This slides compiled from
Computer Network by
Andrew S. Tenenbaum 4th Ed.

Introduction
➢Each of the past three centuries has been dominated by
a single technology
➢The 18th century was the era of the great mechanical
systems accompanying the Industrial Revolution.
➢The 19th century was the age of the steam engine.
➢During the 20th century, the key technology was
information gathering, processing, and distribution.

Introduction - Computer Nework
▪ A collection of autonomous computers
interconnected by a single technology.
▪ Two computers are said to be interconnected if they
are able to exchange information.
▪ The connection need not be via a copper wire; fiber
optics, microwaves, infrared, and communication
satellites can also be used.

Introduction - Computer Nework

Internet and World Wide Web
➢The internet is a massive network of networks, It
connects millions of computers together globally,
forming a network in which any computer can
communicate with any other computer as long as they
are both connected to the internet. Information that
travels over the internet does so via a variety of
languages known as protocols.
➢The World Wide Web, or simply web, is a way of
accessing information over the medium of the internet.

Internet and Distributed System
➢The Internet is not a single network, but
a network of networks
➢In a distributed system, a collection of
independent computers appears to its
users as a single coherent system.
➢A well known example of a distributed
system is the World Wide Web, in which
everything looks like a document (Web
page).

Uses of Computer Network
➢Business Applications
➢Home Applications
➢Mobile Users
➢Social Issues

Business Applications
➢ Many companies have a substantial number of computers. For
example, a company may have separate computers to monitor
production, keep track of inventories, and do the payroll
➢ The client and server machines are connected by a network, as illustrated in
Fig. 1-1. This whole arrangement is called the client-server model.
Fig. 1.1 Client Server Model

Business Applications – cont…
➢ The detailed client server model illustrated in Fig. 1-2.
➢ The company employee can share their information through e-mail.
➢ The company employee can share their ideas through videoconferencing.
➢ Many companies are doing business electronically with other companies,
especially suppliers and customers.
➢ Doing business with consumers over the Internet. Many companies
provide catalogs of their goods and services online and take orders on-
line. It is called e-commerce (electronic commerce).

Home Applications
➢ Initially, for word processing and games.
➢ more popular uses of the Internet for home users are as follows:
1. Access to remote information.
-surfing the World Wide Web for information
2. Person-to-person communication. (person-to-person communication )
-to distinguish it from the client-server model In this form,
individuals who form a loose group can communicate with others
in the group, as shown in Fig. 1-3.
3. Interactive entertainment.
It may be possible to select any movie or television program ever
made, in any country, and have it displayed on your screen
instantly.
4. Electronic commerce.
Home shopping is already popular and enables users to inspect
the on-line catalogs of thousands of companies. (Fig 1.4)

Mobile Users
➢ Mobile computers, such as notebook computers and personal digital
assistants (PDAs), are one of the fastest growing segments of the
computer industry.
➢ Food, drink, and other vending machines are found everywhere.
However, the food does not get into the machines by magic. Periodically,
someone comes by with a truck to fill them. If the vending machines
issued a wireless report once a day announcing their current inventories,
the truck driver would know which machines needed servicing and how
much of which product to bring.

Social Issues
➢newsgroups people can exchange messages with like-
minded individuals.
➢employee rights versus employer rights. Many people read
and write e-mail at work
➢The government does not have a monopoly on threatening
people's privacy. The private sector does its bit too.
(companies to track users' activities in cyberspace )
➢Computer networks offer the potential for sending
anonymous messages
➢A lot of these problems could be solved if the computer
industry took computer security seriously.

Network Hardware
➢ Criteria 1: Transmission Technology
- Broadcasting
- Point to point
➢ Criteria 2: Scale
- Personal Area Network (PAN)
- Local Area Network
- Metropolitan Area Network
- Wide Area Network
- Internetwork
- Wireless network (System Interconnection, Wireless
LAN, Wireless WANs)

Network Hardware Cont…
➢The network hardware can be classified into two criterion one
is transmission technology and another one is scale.
➢ There are two types of transmission technology that are in
widespread use. They are as follows:
1. Broadcast links.
2. Point-to-point links
➢ In computer networking, broadcasting refers to transmitting a
packet that will be received by every device on the network.
➢ Multicast is a group communication where data transmission is
addressed to a group of destination computers
simultaneously. Multicast can be one-to-many or many-to-
many distribution.

