COMPUTER NETWORK_OSI & TCP/IP

ComputerNetworks
OSIModel,TCP/IPModelandProtocols
1
Presented By:
Sweta Kumari Barnwal
Assistant professor
Arka Jain University
Jamshedpur, Jharkhand

Contents
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• Introduction to Layers and Protocols.
• OSI Model
• OSI Model layer functioning.
• TCP/IP Model
• Addressing scheme

OSIModel (Open System Interconnection)
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• Firstly introduced in 1984 by ISO – ‘International
Organization of Standardization’.
• An open system is a set of protocols that allow any two
different systems to communicate.
• Protocols: Set of Rules.
• OSI model itself is not a protocol, it is a model that
define protocols for network communication.
• Seven separate but related layers.
• These 7 layers work collaboratively to transmit the data from
one person to another across the globe.

Organizationof Layers
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7 layers of OSI Model can be further divided
into three subgroup. Named as:
Layer 1
• Physical
• Data link
• Network
Layer 2
• Transpot
Layer 3
• Session
• Presentation
• Application

1. PhysicalLayer
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The lowest layer of the OSI reference model is the physical layer. It
is responsible for the actual physical connection between the
devices. The physical layer contains information in the form
of bits.
Define physical transmission medium (Guided or Unguided).
Interface between devices and transmission medium.
Representation of bits:
PL consist of bits (0s and 1s), so, data -> bits ->signals
(electrical or optical).
Data rate
 Transmission rate (means the number of bits sent per
second).
Synchronization of bits:
Sender and receiver must not receive the same bit rate but must be
at same time (synchronized clock).

Continued…
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Line configuration:
Connection (point-to-point or multipoint) of devices to
the medium.
Physical topology:
Topological Structure (Bus, Star, Mesh etc)
Transmission mode:
Simplex, half-duplex or full duplex.

2. DataLinkLayer
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It is the second layer after physical layer. The data
link layer is responsible for maintaining the data link
between two hosts or nodes. There are several functions
performed by DLL:
Framing:
Arranging bit streams into Data Frames.
Physical addressing (machine address):
Add header to the frame to define the sender and receiver of the data
packet.
Flow Control;
Not to overload receiver
Error Control:
Error Free
Access Control:

Design Issues in Data Link Layer
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The data link layer is divided into two sub-layers :
 Logical Link Control Sub-layer (LLC) –
Provides the logic for the data link , Thus it controls the
synchronization , flow control , and error checking functions of
the data link layer. Functions are –
(i) Error Recovery.
(ii) It performs the flow control operations.
(iii) User addressing.
 Media Access Control Sub-layer (MAC) –
It is the second sub-layer of data-link layer. It controls the flow
and multiplexing for transmission medium. Transmission of data
packets is controlled by this layer. This layer is responsible for
sending the data over the network interface card. Functions are-
(i) To perform the control of access to media.
(ii) It performs the unique addressing to stations directly
connected to LAN.
(iii) Detection of errors.

Continued…
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Design issues with data link layer are :
 Services provided to the network layer –
The data link layer act as a service interface to the network layer. The principle
service is transferring data from network layer on sending machine to the
network layer on destination machine. This transfer also takes place via DLL
(Dynamic Link Library).
 Frame synchronization –
The source machine sends data in the form of blocks called frames to the
destination machine. The starting and ending of each frame should be identified
so that the frame can be recognized by the destination machine . .
 Flow control –
Flow control is done to prevent the flow of data frame at the receiver end. The
source machine must not send data frames at a rate faster than the capacity of
destination machine to accept them.
 Error control –
Error control is done to prevent duplication of frames. The errors introduced
during transmission from source to destination machines must be detected and
corrected at the destination machine.

3. NetworkLayer
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Network layer works for the transmission of data from one host to the
other located in different networks. It also takes care of packet
routing i.e. selection of the shortest path to transmit the packet, from
the number of routes available. The sender & receiver’s IP address
are placed in the header by the network layer.
The functions of the Network layer are :
Routing: The network layer protocols determine which route is
suitable from source to destination. This function of network layer is
known as routing.
Logical Addressing: In order to identify each device on internetwork
uniquely, network layer defines an addressing scheme. The sender &
receiver’s IP address are placed in the header by network layer. Such
an address distinguishes each device uniquely and universally.

