Computer Network Unit I RGPV

Introduction of computer network, Guided and unguided media,
examples of connection oriented,
TCP/IP vs OSI model

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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

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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.

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 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).

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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.

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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.

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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.

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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.).

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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.

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 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.

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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.

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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?

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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.

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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.

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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

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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

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 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

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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.

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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.

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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.

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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.

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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.

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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

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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

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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.

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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.

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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.

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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.

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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.

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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 connection-less 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

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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

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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.

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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

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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.

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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
802.11
protoc
ol
Release
[6]

Freq.
(GHz)
Bandwi
dth
(MHz)
Data
rate pe
r
stream
(Mbit/s
)
[7]

Allowable
MIMOstre
ams
Modulatio
n
Approxim
ate indoor
range
[citatio
n needed]

Approxima
te outdoor
range
[citation
needed]

(m) (ft) (m) (ft)
—
Jun
1997
2.4 20 1, 2 1
DSSS,FH
SS
20 66 100 330
a
Sep
1999
5
20
6, 9, 12,
18, 24,
36, 48,
54
1 OFDM
35 115 120 390
3.7
[A]
— —
5,00
0
16,000
[A]

b
Sep
1999
2.4 20
1, 2,
5.5, 11
1 DSSS 35 115 140 460
g
Jun
2003
2.4 20
6, 9, 12,
18, 24,
36, 48,
54
1
OFDM,DS
SS
38 125 140 460

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n
Oct
2009
2.4/5
20
7.2,
14.4,
21.7,
28.9,
43.3,
57.8,
65,
72.2
[B]
4
OFDM
70 230 250 820
[8]

40
15, 30,
45, 60,
90, 120,
135,
150
[B]

70 230 250 820
[8]

ac
(DRAF
T)
~Feb
2014
5
20
up to
87.6
[9]

8

40
up to
200
[9]


80
up to
433.3
[9]


160
up to
866.7
[9]


ad
Dec
2012
2.4/5/
60

up to
7000


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

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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.

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 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).

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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.

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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

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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:

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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)

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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

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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.