals can be transmitted if each signal is allowed to be transmitted for a definite amount of time. These time slots are so small that all transmissions appear to be

in parallel. 1. Synchronous TDM: Time slots are reassigned and are fixed. Each source is

given it's time slot at every turn due to it. This turn may be once per cycle, or

several turns per cycle, if it has a high data transfer rate, or may be once in a no. of cycles if it is slow. This slot is given even if the source is not ready with data.

So this slot is transmitted empty.



2. Asynchronous TDM: In this method, slots are not fixed. They are allotted dynamically depending on speed of sources, and whether they are ready for transmission.

TCP/IP Reference Model

TCP/IP originated out of the investigative research into networking protocols that the US Department of Defense (DoD) initiated in 1969. In 1968, the DoD Advanced Research Projects

Agency (ARPA) began researching the network technology that is called packet switching.

The original focus of this research was that the network be able to survive loss of subnet hardware, with existing conversations not being broken off. In other words, DoD wanted

connections to remain intact as long as the source and destination nodes were functioning, even if some of the machines or transmission lines in between were suddenly put out of operation. The

network that was initially constructed as a result of this research to provide a communication that could function in war time, and then called ARPANET, gradually became known as the Internet.



The TCP/IP protocols played an important role in the development of the Internet. In the early 1980s, the TCP/IP protocols were developed. In 1983, they became standard protocols for

ARPANET.

Because of the history of the TCP/IP protocol suite, it's often referred to as the DoD protocol suite or the Internet protocol suite.

TCP/IP model layers

Network Access Layer – The lowest layer of the TCP/IP protocol hierarchy. It defines how to use the network to transmit an IP datagram. Unlike higher-level protocols, Network Access

Layer protocols must know the details of the underlying network (its packet structure, addressing, etc.) to correctly format the data being transmitted to comply with the network

constraints. The TCP/IP Network Access Layer can encompass the functions of all three lower layers of the OSI reference Model (Physical, Data Link and Network layers). As new hardware technologies appear, new Network Access protocols must be developed so that

TCP/IP networks can use the new hardware. Consequently, there are many access protocols - one for each physical network standard.

Access protocol is a set of rules that defines how the hosts access the shared medium. Access protocols have to be simple, rational and fair for all the hosts. Functions performed at this level include encapsulation of IP datagram into the frames transmitted by the network, and mapping of

IP addresses to the physical addresses used by the network. One of TCP/IP's strengths is its universal addressing scheme. The IP address must be converted into an address that is

appropriate for the physical network over which the datagram is transmitted.

Internet layer – Provides services that are roughly equivalent to the OSI Network layer. The primary concern of the protocol at this layer is to manage the connections across networks as information is passed from source to destination. The Internet Protocol (IP) is the primary

protocol at this layer of the TCP/IP model.

Transport layer – It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. Two end-to-end transport protocols

have been defined here TCP and UDP Both protocols will be dicussed later.



Application Layer – includes the OSI Session, Presentation and Application layers as shown in the Figure. An application is any process that occurs above the Transport Layer. This includes all

of the processes that involve user interaction. The application determines the presentation of the data and controls the session. There are numerous application layer protocols in TCP/IP,

including Simple Mail Transfer Protocol (SMTP) and Post Office Protocol (POP) used for e-mail, Hyper Text Transfer Protocol (HTTP) used for the World-Wide-Web, and File Transfer Protocol (FTP).

Network standardization

Many Network vendors and suppliers exist, each with its own ideas of how things should be

done. Without coordination, there would be complete chaos, and users would get nothing done. The only way out is to agree on some network standards.

Not only do standards allow different computers to communicate, but they also increase the market for products adhering to the standard. A large market leads to mass production,

economies of scale in manufacturing and other benefits that decrease price and further increase acceptance.

Standards fall into two categories: de facto and de jure.

De facto (Latin for “from the fact”) standards are those that have just happened, without any

formal plan. The IBM PC and its successors are de facto standards for small-office and home computers because dozens of manufactures chose to copy IBM’s machines very closely.

