Computer Netwroks -1
10CS55
Venugopala Rao A S
Dept of CSE
SMVITM Bantakal
Computer Networks-1
Text Books:
 Behrouz A. Forouzan,: Data Communication and Networking, 4th
Edition Tata McGraw-Hill, 2006.
Reference Books:
 Alberto Leon-Garcia and Indra Widjaja: Communication Networks Fundamental Concepts and Key architectures, 2nd Edition Tata McGrawHill, 2004.
 William Stallings: Data and Computer Communication, 8th Edition,
Pearson Education, 2007.
 Larry L. Peterson and Bruce S. Davie: Computer Networks – A Systems
Approach, 4th Edition, Elsevier, 2007.
 Nader F. Mir: Computer and Communication Networks, Pearson
Education, 2007.
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Unit 1
 Data Communications
 Networks
 The Internet
 Protocols & Standards
 Layered Tasks
 The OSI model, Layers in OSI model,
 TCP/IP Protocol suite
 Addressing.
7Hrs
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Data communication
 Communication:
 Sharing of information between two parties.
 Can be local or remote.
 Local communication- usually occurs face to face
 Remote communication- takes place over distance.
 Eg. Telephony, telegraphy, television etc.
 Data:
 Information presented in some form agreed upon two
communicating parties.
 Data communication:
 Exchange of data between two devices via some form of
transmission medium such as wire or cable.
 Here communication system consists of hardware (physical
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Venugopala Rao A S, SMVITM, Bantakal
Data communication
 Fundamental characteristics of communication:
 Four characteristics define the effectiveness of the
communication – Delivery, Accuracy, Timeliness, and Jitter.
data
 Delivery:
 The system must deliver data to the correct destination.
 Data must be received by the intended device or user and only by
that device or user.
 Accuracy:
 The system must deliver the data accurately.
 Data that have been altered in transmission and left uncorrected are
of no use.
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 Timeliness:
Data communication
 The system must deliver data in a timely manner.
 Data delivered late are useless.
 In the case of video and audio, timely delivery means
delivering data as they are produced, without significant
delay. This kind of delivery is called real-time transmission.
 Jitter:
 Jitter refers to the variation in the packet arrival time.
 It is the uneven delay in the delivery of audio or video
packets.
 Eg. Let us assume that video packets are sent every 30 ms. If
some of the packets arrive with 30-ms delay and others with
40-ms delay, an uneven quality in the video is resulted.
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Data communication
 Components of data communication system
 Data communication system has five components as shown
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Data communication
 Message:
 The message is the information (data) to be communicated.
 E.g. text, numbers, pictures, audio, and video.
 Sender:
 The sender is the device that sends the data message.
 E.g. A computer, workstation, telephone handset, camera etc
 Receiver:
 The receiver is the device that receives the message.
 E.g. Computer, workstation, telephone handset, television etc.
 Transmission medium:
 This is the physical path by which a message travels from sender to
receiver.
 E.g. : Twisted-pair wire, coaxial cable, fiber-optic cable, and radio
waves.
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Data communication
 Protocol:
 A protocol is a set of rules that govern data communications.
 It represents an agreement between the communicating devices.
 Without a protocol, two devices may be connected but not
communicating, just as a person speaking Hindi cannot be
understood by a person who speaks only Malayalam.
Data representation:

 Today information is available in various forms namely
 Text Numbers Images Audio-
 Video 9
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Data flow
 Communication between two devices can take up in any of
three ways namely simplex, half duplex and fully duplex.
 Simplex mode:
 Communication is unidirectional.
 Out of two devices only one can transmit and other can receive.
 E.g.
 Keyboard – can only input data to computer
 Monitor – can only display the data
 Here entire channel capacity is used to send the data in one direction
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Data flow
 Half duplex mode:
 Both devices can transmit the data but not at the same time
 When one device is transmitting, other will be in receiving mode
and vice versa as shown below.
 Similar to one-lane traffic.
 E.g. : walkie-talkie
 This mode can be used in situations when there is no need of using
the channel by both devices at the same time
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Data communication
 Full duplex mode:
 Here both devices can transmit and receive data at the same time.
 Signals going in one direction share the capacity of communication
link with the signals going in the other direction.
