OSI Reference Model

OSI Reference Model :-
• Architecture :-

Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer

• Introduction :-

• The International Standard Organization (ISO) is a multinational body decided to worldwide agreement on international standards .

• An ISO standard that covers all network communications is the Open System Interconnection model.

• It was first introduced in the late 1970s.

• An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture.

• OSI model is a layered architecture. It has following layers :-

1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

• Functions of Layers :-

1. Physical Layer :-

• Physical characteristics of interfaces and medium :-
The physical layer defines the characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium.
• Representation of bits :-
The physical layer data consist of a stream of bits with no interpretation. To be transmitted, bits must be encoded into signals. The physical layer defines type of encoding.
• Data Rate :-
It defines the number of bit sent per second.
• Synchronization of bits :-
The sender and receiver not only must use same data rate but also must be synchronizes at the bit level. Sender and receiver clocks must be synchronized.
• Line configuration :-
The physical layer is concerned with connection of devices to media.
• Physical topology :-
The physical topology defines how devices are connected to make network. Devices can be connected using Bus, Star, Mesh, Ring, Tree topologies.

• Transmission mode :-
The physical layer also defines the direction of the transmission between two devices. i.e. Simplex, Half-Duplex or Full-Duplex.

2. Data Link Layer :-

• Framing :-
The data link layer divides the stream of bits received from the network layer into manageable data units called frames.
• Physical Addressing :-
Data link layer adds a header to the frame to define the sender and/or receiver.
• Flow control :-
If the rate of data received is by receiver is less than the rate at which data is send by sender, the data link layer imposes flow control mechanism to avoid overwhelming of receiver.
• Error control :-
The data link layer adds reliability to the physical layer by adding mechanism to detect and retransmit damaged or lost frames.
• Access control :-
When two or more devices are connected to same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

3. Network layer :-
• Logical addressing :-
Physical addressing implemented by data link layer handles addressing problem locally. For global addressing logical addressing is implemented by network layer.
• Routing :-
When independent networks are connected to create internetworks, the connecting devices (router or switches) route the packet to their final destination. Network layer provides this function.

4. Transport Layer :-

• Service-point addressing :-
Source to destination delivery means delivery not only from one computer to another computer but also from specific process running on one computer to process running on another computer.
The transport layer header must therefore include a type of address called a service point address (or port address).
• Segmentation and reassembly :-
The message is divided into transmittable segments, with each segment containing a sequence number. These number enable the transport layer to reassemble the message correctly upon arriving at the destination.
• Connection control :-
Transport layer can be connectionless or connection oriented.
• Flow control :-
Like data link layer, the transport layer is responsible for flow control.
• Error control :-
Like data link layer, the transport layer is responsible for error control.

5. Session layer :-
• Dialog control :-
The session layer allows to two systems to enter into dialog. It allows the communication between two processes to take place in either half-duplex or full-duplex mode.
• Synchronization :-
The session layer allows a process to add checkpoints, or synchronization points, to a stream of data.

6. Presentation layer :-
• Translation :-
In exchanging of data between two processes, it is needed to maintain the form of data correct. The presentation layer is responsible for this.
• Encryption :-
To carry sensitive information the system must be able to ensure a privacy. Encryption means the sender transfer the information to another form and sends the resulting message over the network. Decryption is takes place at receiver end.
• Compression :-
Data compression reduces the number of bits contained within the information. It becomes important in transmission of multimedia such as text, audio, video.

7. Application layer :-

• Network virtual terminal :-
It is a software version of physical terminal, and it allows a user to logon on remote host.

• File transfer, access and management :-
It allows a user to access a file from remote host, control file from remote host.
• Mail services :-
It provides basis for e-mail forwarding and storage.
• Directory services :-
It provides distributed database sources and access for global information about various objects and services.