Similarly, UNIX is the de facto standard for operating system in university computer science departments.

De Jure (Latin for “by law”) standards, in contrast, are formal, legal standards adopted by some authorized standardization body. International standardization authorities are generally divided

into two classes: those established by treaty among national governments, and those comprising voluntary, non treaty organization.



1. Who’s who in the telecommunication world?

The legal status of the world’s telephone companies varies considerably from country to

country. In1865, representatives from many European governments met to from the predecessor

to today’s ITU( International Telecommunication Union).

ITU has three main sectors:

Radio communication Sector (ITU-R)

Telecommunication Standardization Sector (ITU-T)

Development Sector ( ITU-D)

2. Who’s who in the International Standards world?

International standards are produced publish by ISO, a voluntary non treaty organization founded

in 1946.The U.S representative in ISO is ANSI ( American National Standards Institute ),

which despite its name, is a private, non governmental, nonprofit organization. Its members are

manufactures, common carriers, and other interested parties.

NIST (National Institute of Standards and Technology) is part of the U.S. department of

Commerce. It is used to be the National Bureau of Standards. It issues standards that are

mandatory for purchase made by the U.S. Govt. except for those of the Department of Defense,

which has its own standards.

Another major player in the standards world is IEEE (Institute of Electrical and Engineering

Engineers), the largest professional organization in the world. In addition to publishing scores of

journals and running hundreds of conferences each year, IEEE has a standardization group that

develops standards in the area of electrical engineering and computing.

3. Who’s who in the Internet Standards world?

The worldwide internet has its own standardization mechanisms, very different from those of

ITU-T and ISO. ITU-T and ISO meetings are populated by corporate officials and government

civil servants for whom standardization is their job.

When the ARPANET was set up, DoD created an informal committee to oversee it. In 1983, the

committee was renamed the IAB ( Internet Activities Board) and was given a slighter broader

mission, namely, to keep the researchers involved with the ARPANET and the internet pointed

more-or-less in the same direction, an activity not unlike herding cats. The meaning of the

acronym “IAB” was later changed to Internet Architectures Board.



Number Topic 802.1 Overview and architecture of LANs 802.2 Logical Link Control 802.3 Ethernet 802.4 Token Bus 802.5 Token Ring 802.6 Dual queue dual bus 802.7 Technical advisory group on broadband technologies 802.8 802.9 802.10 802.11 802.12 802.13 802.14 802.15 802.16 802.17

Technical advisory group on fiber optic technologies Isochronous LANs Virtual LANs and security Wireless LANs Demand Priority Unlucky Number Cable Modems Personal area network (Bluetooth) Broadband wireless Resilient packet Ring

Connection-Oriented Networks: X.25, Frame Relay, and ATM

Since the beginning of networking, a war has been going on between the people who support

connectionless (i.e., datagram) subnets and the people who support connection-oriented subnets.

The main proponents of the connectionless subnets come from the ARPANET/Internet

community. Remember that DoD’s original desire in funding and building the ARPANET was to

have a network that would continue functioning even after multiple direct hits by nuclear

weapons wiped out numerous routers and transmission lines. Thus, fault tolerance was high on

their priority list; billing customers was not. This approach led to a connectionless design in

which every packet is routed independently of every other packet. As a consequence, if some

routers go down during a session, no harm is done as long as the system can reconfigure itself

dynamically so that subsequent packets can find some route to the destination, even if it is

different from that which previous packets used.

The connection-oriented camp comes from the world of telephone companies. In the telephone

system, a caller must dial the called party’s number and wait for a connection before talking or

sending data. This connection setup establishes a route through the telephone system that is

maintained until the call is terminated. All words or packets follow the same route. If a line or

switch on the path goes down, the call is aborted. This property is precisely what the DoD’s did

not like about it.

