ISO OSI Model

ISO OSI Model
The OSI model includes seven layers. The basic principles that were applied to arrive at the seven layers are as follows:

1)When different level of abstraction is needed a layer should be created.
2)Each layer must follow a well defined function.
3)The role of each layer should be preferred with an eye towards defining internationally standardized protocols.
4)The layer interfaces should be selected to reduce the information flow across the interfaces.

The OSI model itself is not network architecture because in each layer it does not define the exact services and protocols to be used. It just tells function of each layer. However, ISO has also created standard for all the layers, although these are not part of the reference model itself. Each one has been defined as separate International standards.

Different layers of ISO OSI model are as follows

1) Physical Layer
The physical layer is related with delivering raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received as a 1 bit by the other side but not as a 0 bit. The design issues here largely related with mechanical, electrical and procedural interfaces and the physical communication channel which lies below the physical layer.

2) Data Link layer
The important function of the data link layer is to take raw transmission facility and deliver it into a line that appears free of undetected transmission error to the network layer. It achieves this task by having the sender break the input data up into data frames then transmit the frames sequentially and process the acknowledgement frames sent back by the receiver. Since the physical layer takes and delivers a stream of bits without any concern to meaning or structure it is data link layer’s function to create and determine frame boundaries. This can be achieved by attaching special bit patterns to the start and end of the frame. If in the data these bits appear accidentally, special care must be taken to ensure these patterns are not incorrectly interpreted as frame delimiters.

On the line a noise burst can destroy a frame completely. If frame is destroyed the data link layer software on the source machine can retransmit the frame. However, multiple transmissions of the same frame cause the possibility of duplicate frames. If the acknowledgement frame from the receiver to sender were lost then duplicate frame could be sent. It is function of this layer to solve the problems caused by damaged, lost and duplicate frames. The data link layer provides number of different service classes to the network layer, each of a different quality and with a different price.

Another design issue that takes place within data link layer is how to keep a fast transmitter from drowning a slow receiver in data. Some traffic control mechanism must be applied to let the transmitter know how much buffer space the receiver has at the moment. Frequently, this flow control and the error handling are incorporated.
Broadcast networks possess some additional issue in the data link layer: how to control access to the shared channel. The medium access sub layer, a special sub layer of the data link layer related with this problem.

3) Network Layer
The Network layer is designed for controlling the operation of the subnet. An important design issue of network layer is determining how packets are routed from one route to another route. Routes are dependent upon static tables that are "wired into" the network and rarely changed. At the start of each conversation routes are determined. They can be highly dynamic, being determined additionally to reflect the current network load for each packet.

If in the subnet too many packets are present at the same time, they will get in each other’s way, forming bottleneck. The control of such congestion of packets is done by the network layer.
When a packet travel from one source to another destination many problems may arise. The addressing used by the first network may be different from the destination. The destination may not accept the packet at all because it is too large. The protocol used may differ and so on. It is function of network layer to overcome all these problems to permit heterogeneous networks to be interconnected. In Broadcast network, the routing provided is simple, so the network layer is often thin or even nonexistent.

4) Transport Layer
Its main function is to take data from the session layer and divide the data into smaller units and pass these to the network layer and check that the pieces all arrive correctly at the other end. Transport layer perform all this function efficiently and in such way that separates the upper layers from the inevitable changes in the hardware technology.

The transport layer creates a distinct network connection under normal condition for each transport connection required by the session layer. If the transport connection needs a high throughput then the transport layer might create multiple network connection, dividing the data among the network connection to improve throughput. On the other hand if creating or maintaining a network connection is expensive then in order to reduce the cost the transport layer might multiplex several transport connections onto the same network connection. It makes the multiplexing transparent to the session layer.

It also determines what type of services to provide to upper session layer and indirectly to users of the network. The transport connection must be an error free point to point channel that transports message or bytes the order in which they sent. Other transport services are delivery of isolated messages with no guarantee about the order of delivery and broadcasting of message to multiple destinations.
It is a true end to end layer from source to destination.

5) Session layer
The session layer permits users to establish session on different machines. A session permits ordinary data transfer as does the transport layer but it also offers enhanced services useful in some applications. A session is used to permit user to log into a remote timesharing system or to transfer a file between two machines.

Session layer also provide a service to manage dialogue control. Sessions permit traffic to go in dual directions at the same time or in only one direction at a time, the session layer also keeps track of whose turn it is.

Token management is another session service. It is vital that both sides do not attempt the same operation for some protocol at the same time. The session layer provides tokens to manage these activities that can be exchanged. Only the station having token may perform the critical operation.

Synchronization is important session service. The session layer offers a way to insert checkpoints into the data stream so that after a crash only the data transferred after the last checkpoint have to be repeated.

6) Presentation Layer
The presentation layer carry out certain functions that are requested sufficiently often to warrant finding a general solution to them, rater than letting each user solve the problems. It deals with syntax and semantics of the information transmitted.
It manages abstract structure and converts from the representation used inside the computer to the network standard representation and back.

7) Application Layer
The application layer consists of number of protocols that are commonly required.
It performs the function of file transfer. Different file systems represent different file naming conventions and different way of representing text lines. Application layer transfer a file between two different systems which requires handling incompatibilities.
It also provides electronic mail, job entry, directory lookup and various other general purpose facilities.