Overview of an OSI reference Model

OSI Reference Model
Brach: Electronics & Communication Engineering (11)
Semester: B.E (3rd year - 6th Semester)
Subject: Telecommunication Switching System & Networks (TSSN)
GTU Subject Code: 2161103
[UNIT – 06 – As per the GTU Syllabus]
Prepared By:-
Darshan Bhatt
Assistant Professor, EC Dept.
AIT, Ahmedabad, Gujarat

Introduction
3
▪ There are n numbers of users who use computer network and are
located over the world. So to ensure, national and worldwide data
communication, systems must be developed which are compatible to
communicate with each other ISO has developed a standard.
▪ ISO stands for International organization of Standardization.
▪ This is called a model for Open System Interconnection (OSI) and is
commonly known as OSI model.

Features of OSI Model
4
▪ Big picture of communication over network is understandable through
this OSI model.
▪ We can see how hardware and software work together.
▪ We can understand new technologies as they are developed.
▪ Troubleshooting is easier by separate networks.
▪ Can be used to compare basic functional relationships on different
networks.

7 Layers of OSI Model
Physical
Layer
Data Link
Layer
Network
Layer
5
Transport
Layer
Session
Layer
Application
Layer
Presentation
Layer

7 Layer Archietecture of OSI Model
6

7 Layers and their Protocols
7

Principles of OSI Model
8
The OSI reference model has 7 layers. The principles that were applied to arrive at the seven layers can
be briefly summarized as follows:
▪ A layer should be created where a different abstraction is needed.
▪ Each layer should perform a well-defined function.
▪ The function of each layer should be chosen with an eye toward defining internationally
standardized protocols.
▪ The layer boundaries should be chosen to minimize the information flow across the interfaces.
▪ The number of layers should be large enough that distinct functions need not be thrown together in
the same layer out of necessity and small enough that architecture does not become unwieldly.

Physical Layer
Lowest Layer in OSI Model
Layer 1
9

Role of Physical Layer
10
• The physical layer is responsible for the transmission and reception of unstructured raw data
between a device and a physical transmission medium.
• It converts the digital bits into electrical, radio, or optical signals.
• Layer specifications define characteristics such as voltage levels, the timing of voltage changes,
physical data rates, maximum transmission distances, modulation scheme, channel access method
and physical connectors.
• This includes the layout of pins, voltages, line impedance, cable specifications, signal timing and
frequency for wireless devices.
• Bit rate control is done at the physical layer and may define transmission mode as simplex, half
duplex, and full duplex.

Components of
Physical Layer
11
Components of the physical layer include:
• Cabling system components
• Adapters that connect media to physical interfaces
• Connector design and pin assignments
• Hub, repeater, and patch panel specifications
• Wireless system components
• Parallel SCSI (Small Computer System Interface)
• Network Interface Card (NIC)

Data Link Layer
Node-to-Node data transfer
Layer 2
12

Role of Data Link Layer
13
• The data link layer provides node-to-node data transfer—a link between two directly
connected nodes.
• It detects and possibly corrects errors that may occur in the physical layer.
• It defines the protocol to establish and terminate a connection between two physically
connected devices.
• It also defines the protocol for flow control between them.
• Allows a device to access the network to send and receive messages
• Offers a physical address so a device’s data can be sent on the network

Sub-layers of Data Link
Layer
14
IEEE 802 divides the data link layer into two sublayers:
• Medium access control (MAC) layer – responsible for controlling how devices in a
network gain access to a medium and permission to transmit data.
• Logical link control (LLC) layer – responsible for identifying and encapsulating
network layer protocols, and controls error checking and frame synchronization.
• The Point-to-Point Protocol (PPP) is a data link layer protocol that can operate
over several different physical layers, such
as synchronous and asynchronous serial lines.

Network Layer
End-to-end logical addressing system
Layer 3
16

Role of Network Layer
17
• Network Layer routes the signal through different channels from one node to
other.
• It acts as a network controller. It manages the Subnet traffic.
• It decides by which route data should take.
• It divides the outgoing messages into packets and assembles the incoming
packets into messages for higher levels.

Transport Layer
Transferring Variable length data
sequence
Layer 4
19

Role of Transport Layer
20
• The transport layer provides the functional and procedural means of transferring variable-length data
sequences from a source to a destination host, while maintaining the quality of service functions.
• The transport layer controls the reliability of a given link through flow control, segmentation / de-
segmentation, and error control. Some protocols are state- and connection-oriented.
• This means that the transport layer can keep track of the segments and retransmit those that fail
delivery.
• The transport layer also provides the acknowledgement of the successful data transmission and sends
the next data if no errors occurred.
• The transport layer creates segments out of the message received from the application layer.
Segmentation is the process of dividing a long message into smaller messages.

Session Layer
Layer 5
21

Role of Session Layer
22
• The session layer controls the dialogues (connections) between computers.
• It establishes, manages and terminates the connections between the local and remote application. It
provides for full-duplex, half-duplex, or simplex operation, and establishes procedures for check
pointing, suspending, restarting, and terminating a session.
• In the OSI model, this layer is responsible for gracefully closing a session, which is handled in the
Transmission Control Protocol at the transport layer in the Internet Protocol Suite.
• This layer is also responsible for session check pointing and recovery, which is not usually used in the
Internet Protocol Suite. The session layer is commonly implemented explicitly in application
environments that use remote procedure calls.

Presentation Layer
Layer 6
23

Role of Presentation
Layer
24
• The presentation layer establishes context between application-layer entities, in which the
application-layer entities may use different syntax and semantics if the presentation service
provides a mapping between them.
• If a mapping is available, presentation protocol data units are encapsulated into session protocol
data units and passed down the protocol stack.
• This layer provides independence from data representation by translating between application and
network formats.
• The presentation layer transforms data into the form that the application accepts.
• This layer formats data to be sent across a network. It is sometimes called the syntax layer.
• The presentation layer can include compression functions.
• The Presentation Layer negotiates the Transfer Syntax.

Application Layer
Layer 7
25

Role of Application
Layer
26
• Application Layer is the topmost layer.
• Transferring of files disturbing the results to the user is also done in this layer.
• Mail services, directory services, network resource etc are services provided by
application layer.
• This layer mainly holds application programs to act upon the received and to be sent
data.

Merits & Demerits of OSI
Model
Merits
1. OSI model distinguishes well between
the services, interfaces and protocols.
2. Protocols of OSI model are very well
hidden.
3. Protocols can be replaced by new
protocols as technology changes.
4. Supports connection oriented services
as well as connectionless service
27
Demerits
1. Model was devised before the
invention of protocols.
2. Fitting of protocols is tedious
task.
3. It is just used as a reference
model.

Overview of an OSI reference Model

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