Why do the telephone companies like it then? There are two reasons:

1. Quality of Service

2. Billing



By setting up a connection in advance, the subnet can reserve resources such as buffer space and

router CPU capacity. If an attempt is made to set up a call and insufficient resources are

available, the call is rejected and the caller gets a kind of busy signal. In this way, once a

connection has been set up, the connection will get good service. With a connectionless network,

if too many packets arrive at the same router at the same moment, the router will choke and

probably lose packets. The sender will eventually notice this and resend them, but the quality of

service will be jerky and unsuitable for audio and video unless the network is very lightly loaded.

Needless to say, providing adequate audio quality is something telephone companies care about

very much, hence their preference for connections.

The second reason the telephone companies like connection-oriented service is that they are

accustomed to charging for connect time. When you make a long distance call you are charged

by the minute. When networks came around, they just automatically gravitated toward a model

in which charging by the minute was easy to do. If you have to set up a connection before

sending data, that is when the billing clock starts running. If there is no connection, they cannot

charge for it.

It should come as no surprise that all networks designed by the telephone industry have led

connection-oriented subnets. But surprising is that the Internet is also drifting in that direction, in

order to provide a better QoS for audio and video.

Let us examine some connection-oriented networks.

X.25: is an example of connection – oriented network, which was the first public data network.

It was deployed in the 1970’s at a time when telephone service was a monopoly everywhere and

the telephone company in each country expected there to be one data network per country. To

use X.25, a computer first established a connection to the remote computer, that is, placed a

telephone call. This connection was given a connection number to be used in data transfer

packets. Data packets were very simple, consisting of a 3-byte header and up to 128 bytes of

data. The header consisted of a 12-bit connection number, a packet sequence number, an

acknowledgement number, and a few miscellaneous bits, X.25 networks operated for about a

decade with mixed success.

Frame Relay: In the 1980s, the X.25 networks were largely replaced by a new kind of network

called frame relay. The essence of frame relay is that it is a connection-oriented network with no

error control and no flow control. Because it was connection-oriented, packets were delivered in

order. The properties of in-order delivery, no error control, and no – flow control make frame

relay akin to a wide area LAN. Its most important application is interconnecting LANs at

multiple company offices. Frame Relay enjoyed a modest success and is still in use in places

today.

Asynchronous Transfer Mode: is a connection-oriented network. It was designed in the early

1990s and launched amid truly incredible hype. ATM was going to solve all the world’s



networking and telecommunication problems by merging voice, data, cable television, telex,

telegraph, carrier pigeon tin cans connected by strings, tom-toms, smoke signals and everything.

ATM Virtual circuits: Since ATM network are connection oriented sending data requires first

sending a packet to set up the connection. As the set up packet wends its way through the subnet

all the routers on the path make an entry in their internal tables noting the existence of the

connection and reserving whatever resources or needed for it. Connection is often called Virtual

circuits in analogy with the physical circuit used within the telephone system. Most ATM

networks also support permanent virtual circuits which are permanent connection between two

host. They are similar to leased line in the telephone world. Each connection, temporary or

permanent, has a unique connection identifiers. A Virtual Circuit is illustrated in Figure.

Once a connection has been established either site can began transmitting data. The basic idea

behind ATM is to transmit all information in small fixed size packets called cells. The cells are

53 bytes long, of which 5 bytes are header and 48 bytes are payload, as shown in fig. part of the

header is the connection identifier, so the sending end receiving host and all the intermediate

routers can tell which cells belong to which connection. This information allows each router to

know how to route each incoming cell. Cell routing is done in hardware at high speed. In fact,

the mean argument for having fixed size cell is that it is easy to build hardware routers to handle

short, fixed – length cell. Variable – length IP packets have to be routed by software, which is a

slower process. Another plus of ATM is that hardware can be set up to copy one incoming cell to

multiple output lines, a property that is required for handling television program that is being

broadcast to many receivers. Finally, small cells do not block any line for very long, which

makes guaranteeing quality of service easier.