 Sharing can be achieved in two ways
 By having two separate transmission lines one for each side
 By dividing the capacity of the channel to half
 Eg. : Telephone network.
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Networks
 What is a Network?
 Network is a set of devices (called as nodes) connected by
communication link.
 A node can be a computer/Printer/ any other device capable of
sending/ receiving data generated by other nodes.
 Most of the networks use distributed processing – here a task is
divided among several computers so that a large process is shared
among them.
 Network criteria:
 A network should meet certain criteria like performance, reliability
and security.
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Networks
 Performance:
 Measured in terms of transit time and response time
 Transit time- amount of time required for a message to travel from
one device to another
 Response time – time between an inquiry and response to it.
 Various factors influence the performance
 No. of users
 Type of transmission medium
 Capacity of connected hardware
 Efficiency of software
 Performance is also measured in terms of throughput and delay.
 We desire more throughput and less delay for better performance
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Networks
 Reliability
 This is measured in terms of frequency of failure and time taken
by a link to recover from failure.
 Security
 This is one of the major criteria which includes
 Protecting data from unauthorized users
 Protecting data from damage and development
 Implementing policies and procedures to recover from distortion and
data loss.
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Networks
 Physical structure:
 We will learn how a network looks physically.
 Type of connection:
 Any network comprises of two or more devices connected using
some link to transfer data.
 Two possible ways to connect links.
 Point-to-point
 Multipoint
 Point-to-point connection:
 Provides a dedicated link between two devices
 Entire link capacity is reserved for transmission between these two
devices.
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Networks
 In most cases actual cable or wire is used to connect two
devices and in some cases, microwave signals and satellite
links are also used.
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Networks
 Multipoint connection:
 More than two devices share specific link
 Here channel capacity is shared either spatially or temporarily.
 If several devices use the link at the same time then it is spatial
sharing
 If one is used at some time interval and other for some other time
then it is called time shared.
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Networks
 Physical topology:
 Refers to the way in which network is laid physically.
 Topology is geometric representation of relationship of all the links
and linking devices.
 Four topologies possible: mesh, star, bus and ring.
 Mesh:
 Every device has dedicated point-to-point link to every other device.
 In a network of n nodes, there must be n(n-1)/2 duplex mode links
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Networks
 Advantages:
 Guarantees that each connection carries its own data load and thus
eliminates the traffic problem which may occur if connection is
shared by several devices.
 Network is robust- if one link is failed it does not effect the entire
network.
 Privacy and security- when a message travels along a dedicated link,
only intended recipient can see it.
 Point-to-point connection makes it easy to isolate faulty links and
correcting them is easy.
 Disadvantage:
 Main disadvantage is amount of cabling required is more.
 Secondly reconnection and installation is difficult
 Due to more no. of hardware, this is expensive.
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Networks
 Star topology
 Each device has one dedicated point-to-point link only to a central
controller called as hub.
 Here direct traffic between devices not allowed.
 If one device has some data to be sent to some other device then it
has to send to the hub. The hub sends the data to desired recipient.
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Networks
 Advantages:
 Less expensive
 Easy to install and maintain as only one cable is needed to be added
or removed to add a device or to remove a device.
 Robustness- if one link fails only that link is affected and hence it is
easy to isolate faulty link.
 Limitations:
 Entire system is depending on hub, if hub goes down the entire
system will be down.
 Applications:
 Normally used in LAN connections.
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Networks
 Bus topology:
 Example of multi-point topology
 One long cable acts as backbone which links all the devices and all
the nodes are connected by drop lines and taps.
 As signal travels along the backbone cable it is transferred into heat.
As it travels far and far, it gets weaker.
 So there is a limit on number of taps and distance between taps.
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Networks
 Advantages:
 Easy to install.
 As compared to star topology, here redundancy is reduced as single
backbone connects all the devices.
 Limitations:
 Difficulty in reconnection and fault isolation.
 Signal reflection at the tap degrades the signal quality and hence
there is a limit on number of devices to be connected and also on
spacing between devices.
 In case of fault or breakdown entire system collapses
 Applications:
 Used in design of early LANs
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Networks
 Ring topology:
 Each device has a dedicated point-to-point connection with only the
two devices on either side of it.