➢ point-to-point networks consist of many connections between
individual pairs of machines.
➢ To go from the source to the destination, a packet on this type
of network may have to first visit one or more intermediate
machines.
➢ Often multiple routes, of different lengths, are possible, so
finding good ones is important in point-to-point networks.
➢ Point-to-point transmission with one sender and one receiver is
sometimes called unicasting.

➢ The network hardware can be classified by their scale. In Fig. 1-6
we classify multiple processor systems by their physical size.

Personal Area Network
➢ A wireless network connecting a computer with its
mouse, keyboard, and printer is a personal area
network
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

➢ Local area networks, generally called LANs, are privately-
owned networks within a single building or campus of up to a
few kilometers in size.
➢ They are widely used to connect personal computers and
workstations in company offices and factories to share
resources (e.g., printers) and exchange information.
➢ LANs are distinguished from other kinds of networks by three
characteristics:
(1) their size (Distance)
(2) their transmission technology (broadcast or
point to point), and
(3) their topology (Bus or Ring).
Local Area Networks

➢ Various topologies are possible for broadcast LANs. Figure
1-7 shows two of them.
➢ A bus topology is a topology for a Local Area Network (LAN)
in which all the nodes are connected to a single cable.
➢ The cable to which the nodes connect is called a "backbone".
If the backbone is broken, the entire segment fails.
Local Area Networks

➢ A ring network is a network topology in which each node
connects to exactly two other nodes, forming a single
continuous pathway for signals through each node - a ring.
➢ Data travels from node to node, with each node along the way
handling every packet.
Local Area Networks

➢ A metropolitan area network, or MAN, covers a city. The best-
known example of a MAN is the cable television network
available in many cities.
➢ A metropolitan area network (MAN) is similar to a local area
network (LAN) but spans an entire city or campus.
➢ MANs are formed by connecting multiple LANs. Thus, MANs
are larger than LANs but smaller than wide area
networks (WAN).
➢ A metropolitan area network as shown in Figure 1-8.
Metropolitan Area Networks

➢ A wide area network, or WAN, spans a large geographical
area, often a country or continent.
➢ It contains a collection of machines or hosts.
➢ The hosts are connected by a communication subnet.
➢ The subnet is defined as the collection of routers and
communication lines that moved packets from the source host
to the destination host.
➢ The hosts are owned by the customers (e.g., people's personal
computers), whereas the communication subnet is typically
owned and operated by a telephone company or Internet
service provider.
Wide Area Networks

➢ In most wide area networks, the subnet consists of two distinct
components: transmission lines and switching elements or
routers.
➢ Transmission lines move bits between machines. They can be
made of copper wire, optical fiber, or even radio links.
➢ Switching elements are specialized computers that connect three
or more transmission lines
➢ A store-and-forward subnet is a message switching center in
which a message is accepted from the originating
user, i.e., sender, when it is offered, held in a physical storage,
and forwarded to the destination user, i.e., receiver
➢ Relation between host, subnet and router is shown in Figure 1-9.
Wide Area Networks

➢ A stream of packets from sender to receiver is represented in
Figure 1-10.
➢ In this figure, all the packets follow the route ACE, rather than
ABDE or ACDE. ACE is the best route.
Wide Area Networks

➢ A wireless network is a computer network that uses
wireless data connections between network nodes.
➢ wireless networks can be divided into three main categories:
1. System interconnection.
2. Wireless LANs.
3. Wireless WANs.
➢ System interconnection is all about interconnecting the
components of a computer using short-range radio.
➢ In the simplest form, system interconnection networks use the
master-slave paradigm of Fig. 1-11(a). The system unit is
normally the master, talking to the mouse, keyboard, etc., as
slaves.
➢ A WLAN, or wireless LAN, is a network that allows devices to
connect and communicate wirelessly. Fig. 1-11(b).
Wireless Networks