All cells follow the same route to the destination. Cell delivery is not guaranteed but their order

is. If cells 1 and 2 are send that order, then if both arrive in that order, never first 2 then 1. But

either or both of them can be lost along the way. It is up to higher protocol levels to recover from

lost cells. No that although this guarantee is not perfect, It is better than what the internet

provides. There packets cannot only be lost, but delivered out of ordered as well. ATM in

contrast, guarantees never to deliver cells out of order.

ATM network are organized like traditional WANs, with lines and switches. The most common

speed for ATM networks is 155Mbps and 622 Mbps, although higher speed is also supported.

The 155- Mbps speed was chosen because this is about what is needed to transmit high definition

television

The ATM Reference model

ATM has its own reference model, different from the OSI model and also different from the

TCP/IP model. This model is shown fig. It consists of three layers, the physical, ATM, and ATM

adaptation layers, plus whatever users want to put on top of that.



The physical layer deals with the physical medium: voltage, bit timing, and various and other

issues. ATM does not prescribe a particular set of rules but instead says that ATM cells can be

sent on a right or fiber by theme service, but they can also be packaged inside the payload of

other carrier system. In other words, ATM has been designed to be independent of the

transmission medium.

The ATM Layer deals with cells and cell transport. It defines the layout of a cell and tells what

the header fields mean. It also deals with establishment and release of virtual circuits. Congestion

control is also located here.

A ATM layer has been defined to allow users to send packets larger than a cell. The ATM

interface segments these packets, transmits the cells individually, and reassembles them at the

other end. This layer is the AAL (ATM Adaptation Layer).

The user plane deals with transport, flow control, error correction and other user functions. In

contrast, the control plane is concerned with connection management. The layer and plane

management functions relate to resource ,management and interlayer coordination.

The physical layer and AAL layer are each divided into two sublayers, a one at the bottom that

does the work and a convergence sublayer on top that provides the proper interface to the layer

above it.

The PMD (Physical Medium Dependent) sublayer interfaces to the actual cable. It moves the bits

on and off and handles the bit timing. For different carriers and cables, this layer will be

different.

The other sublayer of the physical layer is the TC (Transmission Convergence) sublayer. When

cells are transmitted, the TC layer them as a string of bits to the PMD layer. Doing this is easy.

At the other end, the TC sublayer gets a pure incoming bit stream from the PMD sublayer.Its job

is to convert this bit stream into a cell stream from the PMD sublayer.

The AAL layer is split into a SAR (Segmentation And Reassembly) sublayer and a

CS(convergence Sublayer). The lower layer breaks up packets into cells on the transmission side

and puts them back together again at the destination. The upper sublayer makes it possible to

have ATM systems offer different kinds of services to different applications.

Diagram



Wireless IEEE 802.11

IEEE 802.11 is a set of standards for implementing wireless local area network (WLAN)

computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and

maintained by the IEEE LAN/MAN Standards Committee (IEEE 802). The base version of the

standard was released in 1997 and has had subsequent amendments. These standards provide the

basis for wireless network products using the Wi-Fi brand.

General Description:

The 802.11 family consist of a series of half-duplex over-the-air modulation techniques that use

the same basic protocol. The most popular are those defined by the 802.11b and 802.11g

protocols, which are amendments to the original standard. 802.11-1997 was the first wireless

networking standard, but 802.11b was the first widely accepted one, followed by 802.11g and

802.11n. 802.11n is a new multi-streaming modulation technique.

802.11b and 802.11g use the 2.4 GHz ISM band, operating in the United States under Part 15 of

the US Federal Communications Commission Rules and Regulations. Because of this choice of

frequency band, 802.11b and g equipment may occasionally suffer interference from microwave

ovens, cordless telephones and Bluetooth devices.

The segment of the radio frequency spectrum used by 802.11 varies between countries. In the

US, 802.11a and 802.11g devices may be operated without a license, FCC Rules and

Regulations. Frequencies used by channels one through six of 802.11b and 802.11g fall within

the 2.4 GHz amateur radio band.