 A signal is passed along the ring in one direction, from device to
device, until it reaches its destination.
 Each device in the ring incorporates a repeater.
 When a device receives a signal intended for another device, its
repeater regenerates the bits and passes them further to the next
device.
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Networks
 Advantages:
 Easy to install and reconfigure as each device is connected only to
its neighbors
 Addition or removal of any device requires changing only two
connections
 Only constraint here is about maximum ring length and number of
devices in the ring.
 Fault isolation is simple
 Disadvantage:
 Unidirectional traffic
 In case of any failure in the cable can shut down entire system. This
can be avoided by using dual ring.
 Applications:
 IBM used this in its LAN.
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Networks
 Hybrid topology:
 Combination of various topologies as shown below
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Networks
 Categories of Networks
 Networks are classified based on their coverage into three
classifications: LAN, WAN, MAN
 Local Area Networks:
 Privately owned network which is used to link the devices in a
single office, building, or campus.
 Depending on the need and technology used, LAN can be set up
from very simple to complicated with more systems and peripheral
devices like audio an video etc.
 LAN size is limited to a few kilometres.
 LANs are designed to share resources ( hardware, or software)
between personal computers or workstations.
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Networks
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Networks
 Commonly found in business environments, academic
environments etc
 Usually LAN will use only one type of transmission medium
 Commonly used topologies in LAN are bus, ring and star.
 Early days of LAN had a speed of about 4 to 16Mbps and
today we have LANs with 100 to 1000Mbps.
 Wireless LAN is the latest evolution in LAN Technology.
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Networks
 Wide area networks (WAN):
 Facilitates long distance transmission of data, audio, video etc across
countries, continent or entire world.
 Two classifications:
 Switched WAN
 Point-to-point WAN
 Switched WAN usually connects end systems which consists of
routers that connects to another LAN or WAN
 The point-to-point WAN is normally a line leased from a telephone
or cable TV provider that connects a home computer or a small LAN
to an Internet service provider (ISP).
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Networks
 Examples:
 X.25, a network designed to connect end users, which is being
replaced by high speed networks.
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Networks
 Metropolitan Area Networks:
 Size lies between that of LAN and WAN
 Covers area inside a town.
 Usually designed for users who need high speed connectivity for
internet and have end points spread all over the city.
 Examples:
 High speed DSL line from telephone company
 TV network which can provide cable TV and also high speed
internet.
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Networks
 Interconnection of Networks – Internetwork:
 When more than two networks are connected it is called as
internetwork or simply internet.
 Here networks of different topologies, located at different locations
can be connected using backbone.
 The Internet:
 Using Internet we can send mail, do business transaction, get news
and many more.
 Internet is a collaboration of large number of interconnected
networks.
 Concept of Internet came into existence since 1969 only !!!!!
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 Brief history of Internet:
 Assignment!!!!!!
Internet
 Internet Today:
 Since 1960 till date Internet has come a long way.
 It consists of large number of WANs, LANs which are connected
by joining devices and switches.
 As Internet is growing daily it is not possible to give accurate
picture about it.
 Internet Service Providers(ISP) provide Internet to users.
 Different levels of ISP: International service provider, National
service provider, Regional service provider and local service
provider.
 Note that Internet is run by private companies.
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Internet
 The following figure shows the conceptual view of Internet.
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Internet
 International Internet Service Providers:
 They reside on the top in hierarchy and they connect nations
together to form larger networks.
 National Internet Service providers:
 These are backbone networks created and maintained by specialized
companies.
 The end users are connected to backbones using complex switching
stations called as national access points.
 The data rate is about 600Mbps.
 Regional Internet service providers:
 Smaller ISPs which are connected to one or more National ISPs
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Internet
 Local Internet Service Providers:
 Provide direct service to the end users
 These can be connected to regional ISP or national ISP.
 E.g.
 A company providing Internet service to its employees
 A college/university that maintains its own network which provides
Internet for the users.
 Note that each of these local ISPs are connected either to regional
or national ISP.
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Protocols and Standards
 Protocol:
 Set of rules that govern data communication.
 Protocol defines what is communicated, how it is communicated and
when it is communicated.
