OSI Network Reference Model

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Oct 23, 2013 (3 years and 11 months ago)

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OSI Network Reference Model


The model


Functions of the layers

OSI



Physical Layer



Responsible for movement of data from one node to another
node



Concerned with

-
Physical Characteristics of the medium

-
Representation of bits
-

Electrical, Electromagnetic,
Optical

-
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 )


Framing
-

divides the data from N/W layer into
frames


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
-
to
-
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
-
Mail

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
Duplex


Synchronization


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


Translation


Encoding and Decoding


Sender using a format.


Receiver using different format, So need to Convert


Encryption


for security and privacy purpose


Compression


reducing the number of bits to
sent over network.

Application Layer


Responsible for providing services to
user.


Provides User interfaces and support for
Services, like


E
-
mail,


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


TCP/IP


When Satellites and radio Networks were added to
ARPANET


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

Layers in TCP/IP


Internet Layer


Transport Layer


Application Layer


Host
-
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
Destination


Transport Layer


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


Provides Connection Oriented and Connectionless
services

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
destination


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)


FTP


SMTP (electronic mail)


DNS(Domain Name System)


USENET


HTTP


Host

to Network Layer


Connects Host to the Network


Places IP packets onto the network


Also Called Network Interface layer