History:

802.11 technologies has its origins in a 1985 ruling by the U.S. Federal Communications

Commission that released the ISM band for unlicensed use.

http://en.wikipedia.org/wiki/Wireless_LAN

http://en.wikipedia.org/wiki/IEEE_802.11y-2008

http://en.wikipedia.org/wiki/Institute_of_Electrical_and_Electronics_Engineers

http://en.wikipedia.org/wiki/Local_area_network

http://en.wikipedia.org/wiki/Metropolitan_area_network

http://en.wikipedia.org/wiki/IEEE_802

http://en.wikipedia.org/wiki/Wi-Fi

http://en.wikipedia.org/wiki/Half-duplex

http://en.wikipedia.org/wiki/Modulation

http://en.wikipedia.org/wiki/Hertz

http://en.wikipedia.org/wiki/ISM_band

http://en.wikipedia.org/wiki/Part_15_(FCC_rules)

http://en.wikipedia.org/wiki/Federal_Communications_Commission

http://en.wikipedia.org/wiki/Interference_(communication)

http://en.wikipedia.org/wiki/Microwave_oven

http://en.wikipedia.org/wiki/Microwave_oven

http://en.wikipedia.org/wiki/Cordless_telephone

http://en.wikipedia.org/wiki/Bluetooth

http://en.wikipedia.org/wiki/Radio_frequency

http://en.wikipedia.org/wiki/Amateur_radio

http://en.wikipedia.org/wiki/ISM_band



In 1991 NCR Corporation/AT&T (now Alcatel-Lucent and LSI Corporation) invented the

precursor to 802.11 in Nieuwegein, The Netherlands. The inventors initially intended to use the

technology for cashier systems. The first wireless products were brought to the market under the

name WaveLAN with raw data rates of 1 Mbit/s and 2 Mbit/s.

Vic Hayes, who held the chair of IEEE 802.11 for 10 years and has been called the "father of

Wi-Fi" was involved in designing the initial 802.11b and 802.11a standards within the IEEE.

Protocols:

In 1999, the Wi-Fi Alliance was formed as a trade association to hold the Wi-Fi trademark under

which most products are sold.

802.11 network standards

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needed]

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—

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SS 20 66 100 330

a

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1999

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6, 9, 12,

18, 24,

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54

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g

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http://en.wikipedia.org/wiki/NCR_Corporation

http://en.wikipedia.org/wiki/AT%26T_Corporation

http://en.wikipedia.org/wiki/Alcatel-Lucent

http://en.wikipedia.org/wiki/LSI_Corporation

http://en.wikipedia.org/wiki/Nieuwegein

http://en.wikipedia.org/wiki/WaveLAN

http://en.wikipedia.org/wiki/Vic_Hayes

http://en.wikipedia.org/wiki/Institute_of_Electrical_and_Electronics_Engineers

http://en.wikipedia.org/wiki/Wi-Fi_Alliance

http://en.wikipedia.org/wiki/Wi-Fi

http://en.wikipedia.org/wiki/IEEE_802.11#cite_note-80211timeline-6

http://en.wikipedia.org/wiki/Data_signaling_rate

http://en.wikipedia.org/wiki/Data_signaling_rate

http://en.wikipedia.org/wiki/IEEE_802.11#cite_note-7

http://en.wikipedia.org/wiki/MIMO

http://en.wikipedia.org/wiki/Wikipedia:Citation_needed




http://en.wikipedia.org/wiki/IEEE_802.11_(legacy_mode)

http://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum

http://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum

http://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum

http://en.wikipedia.org/wiki/IEEE_802.11a-1999

http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

http://en.wikipedia.org/wiki/IEEE_802.11#endnote_80211ns_37ghzA1

http://en.wikipedia.org/wiki/IEEE_802.11#endnote_80211ns_37ghzA2

http://en.wikipedia.org/wiki/IEEE_802.11b-1999


http://en.wikipedia.org/wiki/IEEE_802.11g-2003






n

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2009 2.4/5

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135,

150[B]

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Dec

2012

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up to

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The proposed standard had to work in two modes:

1. In the presence of a base station

2. In the absence of a base station

http://en.wikipedia.org/wiki/IEEE_802.11n-2009

http://en.wikipedia.org/wiki/IEEE_802.11#endnote_80211ns_sgiB1


http://en.wikipedia.org/wiki/IEEE_802.11#cite_note-wifiplanet-8

http://en.wikipedia.org/wiki/IEEE_802.11#endnote_80211ns_sgiB2

http://en.wikipedia.org/wiki/IEEE_802.11#cite_note-wifiplanet-8

http://en.wikipedia.org/wiki/IEEE_802.11ac



http://en.wikipedia.org/wiki/IEEE_802.11#cite_note-80211ac_Whitepaper_05112012-9




http://en.wikipedia.org/wiki/IEEE_802.11ad



In the former case, all communication was to go through the base station, called an access point

in 802.11 terminologies. In the latter case, the computers would just send to one another directly.

This mode is now sometimes called ad hoc networking. A typical example is two or more people

sitting down together in a room not equipped with a wireless LAN and having their computers

just communication directly.

The first decision was the easiest: what to call it. All the other LAN standards had numbers like

802.1,802.2, 802,3 up to 802.10, so the wireless LAN standard was dubbed 802.11. The rest was

harder.

First, a computer on Ethernet always listens to the ether before transmitting. Only if the ether is

idle does the computer begin transmitting. With wireless LANs, that idea does not work so well.

Suppose that computer A is transmitting to computer B, but the radio range of A’s transmitter is

too short to reach computer C. If C wants to transmit to B it can listen to the ether before starting,

but the fact that it does not hear anything does not mean that bits transmission will succeed. The

802.1 standard solve this problem.

The second problem that had to be solved is that a radio signal can be reflected off solid objects,

so it may be received multiple times. This interference results in what is called multipath fading.

The third problem is that a great deal of software is not aware of mobility. For example, many

word processors have a list of printers that users can choose from a print a file. When the

computer on which the word processor runs is taken into a new environment, the built- in list of

printers become invalid.

The fourth problem is that if a notebook computer is range of a different away from the ceiling-

mounted base station it is using and into the range of a different base station, some way of

handling it off is needed. Although telephones, it does not occur with Ethernet and needed to be

solved. In particular, the network envisioned consists of multiple cells, each its own base station,

but the base station connected by Ethernet. From the outside, the entire system should look like a

single Ethernet. The connection between the 80.11 system and the outside world is called a

portal.

After some work, the committee came up with a standard in 1977 that addressed these and other

concerns. The wireless LAN it described ran it either 1 Mbps or 2 Mbps. Almost immediately,

people complained that it was too slow, so work began on faster standards. A split developed

within the committee, resulting in two new standards in 1999. The 802.11a standard uses a wider

frequency band and runs at speed up to 54 Mbps. The 802.11b standard uses the same frequency

band as 802.11, but uses a different modulation technique to achieve 11 Mbps. Some people see

this as psychologically important since 11 Mbps is faster than the original wired Ethernet. It is

likely that the original 1 Mbps 802.11 will die off quickly, but it is not yet clear which of the new

standards will win out.

That 802.11 is going to cause a revolution in computing and Internet access is now beyond any

doubt. Airports, train, stations, hostels, shopping malls, and universities are rapidly installing it.



Intranet is shared content accessed by members within a single organization. Extranet is shared content accessed by groups through cross-enterprise boundaries.

Internet is global communication accessed through the Web

Internet

This is the world-wide network of computers accessible to anyone who knows their Internet

Protocol (IP) address - the IP address is a unique set of numbers (such as 209.33.27.100) that

defines the computer's location. To do this your browser (for example Netscape or Internet

Explorer) will access a Domain Name Server (DNS) computer to lookup the name and return an

IP address - or issue an error message to indicate that the name was not found. Once your

browser has the IP address it can access the remote computer. The actual server (the computer

that serves up the web pages) does not reside behind a firewall - if it did, it would be an Extranet.

It may implement security at a directory level so that access is via a username and password, but

otherwise all the information is accessible. To see typical security have a look at a sample secure

directory - the username is Dr and the password is Who(both username and password are case

sensitive).