 There are few key elements of any protocol – Syntax, Semantics,
Timing.
 Syntax:
 The format or structure of the data
 It tells how the data is to be represented.
 E.g. for some protocol, first 8-bits may represent the address of receiver
and remaining part may the actual data.
 Semantics:
 Refers to the meaning of each section of bits
 i.e. how a bit pattern is to be interpreted and what action is to be taken
after interpretation
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Protocols and Standards
 Timing:
 It refers to time at which data is to be sent and also tells with what
speed data is to be sent.
 This is needed because if the receiver is slower that the sender there
may be possibility that the data may be lost.
 Standards:
 Needed to create and maintain an open and competitive market for
equipment manufactures.
 It also guarantees the interoperability of data and telecommunication
technology.
 Provides guidelines to manufactures, vendors, govt. agencies and
also to service providers to ensure the kind of interconnectivities
needed in the present marketplace and also in international
communications.
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Protocols and Standards
 Data communication standards fall into two categories.
 de facto (meaning by fact or by convention)
 de jure ( meaning by law or by regulation)
 de facto standards:
 These are the standards which are not being approved by any
organized body but have been adopted as standards through
widespread use
 Normally defined by manufacturers who wish to define the
functionality of a new product
 de jure standards:
 These have been legislated by some officially recognized body
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Protocols and Standards
 Standards organizations:
 Standards are developed with the help of standards creation
committees, forums and govt. regulatory agencies.
 Standards creation committees:
 There are many standard creation committees namely
 International Organization for Standardization (ISO).
 International Telecommunication Union -Telecommunication Standards Sector
(ITU-T).
 American National Standards Institute (ANSI).
 Institute of Electrical and Electronics Engineers (IEEE).
 Electronic Industries Association (EIA).
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Protocols and Standards
 Forums:
 The standards committees are procedural bodies and hence they are
slow in nature.
 But to cater the need for working models, agreements and also to
facilitate the standardization process, many special interest groups
have developed their own forums.
 These consist of people from various interested organizations and
these work with universities and users to test, evaluate and
standardize new technologies.
 Forums help to speed up the acceptance and use of new technology.
 Some times they also give their conclusion about any new
technology to the standard bodies.
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Protocols and Standards
 Regulatory agencies
 Every communication technology is subjected to the regulation of
govt. agencies.
 Purpose is to protect the public interest by regulating, radio,
television or wire/cable communications.
 E.g.
 Federal Communication Commission (FCC)
 Telecom regulatory Authority of India (TRAI)
 Internet Standards:
 Thoroughly tested specification that is useful to and followed by
those who use Internet.
 A specification undergoes a strict process to be called as Internet
standard.
 Specification starts as Internet draft
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Protocols and Standards
 Internet draft is a working document which has validity for 6
months.
 After recommendationsfrom Internet authorities, drafts are
published as Request for Comment.
 Each RFC is edited and given a number and then made
available for interested parties.
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Layered tasks
 In our daily life concept of layers can be seen in most of the
tasks we perform.
 Let us take up simple example of sending a letter to a friend
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 Hierarchy:
Layered tasks
 In the example, it can be seen that there are various tasks to be
performed at the sender side before letter is sent to the carrier.
 Similarly various tasks at the receiving side before letter reaches the
destination.
 Here the order of tasks are need to be followed as they are given
 Services:
 Each layer in the sending side uses the service of layer immediately
below it.
 i.e. sender uses services of higher layer, higher layer uses service of
middle layer and so on.
 In data communication, the most dominated layered model was Open
System Interconnect (OSI) model.
 But the introduction of TCP/IP protocol suit in 1990 took over the
major role in data communication over OSI model.
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OSI Model
 International standards Organization(ISO), a multinational body
dedicated to worldwide agreement on International standards.
 An ISO standard that covers all aspects of data communication is
OSI model.
 This was proposed in 1970.
 Open system is a set of protocols used for any two systems to
communicate irrespective of their architecture.
 The purpose of OSI model was to show the possibility of
communication between two different systems without changing
the underlying hardware or software.
 Note that OSI model is not protocol, it is a model to understand,
design a network which is flexible, robust and interoperable.