4.TransportLayer
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Transport Layer (Layer 4) :
Transport layer provides services to application layer and takes services from network
layer. The data in the transport layer is referred to as Segments. It is responsible for the
End to End Delivery of the complete message. The transport layer also provides the
acknowledgement of the successful data transmission and re-transmits the data if an
error is found.
 Process-to-process delivery (applications)
 Service-point addressing:
Send data to the correct process on particular computer.
 Segmentation and reassembly
Divide into segments and assign sequence number.
Reassembly at destination according to sequence number.
 Connection control
Connectionless or connection oriented
 Flow control
 Error control
Without damage, loss or duplication

5. SessionLayer
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This layer is responsible for establishment of connection, maintenance of sessions,
authentication and also ensures security.
The functions of the session layer are :
Session establishment, maintenance and termination: The layer allows the two
processes to establish, use and terminate a connection.
Synchronization : This layer allows a process to add checkpoints which are
considered as synchronization points into the data. These synchronization point
help to identify the error so that the data is re-synchronized properly, and ends of
the messages are not cut prematurely and data loss is avoided.
Dialog Controller : The session layer allows two systems to start communication
with each other in half-duplex or full-duplex.

6. PresentationLayer
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It is also called the Translation layer. The data from the
application layer is extracted here and manipulated as per the
required format to transmit over the network.
The functions of the presentation layer are :
Translation : For example, ASCII to EBCDIC.
Encryption/ Decryption : Data encryption translates the data into
another form or code. The encrypted data is known as the cipher text
and the decrypted data is known as plain text. A key value is used
for encrypting as well as decrypting data.
Compression: Reduces the number of bits that need to be
transmitted on the network.

7.ApplicationLayer
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At the very top of the OSI Reference Model stack of layers, we find
Application layer which is implemented by the network applications.
These applications produce the data, which has to be transferred over
the network. This layer also serves as a window for the application
services to access the network and for displaying the received
information to the user.
 User Interface
 File transfer, access and management
 Mail services
 Directory service (database)

Communication Between The Layers In
OSI Model
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TCP/IP(Transmission Control Protocol /
Internet Protocol.
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The OSI Model we just looked at is just a reference/logical model. It
was designed to describe the functions of the communication system
by dividing the communication procedure into smaller and simpler
components. But when we talk about the TCP/IP model, it was
designed and developed by Department of Defense (DoD) in 1960s
and is based on standard protocols. It stands for Transmission
Control Protocol/Internet Protocol. The TCP/IP model is a concise
version of the OSI model. It contains four layers, unlike seven layers
in the OSI model. It consist Four Layers.
 Process/Application Layer
 Host-to-Host/Transport Layer
 Internet Layer
 Network Access/Link Layer
r

TCP/IPLayerFunctioning
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1. Network Access Layer –This layer corresponds to the combination
of Data Link Layer and Physical Layer of the OSI model. It looks out
for hardware addressing and the protocols present in this layer allows
for the physical transmission of data.
2. Internet Layer –This layer parallels the functions of OSI’s Network
layer. It defines the protocols which are responsible for logical
transmission of data over the entire network.
3. Host-to-Host Layer – This layer is analogous to the transport layer
of the OSI model. It is responsible for end-to-end communication and
error-free delivery of data. It shields the upper-layer applications from
the complexities of data.
4. Application Layer –
This layer performs the functions of top three layers of the OSI model:
Application, Presentation and Session Layer. It is responsible for node-
to-node communication and controls user-interface specifications.

Comparison Between OSI &TCP/IP
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Continued…
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TCP/IP OSI
It refers to Transmission Control
Protocol/Internet Protocol.
It refers to Open Systems Interconnection.
It consist 5 layers. It has 7 layers.
It is more reliable It is less reliable
It does not have very strict boundaries. It has strict boundaries
It follows a horizontal approach. It follows a vertical approach.
It uses both session and presentation layer in
the application layer itself.
It uses different session and presentation
layers.
It developed protocols then model. It developed model then protocol.
Transport layer in TCP/IP does not provide
assurance delivery of packets.
In this model, transport layer provides
assurance delivery of packets.
This model network layer only provides
connection less services.
Connection less and connection oriented both
services are provided by network layer in OSI
model.
Protocols cannot be replaced easily in TCP/IP
model.
While in OSI model, Protocols are better
covered and is easy to replace with the change
in technology.

COMPUTER NETWORK_OSI & TCP/IP

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