➢ The fundamental idea is that in the future most homes will be set
up for networking.
➢ Every device in the home will be capable of communicating with
every other device, and all of them will be accessible over the
Internet.
➢ Many devices are capable of being networked. Some of the more
obvious categories (with examples) are as follows:
1. Computers (desktop PC, notebook PC, PDA, shared peripherals).
2. Entertainment (TV, DVD, VCR, camera, stereo, MP3).
3. Telecommunications (telephone, mobile telephone, intercom)
4. Appliances (microwave, refrigerator, clock, lights)
5. Telemetry (utility meter, smoke/burglar alarm)
Home Area Networks

➢ A collection of interconnected networks is called an internetwork
or internet.
➢ A common form of internet is a collection of LANs connected by
a WAN.
➢ The combination of a subnet and its hosts forms a network. In the
case of a LAN, the cable and the hosts form the network.
Internetwork

➢ To reduce their design complexity, most networks are organized
as a stack of layers or levels, each one built upon the one below
it.
➢ The number of layers, the name of each layer, the contents of
each layer, and the function of each layer differ from network to
network.
➢ The purpose of each layer is to offer certain services to the higher
layers
Network Software - Protocol Hierarchies

Network Software - Protocol Hierarchies
➢ A five-layer network is illustrated in Fig.
➢ The entities comprising the corresponding
layers on different machines are called
peers. The peers may be processes,
hardware devices, or even human beings.
➢ virtual communication is shown by dotted
lines and physical communication by solid
lines.
➢ Between each pair of adjacent layers is an
interface.
➢ A set of layers and protocols is called a
network architecture.
➢ A list of protocols used by a certain
system, one protocol per layer, is called a
protocol stack.

Network Software – Design Issues
➢ Every layer needs a mechanism for identifying senders and
receivers. Since a network normally has many computers.
➢ The rules for data transfer. In some systems, data only travel in
one direction; in others, data can go both ways. And also it
specifies the priorities of data transfer either normal or urgent
data transfer.
➢ Error control is an important issue because physical
communication circuits are not perfect. Error correction and
detection mechanism to handle error during transfer.
➢ Not all communication channels preserve the order of messages
sent on them. It is possible to loss of Sequencing.

Network Software – Design Issues Cont…
➢ An issue that occurs at every level is how to keep a fast sender
from swamping a slow receiver with data.
➢ The inability of all processes to accept arbitrarily long messages.
This property leads to mechanisms for disassembling,
transmitting, and then reassembling messages.
➢ When there are multiple paths between source and destination, a
route must be chosen.

Network Software – Connection Oriented and
Connection less services
➢ Connection-oriented service is modeled after the telephone
system.
➢ The sender pushes objects (bits) in at one end, and the receiver
takes them out at the other end.
➢ In most cases the order is preserved so that the bits arrive in the
order they were sent.
➢ A reliable service is implemented through acknowledging
concept.
➢ The acknowledge concept introduces overhead and time delay. It
is not possible to lose the data.
➢ TCP is the example of connection oriented service.

Connection less services – Cont…
➢ Connectionless service is modeled after the postal system.
➢ Each message (letter) carries the full destination address, and
each one is routed through the system independent of all the
others.
➢ Normally, when two messages are sent to the same destination,
the first one sent will be the first one to arrive. Sometimes the
order may be varry.
➢ A non reliable service is implemented with faster file transfer.
➢ It is possible to lose the data.
➢ The Internet Protocol (IP) and User Datagram Protocol (UDP)
are connectionless protocols.

Connection less services – Cont…

Connection less services

Network Software – Service Primitives
➢ A service is formally specified by a set of primitives
(operations) available to a user process to access the service.
➢
➢ As a minimal example of the service primitives that might be
provided to implement a reliable byte stream in a client-server
environment, listed in Figure 1-17.

Network Software – Service Primitives Cont…
➢ First, the server executes LISTEN to indicate that it is
prepared to accept incoming connections.
➢ Next, the client process executes CONNECT to establish a
connection with the server.
➢ The next step is for the server to execute RECEIVE to prepare
to accept the first request.
➢ Then the client executes SEND to transmit its request
followed by the execution of RECEIVE to get the reply.
➢ If the data transferred successfully then it can use
DISCONNECT to terminate the connection.

Network Software – Service Primitives Cont…
➢ Figure. 1-18 briefly summarizes how client-server
communication might work over a connection-oriented
network.