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OSI Model
 The OSI model consists of seven separate but related layers,
each of which defines a part of the process of moving
information across a network
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OSI Model
 Layered Architecture:
 In developing the model, the designers grouped the process of
transmitting data to its most fundamental elements.
 i.e. they identified which networking functions had related uses and
collected those functions into discrete groups known as layers.
 Here each layer defines a family of functions which is distinct from
the functions of the other layers.
 By defining and localizing functionality in this fashion, an
architecture is created that is both comprehensive and flexible.
 Most importantly, the OSI model allows complete interoperability
between incompatible systems.
 This model is shown in a detailed manner in the figure.
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OSI Model
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OSI Model
 Within a single machine, each layer calls upon the services
of the layer just below it.
 E.g.: Layer 3, uses the services provided by layer 2 and
provides services for layer 4.
 Between machines, layer x on one machine communicates
with layer x on another machine.
 This communication is governed by an agreed-upon series of
rules and conventions called protocols.
 The processes on each machine that communicate at a given
layer are called peer-to-peer processes.
 Communication between machines is therefore a peer-topeer process using the protocols appropriate to a given layer.
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OSI Model
 Peer-to-peer process:
 From the fig. it is clear that, at the physical layer, communication is
direct.
 i.e. device A sends bit stream to device B through intermediate
nodes.
 But at the higher level communication comes down through higher
layers on device A and moves upwards in layers of device B
 Each layer in sending side adds its own information to the data it
receives from the layer above it and sends the same to layer below it
 Finally at layer 1, entire message is converted to a form suitable for
transmission to receiving side.
 At receiving side, the message will be unwrapped layer by layer and
passed to next higher layer.
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OSI Model
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OSI Model
 Interfaces Between Layers:
 The passing data and network information between layers is made
possible by an interface between each pair of adjacent layers.
 Each interface defines the information and services a layer must
provide for the layer above it.
 As long as a layer provides the expected services to the layer above
it, the specific implementation of its functions can be modified or
replaced without requiring changes to the surrounding layers.
 Organization of layers:
 Seven layers are grouped into three groups
 Physical, data link an network- network support layers.
 This is because they deal with physical aspects of data movement
like physical connection, electrical specification, physical addressing
etc.
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OSI Model
 Session, presentation and application layers – user support
layers.
 They allow interoperability among unrelated software
systems.
 Layer 4, the transport layer links the above said two
subgroups and also ensures that, data transmitted by lower
layer is in a form which can be used by upper layers.
 Upper layers of OSI model are always implemented using
software only.
 Lower layers except physical layer are combination of both
hardware and software.
 Physical layer is commonly hardware.
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OSI Model
 Data Exchange using OSI model
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OSI Model
 In the figure, D7 means data unit at layer 7, D6 means data unit
at layer 6 and so on.
 Communication process starts at layer 7, and moves downwards
layer by layer.
 Each layer adds some information to the data it receives from
higher layer. These are known as headers or trailers.
 Finally after reaching layer 1, data is converted to EM signal and
transported via physical link.
 At receiving end, data is converted back to digital form and
moves upwards through layers.
 At every layer, the headers and trailers attached to the data by the
corresponding layer is removed and performs layer specific tasks
and sends data upwards for next layer.
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OSI Model
 Encapsulation:
 From the figure it can be seen that, data portion at any given layer N
contains the whole packet (data + header/trailer) from the layer (N-1).
 This is called as encapsulation.
 For example, a packet from layer 6 which consists of header and
data is together considered as single data unit and layer 5 adds its
header to this data packet.
 Layer (N-1) will not be aware of which part of encapsulated packet
is data an which part is header. It considers whole packet received
from layer N as single data unit.
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Layers in OSI Model
 Physical layer:
 The physical layer coordinates the functions required to carry a bit
stream over a physical medium.
 It deals with the mechanical and electrical specifications of the
interface and transmission medium.
 It also defines the procedures and functions of physical devices and
interfaces for the transmission to take place.
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Layers in OSI Model
 Other responsibilities of physical layer are Physical characteristics of interfaces and medium: The physical
layer defines the characteristics of the interface between the devices
and the transmission medium. It also defines the type of
transmission medium.
