OSI Network Reference Model

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Oct 23, 2013 (4 years and 8 months ago)


OSI Network Reference Model

The model

Functions of the layers


Physical Layer

Responsible for movement of data from one node to another

Concerned with

Physical Characteristics of the medium

Representation of bits

Electrical, Electromagnetic,

Data rate

Line Configuration

P2P, Multipoint.

Synchronization of bits between sender and receiver.

Physical Topology

BUS , RING etc…

Transmission Mode

Simplex, Duplex, Half Duplex

Data Link Layer

Responsible for moving frames from one
hop/node to the other.( Next hop/node delivery )


divides the data from N/W layer into

Physical Addressing

Sender and Rx if the Rx is
within the N/W, otherwise next hop addressing.

Data Flow Control on the sender

Error detection/control with the use of trailer

Access control

If there are Multiple users in Multi
Link system, then who has the access of Line

Data Link Layer

Network Layer

Source to Destination Delivery of Packets

Not required in peer
peer delivery

Adds Logical addresses of sender and Rx.

Routing for internetwork communication.

Network Layer

Transport Layer

Process to Process Delivery of message b/w Tx & Rx

Transport Layer adds a
Service Point or Port Address
which is for each process. Eg
Yahoo Messenger, Firefox

Segmentation and Reassembly

Divide the message
received from Session layer in to Segments and number
them to make a sequence for reassembly at the
receiving side

Connection Control

Connection Oriented Transport Layer, like Telephone

Connectionless Transport Layer, like E

Transport Layer

Flow Control

Process to Process flow control at the end machines

Error Control

Process to Process. Controls duplication of message

Session Layer

Network dialog controller. Establishes,
maintains and Synchronizes interaction
b/w communicaing systems

Dialog Controlling

Half Duplex or Full


Allows a process to
add checkpoints to a stream of data

Presentation Layer

Concerned with Syntax and Semantics of the
information exchanged b/w the two
communication systems


Encoding and Decoding

Sender using a format.

Receiver using different format, So need to Convert


for security and privacy purpose


reducing the number of bits to
sent over network.

Application Layer

Responsible for providing services to

Provides User interfaces and support for
Services, like


File transfer

etc …

TCP/IP Reference Model

TCP/IP Reference Model

TCP/IP model forms the base of present day Internet

It was developed prior to OSI Model

Two Main Protocols:

TCP: Transmission Control Protocol

IP: Internet Protocol


When Satellites and radio Networks were added to

Failure of existing Protocols to handle such a network
led to the introduction of a new Reference Model

Layers in TCP/IP

Internet Layer

Transport Layer

Application Layer

to Network Layer

Internet layer

Similar to Network Layer of OSI Model

Permits hosts to inject packets into network and
make them travel independently to the destination

Similar to Snail Mail

Different Countries have Different Stamps

Preferred envelope sizes

Delivery Rules

Internet layer

Specifies a specific format for a packet

Packets can arrive in a different order at the
destination, so a higher layer arranges them in a
proper order

Packet Routing is a Major Issue

Internet layer

Congestion Control

(IP)Internet Protocol is important protocol

Responsible for delivering IP packets to their

Transport Layer

Allow two processes on source and destination hosts
to carry on a conversation

Provides Connection Oriented and Connectionless

Transport Layer

TCP (Transmission Control Protocol)

Reliable Connection Oriented Service

UDP(User Datagram Packet)

Unreliable Connectionless Services

Transport Layer

TCP provides:

Error free delivery of byte streams from source to

Fragments byte stream into parts

Reassembles fragments at receiving end

Flow Control

Transport Layer

UDP provides:

Applications that do not want TCP’s Sequencing and
Flow Control

Used where prompt delivery is more important that
accurate delivery eg Speech and Video

Application Layer

Provides services to different User Applications

Contains all the higher level protocols

Virtual terminal (TELNET
Terminal Network)


SMTP (electronic mail)

DNS(Domain Name System)




to Network Layer

Connects Host to the Network

Places IP packets onto the network

Also Called Network Interface layer