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The OSI Model: Data Transmission

In the previous post we have seen the detail architecture of OSI reference model under the heading Network architecture: The OSI reference Model.  now we will see how data can be transmitted in the OSI model.

The OSI Reference Model

This model is developed by the International Standards Organization (ISO) as the first step toward international standardization of the protocols used in the various layers.
The model is called the ISO-OSI (Open System Interconnection) Reference Model because it deals with connecting open systems- that is, systems that are open for communication with other systems.
The OSI model has seven layers. In layers the principal ideas are as follows
1. A layer should be created where a different level of abstraction is needed.
2. Each layer should perform a well defined function.
3. The function of each layer should be defined international standardized protocols.
4. The layer boundaries should be chosen to minimize the information flow across the interfaces.
5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy.

The OSI model itself is not network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer should do.
OSI reference model

The Physical Layer

The Physical layer is concerned with transmitting raw bits over a communication channel.
The design issues here largely deal with mechanical, electrical, and procedural interfaces, and the physical transmission medium, which lies below the physical layer.

The Data Link Layer

The main task of this layer is to take a raw transmission facility and transform it into a line that appears free of undetected transmission errors to the network layer. The sender breaks the input data into data frames, transmit the frames sequentially, and process the acknowledgment frames sent back by the physical layer. The data link layer is to create and recognize frame boundaries by attaching special bit patterns to the beginning and end of the frame.

Data Link Layer handles following points:

1. A noise burst on the line can destroy a frame completely. In this case, this layer software on the source machine can retransmit the frame till the receiver sends back the acknowledgment frame.
2. Flow regulation and the error handling by using some traffic regulation mechanism.
3. A Clever solution (piggybacking) used to transmit data in both directions.
4. A special sub layer of the data link layer called medium access sub layer, deals with to control access to the shared channel.

The Network Layer

The network layer is concerned with controlling the operation of the subnet.
A key design issue is concerned with routing of data packets from source to destination.
If too many packets are present in the subnet at the same time, the control of such congestion solved by network layer.
This layer also handles some accounting function to count how many packets or characters or bits are sent by each customer, to produce billing information.
It handles protocol incompatibilities between different networks.

The Transport Layer

The basic function of the transport layer is to accept data from the session layer, split it up into smaller units, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.
The transport layer creates multiplex several transport connections on to the same network required by the session layer to reduce the cost. In all cases, the transport layer is required to make the multiplexing transparent to the session layer.
The transport layer also determines what type of service to provide the session layer or the users of the network.
1. An error-free point-to-point channel that delivers messages or bytes in the order in which they were sent.
2. Transport of isolated messages with no guarantee about the order of delivery, and broadcasting of messages to multiple destinations.
3. It handles flow control between hosts i.e. a mechanism to regulate the flow of information, so that a fast host cannot overrun a slow one.

The Session Layer

The session layer allows users on different machines to establish sessions between them to allows ordinary data transport, log into a remote time sharing system or to transfer a file between two machines.
This layer provides different services.
1. To manage dialogue control. Sessions can allow traffic to go in both directions at the same time, or in only one direction at a time.
2. Token management. For some protocols, it is essential that both sides do not attempt the same operation at the same time. To manage these activities, the session layer provides tokens that can be exchanged.
3. Synchronization. The session layer provides 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.

The Presentation Layer

The presentation layer is concerned with the syntax and semantics of the information transmitted.
A typical example of a presentation service is encoding data in a standard agreed upon way. This layer manages these abstract data structures and converts from the representation used inside the computer to the network standard representation and back.

The Application Layer

The application layer contains a variety of protocols that are commonly needed
FTP (File Transfer Protocol): transferring a file between two different systems requires handling these and other incompatibilities.
Electronic mail, remote job entry, directory look up, and various other general purpose and special-purpose facilities.
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Service Primitives:

A service is formally specified by a set of primitives (operations) available to a user or other entity to access the service.

Connection Oriented and Connectionless Services

Computer network layers can offer two different types of services: Connection-oriented and connectionless services through witch one host can connect to another for exchanging their information. these are as follows.

Relation between layers at an interface

 The important terminology, relations between the layers and their interfaces are as follows. 
 Important Terminology
Simplex Communication:  In some systems, data only travel in one direction called simplex communication.

Network Software: The Protocol Hierarchy

The first computer networks were designed with the hardware as the main concern and the software as an afterthought.This strategy no longer works. Network software is now highly structured.

To reduce their design complexity, most networks are organized as a series or hierarchy of  layers or levels.

Classification of Computer Networks

The brief classification of computer networks based on transmission mode as well as based on network scale is as follows.

The Client-Server Model

Small computers (personal computers) have a much better price/performance ratio than large ones.

Mainframes are ten times faster than personal computers, but their cost is thousand times more. This imbalance has caused many system designers to build systems consisting of personal computers, one per user, with data kept on one or more shared machines called file server.

Computer Networks Application

There are two types of network applications.
1. Business Network application
2. personal Network application
A brief applications of computer networks are as follows.v Business Network Application

Difference between Computer Networks and Distributed Systems

Do you know, what is the difference between Computer Networks and Distributed Systems.

• Computer Networks:

– A computer network is an interconnected collection of autonomous computers able to exchange information.
– A computer network usually require users to explicitly login onto one machine, explicitly submit jobs remotely,
explicitly move files/data around the network.

• Distributed Systems:

– The existence of multiple autonomous computers in a computer network is transparent to the user.
– The operating system automatically allocates jobs to processors, moves files among various computers without
explicit user intervention.



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