 Representation of bits: The physical layer data consists of a stream
of bits (sequence of 0’s or l’s) with no interpretation. To be
transmitted, bits must be encoded into signals---electrical or optical.
The physical layer defines the type of encoding (how 0’s and 1’s are
changed to signals).
 Data rate: (The transmission rate) - the number of bits sent each
second—is defined by the physical layer. In other words, the
physical layer defines the duration of a bit.
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Layers in OSI Model
 Synchronization of bits: The sender and receiver not only must use the
same bit rate but also must be synchronized at the bit level. In other
words, the sender and the receiver clocks must be synchronized.
 Line configuration: The physical layer is concerned with the
connection of devices to the media. In a point-to-point configuration,
two devices are connected through a dedicated link. In a multipoint
configuration, a link is shared among several devices.
 Physical topology: The physical topology defines how devices are
connected to make a network. Devices can be connected by using a
mesh topology, a star topology, a ring topology, a bus topology, or a
hybrid topology.
 Transmission mode: The physical layer also defines the direction of
transmission between two devices: simplex, half-duplex, or fullduplex.
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Layers in OSI Model
 Data link layer:
 Data link layer makes physical layer to be error free for the upper
layer (Network layer).
 Other functions of data link layer include Framing: The data link layer divides the stream of bits received from the
network layer into manageable data units called frames.
 Physical addressing: The data link layer adds a header to the frame to
define the sender and/or receiver of the frame. If the frame is intended for
a system outside the sender's network, the receiver address is the address
of the device that connects the network to the next one.
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Layers in OSI Model
 Flow control: If the rate at which the data are absorbed by the
receiver is less than the rate at which data are produced in the
sender, the data link layer imposes a flow control mechanism to
avoid data loss at the receiver.
 Error control: The data link layer adds reliability to the physical
layer by adding mechanisms to detect and retransmit damaged or
lost frames. It also uses a mechanism to recognize duplicate
frames. Error control is normally achieved through a trailer added
to the end of the frame.
 Access control: When two or more devices are connected to the
same link, data link layer protocols are necessary to determine
which device has control over the link at any given time.
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 Network layer:
Layers in OSI Model
 Responsible for the source-to-destination delivery of a packet, possibly
across multiple networks (links).
 The network layer ensures that each packet gets from its point of origin to
its final destination.
 If two systems are connected to the same link, there is usually no need for
a network layer. But if the two systems are attached to different networks
(links) with connecting devices between the networks (links), there is often
a need for the network layer to accomplish source-to-destination delivery.
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Layers in OSI Model
 Other functions of network layer:
 Logical addressing:
 The physical addressing implemented by the data link layer handles
the addressing problem locally.
 If a packet passes the network boundary, we need another addressing
system to help distinguish the source and destination systems.
 The network layer adds a header to the packet coming from the
upper layer that, among other things, includes the logical addresses
of the sender and receiver.
 Routing: When independent networks or links are connected to
create internetworks (network of networks) or a large network,
the connecting devices (called routers or switches) route or
switch the packets to their final destination. The network layer
provides this mechanism.
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Layers in OSI Model
 Transport layer:
 Responsible for process to process delivery of the entire message.
 i.e. this layer ensures that the whole message arrives from source to
destination intact and also in the same order as it is sent.
 It also performs error control and flow control operations
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Layers in OSI Model
 Other responsibilities:
 Service point addressing:
 W.k.t. computers run several programs at the same time.
 In this context, source to destination delivery implies that delivery of
messages from a specific process in source to a specific process in
the destination.
 The transport layer header must include a type of address called a
service-point address (or port address).
 Based on this address, the network layer gets each packet to the
correct computer, the transport layer gets the entire message to the
correct process on that computer.
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Layers in OSI Model
 Segmentation and reassembly:
 Messages are divided in small pieces so that transportation becomes
easy.
 Each piece will have a sequence number which is used for
reassembling them at destination.
 In case any number in the sequence is missing, this indicates that,
corresponding packet is lost.
 This can be obtained by the retransmission from the source.
 Flow control:
 Similar to data-link layer, this layer also performs flow control.
 But difference here is, the flow control here is done from end to end.
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Layers in OSI Model
 Error control:
 Error control is done process to process.