Network Software – Relationships of Service and
Protocol
➢ Services and protocols are distinct concepts, although they
are frequently confused.
➢ A service is a set of primitives (operations) that a layer
provides to the layer above it.
➢ The service defines what operations the layer is prepared to
perform on behalf of its users.
➢ A service relates to an interface between two layers, with the
lower layer being the service provider and the upper layer
being the service user.
➢ A protocol is a set of rules governing the format and
meaning of the packets, or messages that are exchanged by
the peer entities within a layer.

Network Software – Relationships of Service and
Protocol
➢ services relate to the interfaces between layers, as illustrated
in Fig. 1-19. In contrast, protocols relate to the packets sent
between peer entities on different machines.

Reference Model
➢ OSI Reference Model
➢ TCP/IP Reference Model
➢ Difference between OSI and TCP/IP

OSI Reference Model
➢ The OSI model (minus the physical medium) is shown in
Fig. 1-20.
➢ This model is based on a proposal developed by the
International Standards Organization (ISO) as a first step
toward international standardization of the protocols used in
the various layers (Day and Zimmermann, 1983).
➢ It was revised in 1995 (Day, 1995). The model is called the
ISO OSI (Open Systems Interconnection) Reference Model
because it deals with connecting open systems—that is,
systems that are open for communication with other systems.

OSI Reference Model – Cont…

OSI Reference Model
The OSI model has seven layers. The principles of the seven
layers can be summarized as follows:
➢ Layers created for different abstractions
➢ Each layer performs well-defined function
➢ Function of layer chosen with definition of international
standard protocols in mind
➢ Minimize information flow across the interfaces between
boundaries
➢ Number of layers optimum

OSI Reference Model - The Physical Layer
➢ The physical layer is concerned with transmitting raw bits
over a communication channel.
➢ The design issues have to do with making sure that when one
side sends a 1 bit, it is received by the other side as a 1 bit,
not as a 0 bit.
➢ The design issues here largely deal with mechanical,
electrical, and timing interfaces, and the physical
transmission medium, which lies below the physical layer.

OSI Reference Model - The Data Link Layer
➢ The main task of the data link layer is to transform the input
data into data frames (typically a few hundred or a few
thousand bytes) and transmit the frames sequentially to the
network layer.
➢ If the service is reliable, the receiver confirms correct
receipt of each frame by sending back an acknowledgement
frame.
➢ The design issues here, how to keep a fast transmitter from
drowning a slow receiver in data.
➢ Broadcast networks have an additional issue in the data link
layer: how to control access to the shared channel. A special
sublayer of the data link layer, the medium access control
sublayer, deals with this problem.

OSI Reference Model - The Network Layer
➢ The network layer controls the operation of the subnet.
➢ A key design issue is determining how packets are routed
from source to destination.
➢ When a packet has to travel from one network to another to
get to its destination, many problems can arise.
• The addressing used by the second network may be
different from the first one.
• The second one may not accept the packet at all because
it is too large. The protocols may differ, and so on.
• It is up to the network layer to overcome all these
problems to allow heterogeneous networks to be
interconnected.

OSI Reference Model - The Transport Layer
➢ The basic function of the transport layer is to accept data
from the network layer and split it up into smaller units if
need be, pass these to the network layer, and ensure that the
pieces all arrive correctly at the other end.
➢ The transport layer also determines what type of service to
provide to the session layer, and, ultimately, to the users of
the network.
➢ The most popular type of transport connection is an error-
free point-to-point channel that delivers messages or bytes in
the order in which they were sent.

OSI Reference Model - The Transport Layer
➢ A program on the source machine carries on a conversation
with a similar program on the destination machine, using the
message headers and control messages.
➢ The difference between layers 1 through 3, which are
chained, and layers 4 through 7, which are end-to-end, is
illustrated in Fig. 1-20.

OSI Reference Model - The Session Layer
➢ The session layer allows users on different machines to
establish sessions between them.
➢ Sessions offer various services, including
▪ Dialog control (keeping track of whose turn it is to
transmit)
▪ Token management (preventing two parties from
attempting the same critical operation at the same time)
▪ Synchronization (checkpointing long transmissions to
allow them to continue from where they were after a
crash).

