ISO - OSI Model in Communication Networks
ISO stands for International organization of Standardization. This is called a model for Open System Interconnection (OSI) and is normally called as OSI model.
The ISO-OSI model consists of seven layer architecture. It defines seven layers or levels in a complete communication system.
Lets know the features of OSI Model :
1. OSI model enables us to easily understand the big picture of network.
2. Through OSI models we can understand how hardware and software work together.
3. Through OSI models we can understand new technologies as they are developed.
4. Through OSI models troubleshooting is easier by separate networks.
5.Through OSI models can be used to compare basic functional relationships on different networks.
Each of all 7 layers of OSI model have distinct functions responsible to complete the communication.
Functions of Different Layers are described as follows :-
Layer 1: The Physical Layer :
This layer activates, maintain and deactivate the physical connection. This layer is responsible for :-
1. Movements of individual bits from one node to next.
2. Physical characteristics of interface and medium.
3. Representation of bits.
4. Data rate.
5. Synchronization of bits.
6. Line configuration.
7. Physical topology.
8. Transmission mode.
Layer 2: Data Link Layer :
Data link layer synchronizes the information which is to be transmitted over the data. This layer is responsible for :-
1. Moving frames from one node to next (node to node delivery).
2. Framing
3. Physical addressing
4. Flow and error control
5. Access control : when two or more devices are connected to the same link, this layer protocols are necessary to determine which device has control over the link at any given time.
Layer 3: The Network Layer :
This layer routes the signal through different channels to the other end. It acts as a network controller. This layer is responsible for :-
1. Source to destination delivery of individual packet.
2. Logical addressing.
3. Routing : by which route data should take.
4. Congestion control : controls the traffic of the network
5. Billing.
This layer divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.
Layer 4: Transport Layer :
This layer decides if data transmission should be on parallel path or single path. Functions such as multiplexing, segmenting or splitting on the data done by layer four that is transport layer.
1. This layer is responsible for Process to Process delivery of entire message.
2. Ensures that whole message arrives intact and in-order.
3. Overseeing both Error Control and Flow Control.
4. This layer is responsible for Segmentation and Reassembly.
Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer.
The transport layer can be either Connection Less or Connection Oriented.
A connection less transport layer treats each segment as in independent packet and delivers it to the transport layer at the destination machine.
A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all data are transmitted, the connection is terminated.
Layer 5: The Session Layer :
Session layer manages and synchronize the conversation between two different applications.
1. Responsible for dialog control and synchronization.
2. It establishes, maintains and synchronizes the interaction amongst communication systems.
Transfer of data from one destination to another session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.
Layer 6: The presentation Layer :
Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data.
1. Responsible for translation, compression and encryption.
Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.
Layer 7: Application Layer :
It is the top layer and responsible for providing services to the users.
1. Enables the users (whether human or software) to access the network.
2. It provides user interface and supports for services such as e-mail, remote file access & transfer, shared database management etc.
3. Manipulation of data (information) in various ways is done in this layer.
4. Transferring of files disturbing the results to the user is also done in this layer.
Merits of OSI reference model:
1. OSI model distinguish between the services, interfaces and protocols.
2. Protocols of OSI model are very well hidden.
3. They can be replaced by new protocols as technology changes.
4. Supports connection oriented as well as connectionless service.
Demerits of OSI reference model:
1. Model was devised before the invention of protocols.
2. Fitting of protocols is tedious task.
