Functions of Osi Layers

7/27/2019 Functions of Osi Layers

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Tx Rx





• When data is transmitted from source to

destination each layer attaches additionalinformation called as header or trailer.

• Header will be the added before the data and

trailer will be added at the end of the data.

Data Trailer

Header Data

Protocol Data Unit



• A PDU can contain a header added at the beginning of the data

or a trailer appended at the end of the data.• The header or trailer contains control information like size of

the data, timing, address where to be sent and other details

related to the ‘data’.

•

At the transmitter side headers are added to the data at eachlayer. Whereas at the receiver side the headers are removed

from the data.

• The process of adding headers to the data is called

ENCAPSULATION.

• The process of removing headers from the data is called as

DECAPSULATION.





FUNCTIONS OF OSI LAYERS



APPLICATION LAYER

• It is the top layer of the OSI model.

• This layer is closest to the user and

provides different user interfaces.

• It provides various interfaces for sending and receiving data.

• The main functions of application

layer are

File Transfer, Access and ManagementMail services

Database Management

Accessing data from a remote computer



PRESENTATION LAYER

• Presentation layer deals with data format i.e.,syntax and semantic.

• This layer looks into the way a particular datacan be represented.

• It does the process of compression in order toreduce the size of the data and encryption isdone for maintaining secrecy of the data to be

transmitted.• Different computers use different encoding

techniques.

• Presentation layer at the sender converts theinformation from sender dependent format intoa common format. At the receiving side the

common format is changed into receiver dependent format.

• The main functions of presentation layer are Data Conversion

Data compression/decompression

Data encryption/decryption



SESSION LAYER

• Session layer enables two systems tohold on-going communication across a

network such that they are able toexchange data for that duration of time.

• It establishes, maintains andsynchronizes the interaction betweentwo communication system.

• It defines how to start, control and endconversations which are also called assessions between two systems.

• Because of the above function sessionlayer is also called as network dialogue

controller.• The main functions of session layer are

Initiate session

Maintain session

Terminate session



Dialog control:

It determines which system

will communicate, when will

they communicate and for how

long.

1) Simplex: It is a form of

communication where only

one device acts as a sourceand other device acts only as

a destination.

Eg: Keyboard and Monitor

2) Half-Duplex: It is a form of

communication which is

bidirectional but both can’t transmit data at the same

time.

If one acts as a transmitter at

certain period of time then

the other acts as a

transmitter for another

period of time.

Eg: Walkie-Talkie

2

Tx Rx 1 2t1

t2



3) Full-Duplex: It is a form of communication where both

the devices can send or receive data at the same time.

Eg: Telephone or mobile

1 2

t



TRANSPORT LAYER • It manages transmission or transport of data between process to process.

• The main functions of the transport layer are

Segmentation and Reassembling

Multiplexing

Error control

Flow control



1 1

2

3

2

3

•The transport layer is responsible for dividing the entire data

into segments and this process is called segmentation.

•The Transport layer regulates information flow from process to

process ensuring an end to end communication



NETWORK LAYER • The functions of the network layer are

Routing : It looks into the path through which the data can be sent. It

includes shortest path and also avoids congestion.

Message priority: The network layer should route a data according

to its priority. A data with high priority is delivered first to thedestination.

Logical addressing: Logic address is used by the network layer to

identify a particular network among other networks.



Personal Computers





DATA-LINK LAYER

• In this layer frames are formed.

• Frames are group of information containing the actual

data, address, error checking etc…..

• This makes the physical layer ready to accept data for

transmission onto the channel.• The main functions of data link layer are

Framing

Error control

Flow control

Physical addressing



PHYSICAL LAYER

• It is the bottom layer of OSI model.

• It deals with hardware specifications of connectors,

terminals, type of medium etc….

• It is the layer connected directly to the

communication channel.• It is concerned with transmission of bits in 1’s and 0’s

over the channel.





Functions of Osi Layers

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