OSI Reference Model - The Presentation Layer
➢ The presentation 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, the data structures to be
exchanged can be defined in an abstract way, along with a
standard encoding to be used ''on the wire.''
➢ The presentation layer manages these abstract data structures
and allows higher-level data structures (e.g., banking
records), to be defined and exchanged.

OSI Reference Model - The Application Layer
➢ The application layer contains a variety of protocols that are
commonly needed by users.
➢ One widely-used application protocol is HTTP (Hyper Text
Transfer Protocol), which is the basis for the World Wide
Web.
➢ When a browser wants a Web page, it sends the name of the
page it wants to the server using HTTP.
➢ The server then sends the page back. Other application
protocols are used for file transfer, electronic mail, and
network news.

TCP/IP Reference Model
➢ The ARPANET (Advanced Research Project Agency
NETwork) was a research network sponsored by the DoD
(U.S. Department of Defense).
➢ It eventually connected hundreds of universities and
government installations, using leased telephone lines.
When satellite and radio networks were added later, the
existing protocols had trouble interworking with them, so
a new reference architecture was needed.
➢ Thus, the ability to connect multiple networks in a seamless
way was one of the major design goals from the very
beginning. This architecture later became known as the
TCP/IP Reference Model.

TCP/IP Reference Model - The Internet Layer
➢ The internet layer defines an official packet format and
protocol called IP (Internet Protocol).
➢ The TCP/IP internet layer is similar in functionality to the
OSI network layer. Figure 1-21 shows this correspondence.

TCP/IP Reference Model - The Transport Layer
➢ The layer above the internet layer in the TCP/IP model is
now usually called the 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.
➢ The first one, TCP (Transmission Control Protocol), is a
reliable connection-oriented protocol that allows a byte
stream originating on one machine to be delivered without
error on any other machine in the internet.

TCP/IP Reference Model - The Transport Layer
➢ The second protocol in this layer, UDP (User Datagram
Protocol), is an unreliable, connectionless protocol for
applications that do not want TCP's sequencing or flow
control and wish to provide their own.
➢ The relation of IP, TCP, and UDP is shown in Fig. 1-22.

TCP/IP Reference Model - The Application Layer
➢ The TCP/IP model does not have session or presentation
layers. On top of the transport layer is the application layer.
➢ It contains all the higher-level protocols. The early ones
included virtual terminal (TELNET), file transfer (FTP), and
electronic mail (SMTP), as shown in Fig. 1-22.
➢
➢ The virtual terminal protocol allows a user on one machine
to log onto a distant machine and work there.
➢ The file transfer protocol provides a way to move data
efficiently from one machine to another.
➢ Electronic mail was originally just a kind of file transfer, but
later a specialized protocol (SMTP) was developed for it.

TCP/IP Reference Model
The Host-to-Network Layer
➢ Below the internet layer is a great void.
➢ The TCP/IP reference model does not really say much about
what happens here, except to point out that the host has to
connect to the network using some protocol so it can send IP
packets to it.
➢ This protocol is not defined and varies from host to host and
network to network.

A Comparison of the OSI and TCP/IP Reference Models
➢ The OSI and TCP/IP reference models have much in
common.
➢ Both are based on the concept of a stack of independent
protocols. Also, the functionality of the layers is roughly
similar.
➢ Despite these fundamental similarities, the two models also
have many differences. Three concepts are central to the
OSI model:
1. Services.
2. Interfaces.
3. Protocols.

➢ Each layer performs some services for the layer above it.
The service definition tells what the layer does, not how
entities above it access it or how the layer works. It defines
the layer's semantics.
➢ A layer's interface tells the processes above it how to access
it. It specifies what the parameters are and what results to
expect. It, too, says nothing about how the layer works
inside.
➢ Finally, the peer protocols used in a layer are the layer's
own business. It can use any protocols it wants to, as long as
it gets the job done (i.e., provides the offered services). It can
also change them at will without affecting software in higher
layers.

➢ The difference between the two models is the number of
layers:
➢ The OSI model has seven layers; and
➢ The TCP/IP has four layers. Both have (inter)network,
transport, and application layers, but the other layers are
different.
➢ Another difference is type of communication.
➢ The OSI model performs the connection oriented
communication in the transport layer.
➢ The TCP/IP model performs connectionless
communication in the network layer. This choice is
especially important for simple request-response
protocols.