Intranet

This is a network that is not available to the world outside of the Intranet. If the Intranet network

is connected to the Internet, the Intranet will reside behind a firewall and, if it allows access from

the Internet, will be an Extranet. The firewall helps to control access between the Intranet and

Internet to permit access to the Intranet only to people who are members of the same company or

organization.

In its simplest form, an Intranet can be set up on a networked PC without any PC on the network

having access via the Intranet network to the Internet.

For example, consider an office with a few PCs and a few printers all networked together. The

network would not be connected to the outside world. On one of the drives of one of the PCs

there would be a directory of web pages that comprise the Intranet. Other PCs on the network

could access this Intranet by pointing their browser (Netscape or Internet Explorer) to this

directory - for example

Extranet

An Extranet is actually an Intranet that is partially accessible to authorized outsiders. The actual

server (the computer that serves up the web pages) will reside behind a firewall. The firewall

helps to control access between the Intranet and Internet permitting access to the Intranet only to

people who are suitably authorized. The level of access can be set to different levels for

individuals or groups of outside users. The access can be based on a username and password or

an IP address (a unique set of numbers such as 209.33.27.100 that defines the computer that the

user is on).

http://www.hcidata.com/dr/12345678/index.htm

http://www.hcidata.com/dr/12345678/index.htm



System Network Architecture (SNA)

SNA is a computer networking architecture developed by IBM to provide a network structure for IBM mainframe, midrange, and personal computer systems. SNA defines a set of proprietary

communication protocols and message formats for the exchange and management of data on IBM host networks.

SNA defines methods that accomplish the following: Terminal access to mainframe and midrange computer applications. File transfer of data between computer systems.

Printing of mainframe and midrange data on SNA printers. Program-to-program communications that allow applications to exchange data over the

network.

SNA can be implemented in the following two network models:

Hierarchical The hierarchical SNA networking model, also called subarea networking, provides geographically disparate terminal users access to centralized mainframe processing

systems. In the hierarchical networking model, centralized host-based communication systems must provide the networking services for all users on the network. Peer-to-Peer The more recently developed Advanced Peer-to-Peer Networking (APPN) model

makes use of modern local area network (LAN) and wide area network (WAN) resources and client/server computing. APPN networking enables a form of distributed processing by allowing

any computer on the network to use SNA protocols to gain access to resources on any other computer on the network. Computers on an APPN network do not have to depend on mainframe-based communication services.

Because of the large installed base of legacy applications that run on IBM mainframe and midrange systems, both of these SNA networking models continue to be widely used in

enterprise networks.



SNA is IBM’s proprietary networking architecture, developed in the mid 1970s. SNA describes general characteristics of computer hardware and software required for interconnection.

Physical Control

This deals with electrical, mechanical, and procedural characteristics of the media and interfaces to the physical media, and is similar to the OSI physical layer.

Data Link Control

Similar to the data link layer, SDLC is defined here to allow for communication.

Path Control

Similar to the network layer, flow control and routing are defined and function here.

Transmission Control

Similar to the transport layer, the transmission control layer provides a connection service

from end to end that is reliable.

Data Flow Control

Request and response processing is done here (similar to the session layer).

Presentation Services

Resource sharing and data translation algorithms are performed here.

Transaction Services (NAU Services)

Application services are provided through programs (similar to the application layer).

SNA OSI Model

Transaction Services Application

Presentation Services Application, Presentation, and Session

Data Flow Control Session

Transmission Control Transport and Network

Path Control Network and Data Link

Data Link Control Data Link



Physical Control Physical

Architectural Components of an SNA System

Host nodes

Host nodes can be midrange and mainframe computers. Host nodes can control domains, which include one or more subareas. Each subarea contains nodes and peripherals.

Communication controller nodes

Also known as front end processors (FEPs), these devices route and control the flow of

data in a hierarchical structure.

Peripheral nodes

Peripheral nodes can include printers, terminals, and basically any client device.