 Transport layer of sending side, makes sure that the message is
received at the destination without any error caused due to damage,
loss or duplication.
 Usually error correction is done via retransmission.
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Layers in OSI Model
 Session layer:
 In some processes, services provided by physical, data link and
network layers may not be sufficient.
 In such cases session layer is useful for providing required
additional services.
 Session layer establishes, maintains and synchronizes the interaction
among communicating systems.
 Responsibilities of Session layer:
 Dialog control- this layer enables two systems to enter into
communication(dialog)
 Communication may be half duplex or full duplex.
 Synchronization:- this layer adds check points/synchronization
points to the stream of data.
 E.g. sending a file of say 5000 pages with check points after every
500 pages
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Layers in OSI Model
 Presentation layer:
 Concerned with syntax and semantics of the information being
exchanged between two systems.
 Main functions of this layer include translation, encryption and
compression.
 Translation:
 It is known that different machines work with different types of data
encoding formats.
 So, presentation layer in sender side converts the message from
sender dependent format to a common format.
 On the other hand, presentation layer converts this back to receiver
dependent format.
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Layers in OSI Model
 Encryption:
 In order to protect the privacy of the data, at the sending side
original data will be transformed to another form and this modified
data is sent across network.
 in the receiving end, decryption is done which is a reverse operation
of encryption.
 Data compression:
 Used to reduce the number of bits in the information
 Becomes useful in transmitting messages such as images audio n
video etc.
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Layers in OSI Model
 Application layer:
 Enables the user (human/software) to access the network.
 Provides user interfaces and supports for services like e-mail, file
transfer, remote file access, shared database management an other
distributed information services.
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Layers in OSI Model
 Specific services provided:
 Network virtual terminal:
 Software version of a physical terminal
 Allows user to log on to a remote system
 File transfer, access and management
 Allows to access files from a remote host, retrieve files from a
remote host to local system, and also to manage files located in
remote host locally.
 Mail services:
 Provides platform for e-mail forwarding and storage.
 Directory Services:
 This provides distributed database sources and access for global
information about various objects and services.
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Layers in OSI Model
 Summary of layers:
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TCP/IP Protocol suit
 TCP/IP Protocol suit:
 Has 4 layers namely host-to-network, internet, transport and
application layers.
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TCP/IP Protocol suit
 TCP/IP is a hierarchical protocol made up of interactive
modules, each of which provides a specific functionality.
 The modules are not necessarily interdependent.
 The layers of the TCP/IP protocol suite contain relatively
independent protocols that can be mixed and matched
depending on the needs of the system.
 The term hierarchical means that each upper-level protocol
is supported by one or more lower-level protocols.
 At the transport layer, TCP/IP defines three protocols:
Transmission Control Protocol (TCP), User Datagram
Protocol (UDP), and Stream Control Transmission Protocol
(SCTP).
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TCP/IP Protocol suit
 At the network layer, the main protocol defined by TCP/IP is
the Internetworking Protocol (IP); there are also some other
protocols that support data movement in this layer.
 Physical and Data Link Layers
 At the physical and data link layers, TCP/IP does not define any
specific protocol. But it supports all the standard and proprietary
protocols.
 A network in a TCP/IP internetwork can be a local-area network or a
wide-area network.
 Internetworking layer:
 In this layer, TCP/IP supports the Internetworking Protocol IP, in
turn, uses four supporting protocols: ARP, RARP, ICMP, and IGMP.
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TCP/IP Protocol suit
 Internetworking Protocol (IP) :
 This is the transmission mechanism used by the TCP/IP protocols.
 It is an unreliable and connectionless protocol--a best-effort delivery
service.
 Best effort means that IP provides no error checking or tracking. IP
assumes the unreliability of the underlying layers and does its best to
get a transmission through to its destination, but with no guarantees.
 IP transports data in packets called datagrams, each of which is
transported separately.
 Datagrams travel along different routes and can arrive out of
sequence or be duplicated.
 IP does not keep track of the routes and has no facility for reordering
datagrams once they arrive at their destination.
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TCP/IP Protocol suit
 Address Resolution Protocol(ARP):
 ARP is used to associate a logical address with a physical address.