OSI Reference Model TCP/IP Reference Model
The Open System Interconnection
(OSI) model defines a networking
framework to implement protocols
in seven layers.
Transmission Control Protocol/Internet
Protocol (TCP/IP) is the language a
computer uses to access the internet.
It is a grandparent of All network
models.
It is a network model used in current
internet architecture.
7 Layer Architecture 4 Layer Architecture
It supports both connectionless and
connection-oriented communication
in the network layer, but only
connection oriented communication
in the transport layer.
The TCP/IP model has only one mode
in the network layer (connectionless)
but supports both modes in the
transport layer.

Example Networks
➢ The Internet
➢ The ARPANET
➢ NSFNET
➢ Internet Usage
➢ Architecture of the Internet
➢ Connection-Oriented Networks
➢ X.25,
➢ Frame Relay
➢ ATM
➢ Ethernet
➢ Wireless LANs: 802.11

Example Networks
➢ The subject of computer networking covers many
different kinds of networks, large and small, well
known and less well known.
➢ They have different goals, scales, and technologies.

The Internet
➢ The Internet is not a network at all, but a vast collection of
different networks that use certain common protocols and
provide certain common services.

The Internet - The ARPANET
➢ Nuclear War -
➢ Non-Distributed, i.e. switching telephone system used for
communication.
➢ Distributed system using digital packet switching system
proposed by borens, employee of RAND.
➢ Packet switching subnet established by robert by using Donald
Davies and Wesley Clark idea.
➢ Software implemented for Subnet host communication by using
BBN (Bolt, Beranek and Newman).
➢ Robert had problem to write software for host to host
communication. To solve this problem, he conducted a meeting
with students and network experts.
➢ Experimental Network established.
➢ Cerf and Kahn implemented a TCP/IP protocol for connecting
multiple network.

➢ The story begins in the late 1950s.
➢ At the height of the Cold War, the DoD (Department of Defence)
wanted a command-and-control network that could survive a
nuclear war.
➢ At that time, all military communications used the public
telephone network, which was considered vulnerable (affected).
➢ The structure of telephone system used by military as shown in
Fig. 1-25(a). Here the black dots represent telephone switching
offices, each of which was connected to thousands of telephones.
➢ These switching offices were, in turn, connected to higher-level
switching offices (toll offices), Note: There is no direct
connection from one switching office to another one switching

➢ Around 1960, the DoD (Department of Defence) awarded a
contract to the RAND Corporation to find a solution.
➢ One of its employees, Paul Baran, came up with the highly
distributed and fault-tolerant design of Fig. 1-25(b).
➢ Since the paths between any two switching offices were now
much longer than analog signals could travel without distortion.
➢ Baran proposed using digital packet-switching technology
throughout the system. But, it was unable to implement due to
rejection of AT & T.

➢ In 1963, ARPA, the Advanced Research Projects Agency is a
branch of DoD is established.
➢ Initially, ARPA had no scientists or laboratories. It did its work by
issuing grants and contracts to universities and companies whose
ideas looked promising to it.
➢ In 1967, Computer Network expert Wesley Clark, suggested to
building a packet-switched subnet, giving each host its own
router, as illustrated in Fig. 1-10.
➢ In 1967, Robert published a research paper in the topic of
Multiple Computer Networks and Inter computer
Communication.

➢ At the same time, the similar system is designed and
implemented by the direction of Donald Davies at the National
Physical Laboratory (NPL) in England.
➢ It demonstrated that packet switching could be made to work.
Furthermore, it cited Baran's now discarded earlier work.
➢ From these ideas, Robert try to build ARPANET from the idea
gathered from Donald Davies.
➢ The subnet would consist of minicomputers called IMPs
(Interface Message Processors) connected by 56-kbps
transmission lines.
➢ For high reliability, each IMP would be connected to at least two
other IMPs.

➢ The subnet was to be a datagram subnet, so if some lines and
IMPs were destroyed, messages could be automatically rerouted
along alternative paths.
➢ In 1968, ARPA selected and awarded to BBN (Bolt, Beranek and
Newman) for building the subnet and write the subnet software.
➢ The software was split into two parts: subnet and host.
➢ The subnet software consisted of the IMP end of the host-IMP
connection, the IMP-IMP protocol, and a source IMP to
destination IMP protocol designed to improve reliability.
➢ The original ARPANET design is shown in Fig. 1-26.