Physical units

PUs are a combination of hardware, software, and firmware that manage and monitor the resources of a node. The following chart is a list of Pus.

Types of PUs

Type 1.0 Terminal node

Type 2.0 Terminals, printers, or any peripheral that can communicate with only a mainframe

Type 2.1 Minis, gateway devices, or any device that can communicate with a mainframe or another Type 2.1 device.

Type 4.0 Communication controllers that link lost mainframes and

cluster controllers or other type-2 devices

Type 5.0 Host computers

Logical Units ("Entities" of the network)

Units that have the ability to establish a connection with another logical unit and

exchange information. The following chart is a list of LU types:



Type 0 General purpose, used in program-to-program connections

Type 1 Card readers, printers, and batch-type terminals

Type 2 Terminals (like the 3270)

Type 3 3270 printers, similar to type 1

Type 4 Used in old peer-to-peer connections (6670 to 6670 connections and 5250 printers)

Type 6.0 and Type 6.1 Used in CICS to CICS or IMS connections between

mainframes

Type 7 Lus are 5270 display stations

Advanced Peer-to-Peer Networking (APPN)

The APPN protocol suite is a new SNA architecture that uses computer processing power

throughout the network using mainframes, minicomputers, and PCs. The following chart displays how the SNA architecture relates to the OSI model.

Digital Network Architecture (DNA)



DNA supports Digital Equipment Corporation (DEC) proprietary protocols and standards-based protocols. Products using DNA are referred to as DECnet products.

DNA Protocols

Ethernet version 2. Uses CSMA/CD, coax cable, and is based on the 3 Mbps Xerox Research Ethernet protocol. Ethernet 2 frames are slightly different from the Ethernet IEEE/ISO frame format.

High-Level Data Link Control (HDLC)

HDLC supports synchronous and asynchronous communication. It is a data link layer protocol

and defines both the format of the data frames and the commands needed to establish frame transfer.

Digital Data Communications Message Protocol (DDCMP)

Operates under asynchronous and synchronous communication and can be used in full- or half-

duplex communication.

Connectionless Network Service (CLNS)

Supported at the network layer, CLNS supports connection-oriented and connectionless network services. DNA Phase V (current version) uses CLNS.

Connection-Oriented Network Service (CONS)

Functions at the network layer, but for CLNS is more often used.

Connection Oriented Transport Protocol Specification

ISO 8073. Used to provide reliable connections at the transport layer.

Network Services Protocol (NSP)

Provides a connection-oriented, flow-controlled service through subchannels.

Session Control

Responsible for providing an interface between applications and transport layer protocols. Provides the following proprietary functions:

Address/name resolution

Protocol stack selection Transport connection management Connection identifier addressing



FTAM and Data Access Protocol (DAP)

ISO 8571. May be implemented differently by every vendor who writes FTAM applications. Used as a protocol for file services. DAP’s big difference compared to other protocols is its

ability to access indexed files.

Session Protocol Specification

ISO 8327. Provides a negotiated connection establishment and release and half-duplex data transfer. This protocol can use more than one transport layer connection for each session. Tokens

are used so a session’s dialog can be reset to any synchronization point.

Abstract Notation Syntax One

ISO 8824 (ASN.1) with ISO 8825 Basic Encoding Rules (BER). ASN.1 performs character code translation; BER defines rules for translating to and from ASN.1.

Network Virtual Terminal Service (NVTS)

Allows data to be translated to from local format to a network format before they are transmitted

to the host.

Message Handling System (MHS)

MAILbus Product Family and X.400. Provides specifications for DECnet message services. These are not protocols. MAILbus is a proprietary E-mail service developed by DEC. X.400

relates to how messages are stored and forwarded between different devices on an internetwork.

Naming Service and X.500 Directory

A portion of DNA that performs address/name resolution. X.500 is a directory service recommendation.

computer network unit i rgpv

Engineering

telephone networks

telephone system

telephone industry

bell telephone company

telephone receiver

telephone service

bhopal computer network

breakup of american