 On a typical physical network, such as a LAN, each device on a link
is identified by a physical address, usually imprinted on the network
interface card (NIC).
 ARP is used to find the physical address of the node when its
Internet address is known.
 Reverse Address Resolution Protocol (RARP):
 The RARP allows a host to discover its Internet address when it
knows only its physical address.
 It is used when a computer is connected to a network for the first
time or when a diskless computer is booted.
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TCP/IP Protocol suit
 Internet Control Message Protocol (ICMP):
 The ICMP is a mechanism used by hosts and gateways to send
notification of datagram problems back to the sender.
 ICMP sends query and error reporting messages.
 Internet Group Message Protocol (IGMP):
 The IGMP is used to facilitate the simultaneous transmission of a
message to a group of recipients.
 Transport Layer
 Transport layer was represented in TCP/IP by two protocols: TCP
and UDP.
 UDP and TCP are transport level protocols responsible for delivery
of a message from a process to another process.
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TCP/IP Protocol suit
 User Datagram Protocol (UDP):
 The UDP is the simpler of the two standard TCP/IP transport
protocols.
 It is a process-to-process protocol that adds only port addresses,
checksum error control, and length information to the data from the
upper layer.
 Transmission Control Protocol (TCP)
 The TCP provides full transport-layer services to applications. TCP
is a reliable stream transport protocol.
 The term stream, means connection-oriented- A connection must be
established between both ends of a transmission before either can
transmit data.
 At the sending end of each transmission, TCP divides a stream of
data into smaller units called segments.
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TCP/IP Protocol suit
 Each segment includes a sequence number for reordering after
receipt, along with an acknowledgment number for the segments
received.
 Segments are carried across the internet inside of IP datagrams.
 At the receiving end, TCP collects each datagram as it comes in and
reorders the transmission based on sequence numbers.
 Stream Control Transmission Protocol (SCTP):
 New protocol introduced in transport layer.
 This provides support for newer applications such as voice over the
Internet.
 This combines the best features of UDP and TCP.
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TCP/IP Protocol suit
 Application layer:
 The application layer in TCP/IP is equivalent to the combined
session, presentation, and application layers in the OSI model.
 Many protocols like HTTP, FTP etc are defined at this layer.
 Addressing:
 Four levels of addresses are used in an internet employing the
TCP/IP protocols:
 Physical (link) addresses,
 Logical (IP) addresses,
 Port addresses, and
 Specific addresses
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Addressing
 Each address is related to a specific layer in the TCP/IP
architecture, as shown below
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Addressing
 Physical Addresses
 The physical address, (link address) is the address of a node as
defined by its LAN or WAN.
 Lowest-level address.
 The size and format of these addresses vary depending on the
network.
 E.g.
 Ethernet uses a 6-byte (48-bit) physical address that is imprinted on
the network interface card (NIC).


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07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address
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Addressing
 Logical Addresses
 Physical addresses are not adequate in an internetwork environment
where different networks can have different address formats.
 A universal addressing system is needed in which each host can be
identified uniquely, regardless of the underlying physical network.
 The logical addresses are designed for this purpose. A logical
address in the Internet is currently a 32-bit address that can uniquely
define a host connected to the Internet.
 No two publicly addressed and visible hosts on the Internet can have
the same IP address.
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 Port Addresses
Addressing
 The logical address and the physical address are necessary for data
to travel from a source to the destination host.
 We know that, Computers can run multiple processes at the same
time.
 The main objective of Internet communication is a process
communicating with another process.
 E.g.: Computer A can communicate with computer C by using
TELNET. At the same time, computer A communicates with
computer B by using the File Transfer Protocol (FTP).
 For these processes to receive data simultaneously, there should be a
method to label the different processes.
 In the TCP/IP architecture, the label assigned to a process is called a
port address.
 A port address in TCP/IP is 16 bits in length.
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Addressing
 Specific Addresses:
 Some applications have user-friendly addresses that are designed
for that specific address.
 The e-mail address – defines the recipient of an e-mail
 abc@wxyz.com
 Universal Resource Locator (URL) address- used to find a
document on the World Wide Web.
 E.g. www. sode-edu.in
 These addresses get changed to the corresponding port and
logical addresses by the sending computer.
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