➢ Outside the subnet, software was also needed, namely, the
host end of the host-IMP connection, the host-host protocol,
and the application software.
➢ It soon became clear that BBN felt that when it had accepted
a message on a host-IMP wire and placed it on the host-IMP
wire at the destination, its job was done.
➢ Robert had problem to write software for host to host
communication.
➢ To solve this problem, he conducted a meeting with students
and network experts.
➢ Finally, the experimental network went on the air in
December 1969 with four nodes: at UCLA (, UCSB, SRI, and
the University of Utah.

➢ These four were chosen because all had a large number of
ARPA contracts, and all had different and completely
incompatible host computers.
➢ Figure 1-27 shows how rapidly the ARPANET grew in the
first 3 years.

➢ This experiment also demonstrated that the existing
ARPANET protocols were not suitable for running over
multiple networks.
➢ In 1974, Cerf and Kahn implemented a TCP/IP protocol.
➢ During the 1980s, additional networks, especially LANs,
were connected to the ARPANET.
➢ As the scale increased, finding hosts became increasingly
expensive, so DNS (Domain Name System) was created to
organize machines into domains and map host names onto IP
addresses.

The Internet - NSFNET
➢ By the late 1970s, NSF (the U.S. National Science
Foundation) saw the enormous impact the ARPANET was
having on university research, allowing scientists across the
country to share data and collaborate on research projects.
➢ NSF decided to build a backbone network to connect its six
supercomputer centers, in San Diego, Boulder, Champaign,
Pittsburgh, Ithaca, and Princeton.
➢ Each supercomputer was given a little brother, consisting of
an LSI-11 microcomputer called a fuzzball.
➢ The fuzzballs were connected with 56-kbps leased lines and
formed the subnet, the same hardware technology as the
ARPANET used.

The Internet - NSFNET
➢ The software technology was different however: the fuzzballs
spoke TCP/IP right from the start, making it the first TCP/IP
WAN.
➢ NSFNET allows to thousands of universities, research labs,
libraries, and museums to access any of the supercomputers
and to communicate with one another.
➢ This setup is called NSFNET.
➢ After the success of NSFNET, the data can be transferred 448
kbps by using fiber optic channels and 1.5 mbps by using
router in 1990 and it is updated at the speed of 45 mbps by
ANSNET (Advanced Networks and Services).

The Internet - Internet Usage
➢ The number of networks, machines, and users connected to
the ARPANET grew rapidly after TCP/IP became the only
official protocol on January 1, 1983.
➢ When NSFNET and the ARPANET were interconnected, the
growth became exponential.
➢ Many regional networks joined up, and connections were
made to networks in Canada, Europe, and the Pacific.
➢ Traditionally (meaning 1970 to about 1990), the Internet and
its predecessors had four main applications:

1. E-mail. The ability to compose, send, and receive electronic
mail has been around since the early days of the ARPANET
and is enormously popular.
2. News. Newsgroups are specialized forums in which users
with a common interest can exchange messages. Thousands of
newsgroups exist, devoted to technical and nontechnical topics,
including computers, science, etc.
3. Remote login. Using the telnet, rlogin, or ssh programs, users
anywhere on the Internet can log on to any other machine on
which they have an account.
4. File transfer. Using the FTP program, users can copy files
from one machine on the Internet to another. Vast numbers of
articles, databases, and other information are available this way.

➢ In the 1990, the WWW (World Wide Web) is invented by
CERN physicist Tim Berners-Lee.
➢ Mosaic browser, written by Marc Andreessen at the National
Center for Supercomputer Applications.
➢ With the help of browser the user can share the audio, video,
image and text data.
➢ During the 1990s was fueled by companies called ISPs
(Internet Service Providers).
➢ These are companies that offer individual users at home the
ability to call up one of their machines and connect to the
Internet, thus gaining access to e-mail, the WWW, and other
Internet services.

➢ These companies signed up tens of millions of new users a
year during the late 1990s, completely changing the character
of the network from an academic and military playground to
a public utility, much like the telephone system.

