OSI reference model

The OSI model is based on ISO and is introduced in the year 1983 and was revised in 1995 .

This is also known as ISO-OSI model(International Standards Organization-Open System Interconnection model)

and is used to connect open systems(open- they are ready for communication)

The OSI model has 7 layers. These layers are formed by considering the following things

1.  A layer should be created where a different abstraction is required.
2. Each layer should perform a well defined function.
3. A layer  boundaries should be chosen to minimize the information flow across the interface.
4. The function of each layer chosen by keeping an eye toward defining international standardized protocols.
5. The no.of layers chosen  that same function is not performed in the each layer and the function performed is not so small.

Now the model looks like this

Physical layer:-

Physical layer is connected with 

• Transmitting raw bits over a communication channel.
• i.e, the design issue makes sure that sending ‘1’  must be received as ‘1’ itself  but not as ‘0’.
• How many volts is required to represent 1 and 0?
• How many Nano Seconds a bit lasts?
• whether transmission may proceed in both the directions (or) uni-directional and how the initial connection is established?
• whether to terminate the connection (or) not?
• i.e, most of the design issues deal with mechanical, electrical and timing interfaces & the physical transmission medium.

 At physical layer  the data rate, synchronization of bits, line configuration(point-to-point,Broadcasting) and the topology used and Transmission mode( simplex/duplex) are specified.

Data Link Layer:-

It gets services from physical layer and offers services to the N/w layer.

The DLL makes the raw transmission as reliable and is responsible for node to node delivery . It makes physical layer appears error free to the upper layer.

the main functions of DLL are:

Framing:- The DLL divides the stream of bits received from N/w layer into manageable data units called frames.

Physical addressing:- If frames are distributed to different systems on the N/w , DLL adds header to the frame to define physical address of sender and receiver of the frame.

Flow control:- DLL also keep a fast Transmitter from drowning a slow receiver in data. therefore it requires a mechanism for controlling the flow to prevent overwhelming of the receiver.

Error control:-

DLL provides a mechanism to 

• detect damaged (or) lost frames and to re transmit this damaged (or) lost frames.
• and needs a mechanism to prevent duplication of frames. error control is normally achieved through a trailer added at the end of the frame.
• it accepts data from N/w layer and break up that data into data frames and transmit the frames sequentially. If transmission is reliable it is observed by acknowledgement frame.
• MAC layer in DLL takes care of how to share channel and control the access in case of broad casting used for end-to-end (or) node-to-node delivery.

Network Layer:-

• It controls the operation of subnet. The processes involved in N/w layer are switching and Routing.
• Switching:- refers to make a temporary connection between physical links for N/w transmission.
• Routing:- means choosing a best path for sending a packet from one node to another when  number of paths are available. This can be done by Routing algorithms.
• congestion control:- If too many packets are present in the subnet at the same time. They will from bottlenecks  forming a congestion which can be controlled by using a congestion control algorithms.
•  when a packet has to travel from one n/w to another to get its destination many problems are like

i. The addressing used by second N/w may differ from first one.

ii. The second one may not accept the packet at all because it is too long.

iii. The protocols used may differ.

N/w layer may takes care of all the above issues in order to provide inter connection between heterogeneous structures.

In Broadcast N/ws the routing problem is simple often thin (or) non-existent.

Transport Layer:-

• Transport layer (TL) accepts data from session layer and makes into smaller units called TPDU (Transport Protocol Data Units).
• TL passes this TPDU into N/w layer and makes sure that these pieces are all arriving at the other end correctly.
• TL determines what type of service to provide to the session layer.
• The most popular type of transport connection is an error free point to point channel that derives messages (or) bytes as the order in which they were sent.
• The other possible service is transporting isolated messages and broadcasting messages to multiple destinations in which layer 1 to 3 are chained and layers 4 to 7 are end-to-end.

Session layer:-

• session layer allows users on different machines to establish sessions between them and offers services as follows
• dialog control: – keeping track of whose turn it is to transmit.
• token management: -If two parties attempting for the same operation at the some time.

ex:- Bank–>cash withdrawl–> giving some tokens.

• Synchronization:- keeping check points when long transmission needs to transmit bulk data .

ex:-while sending 2000 pages of a file keep check points after every 100 pages makes transmission efficient.

Presentation layer:-

• Mostly deals with syntax and semantics of data to be transmitted.
• for communicating computers with different data structures can be possible through presentation layer.
• for encryption and decryption.

Application layer:-

It contains different protocols that are commonly needed by users

• accessing webpages using HTTP(or) TCP.
• email.
• directory services.
• for N/w news.
• FTAM- File Transfer Access and Management.
• to access files in remote computer.
• to retrieve files from remote computer.
• to manage and control remote computer.