The Internet - Architecture of the Internet
➢ The client calls his or her isp over a telephone line.
➢ Modem in the computers, converts the digital signal to analog
signal and pass it to the telephone line.
➢ The analog signals are removed from the telephone system
and injected to the ISP’s POP (Point of Presence) regional
network.
➢ Now the system is digital and packet switched.
➢ The ISP's regional network consists of interconnected routers
in the various cities the ISP serves.

➢ If the packet is destined for a host served directly by the ISP,
the packet is delivered to the host. otherwise, it is handed
over to the ISP's backbone operator.
➢ Telephone companies like AT&T and Sprint is called
Backbone operator.
➢ They operate large international backbone networks, with
thousands of routers connected by high-bandwidth fiber
optics. NAP (Network Access Point) is a room full of routers,
at least one per backbone.

Connection-Oriented Networks: X.25, Frame Relay, and
ATM
➢ The connection-oriented network comes from the world of
telephone companies. 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 did not like
about it.
➢ Why do the telephone companies like it then? There are two
reasons:
➢ 1. Quality of service. With a connection oriented network,
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.
➢ 2. Billing.: the telephone companies like connection-oriented
service is that they are accustomed to charging for connect
time.

Connection-Oriented Networks: X.25 and Frame Relay
➢ X.25 was a standard suite of protocols used for packet-
switched communications over a wide area network—
a WAN.
➢ The first example of a connection-oriented network is X.25,
which was the first public data network.
➢ 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 (because multiple connections could be
open at the same time).
➢ Data packets were very simple, consisting of a 3-byte header
and up to 128 bytes of data.

Connection-Oriented Networks: X.25 and Frame Relay
➢ The header consisted of a 12-bit connection number, a packet
sequence number, an acknowledgement number, and a few
miscellaneous bits.
➢
➢ 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 (if they were delivered at all). The properties of in-
order delivery, no error control, and no flow control make
frame relay to a wide area LAN.

Connection-Oriented Networks: Asynchronous Transfer
Mode
➢ The second example of a connection-oriented network is
ATM (Asynchronous Transfer Mode).
➢ In the telephone system, most transmission is synchronous
and ATM is not. ATM was designed in the early 1990s.
ATM Virtual Circuits
➢ Connections are often called virtual circuits, in analogy with
the physical circuits used within the telephone system.
➢ Each connection, temporary or permanent, has a unique
connection identifier. A virtual circuit is illustrated in
Fig. 1-30.

Mode
Figure 1-30. A virtual circuit

Mode
➢ Once a connection has been established, either side can
begin 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. 1-31.

1.5.3 Ethernet
➢ Ethernet is the traditional technology for connecting wired
local area networks (LANs), enabling devices to
communicate with each other via a protocol
➢ Aloha was the basis for Ethernet, a local area network
protocol. Norman Abramson is the inventor of the Aloha
system,. In pure Aloha, a user can transmit at any time but
risks collisions with other users' messages.
➢ A sketch of Ethernet architecture is given in Fig. 1-34.
Ethernet had a major improvement over ALOHANET:
before transmitting, a computer first listened to the cable to
see if someone else was already transmitting.

1.5.3 Ethernet
➢ If so, the computer held back until the current transmission
finished. Doing so avoids interfering with existing
transmissions, giving a much higher efficiency.
➢ ALOHANET did not work like this because it was
impossible for a terminal on one island to sense the
transmission of a terminal on a distant island. With a single
cable, this problem does not exist.

1.5.4 Wireless LANs: 802.11
➢ Almost as soon as notebook computers appeared, many
people had a dream of walking into an office and magically
having their notebook computer be connected to the Internet.
This work rapidly led to wireless LANs being marketed by a
variety of companies.
➢ The IEEE committee that standardized the wired LANs was
given the task of drawing up a wireless LAN standard.
➢ The standard it came up with was named 802.11. A common
slang name for it is WiFi (Wireless Fidelity).
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.

1.5.4 Wireless LANs: 802.11
➢ In the former case, all communication was to go through the
base station, called an access point in 802.11 terminology.
➢ In the latter case, the computers would just send to one
another directly. This mode is now sometimes called ad hoc
networking.
➢ The two modes are illustrated in Fig. 1-35.
➢

Lecture Notes - Introduction to Computer Network

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Lecture Notes - Introduction to Computer Network