7 Layers of OSI Reference Model (With Diagram) | Networking |Computer Science

The seven layers identified by ISO in the OSI reference model from bottom to top are: 1. Physical Layer 2. Data Link Layer 3. Network Layer 4. Transport Layer 5. Session Layer 6. Presentation Layer 7. Application Layer.

The physical layer is concerned with transmission of data through communication channels, these communication channels do not themselves form part of the physical layer. This OSI reference model is presented in Fig. 5.1, along with the protocols required for each layer.

It must be remembered that this division into layers is notional and not physical, and has been done for ease in design and analysis. Each layer provides some service to the layer above it and has a layer boundary through which it communicates with it.

Each layer and its functions are discussed one by one below:

1. Physical Layer:

The function of the physical layer is to ensure the sending of messages to another host. The communication channel that sends the message is not part of the physical layer. The function of this layer is to make sure that when a message is sent, it is received by the addressee correctly.

This will entail questions like, how the initial connection is established and how it is disconnected, whether the communication will be simplex or half duplex or full duplex, and so on.

The last bit of information is decided by many issues concerned such as the nature of the lines, the method utilised by the channel for transmission. Other issues that will concern the physical layer will be the interfaces to be utilised and the protocols to be used.

2. Data Link Layer:

As shown in Fig. 5.1, the Data Link Layer is considered to be the layer just above the Physical Layer. This layer provides services to the layer above it, that is, the Network Layer. It operates on frames, into which each message is broken up by attaching some character at the start and end of each frame, to delimit each frame.

It transmits the frames in the correct sequence. The receiver of the message also sends a frame as acknowledgement of receipt, which is interpreted and acted upon by the Data Link Layer.

Because of the possibility of transmission errors and also because the receiver of the message may need to regulate the rate of inflow of the message, synchronization and interfacing methods may not be adequate by themselves and some method to control the transmission, reception (and hence flow control) as well as error control and error correction must be provided by this layer, this achieved by the data link control protocol.

This layer checks for any transmission errors that have remained undetected so far. For example, if the acknowledgement frame does not reach the addressed machine (due to some error such as a voltage surge that destroys the frame), then the Data Link Layer of the sending machine sends that frame again.

Of course, elaborate checks may be required, to make sure that the frame is not being transmitted again unnecessarily. Other issues with which the Data Link Layer is concerned include the clash between data and acknowledgement frames and matching of speeds between despatch and receipt of frames. 

3. Network Layer:

The layer hierarchically above the Data Link Layer has been designated the Network Layer. It decides and controls the route of transmission of each packet in the communications network.

Different algo­rithms have been developed to do this; a suitable algorithm is given to this layer, based on which it performs this function. As a corollary, it also ensures that congestion of message traffic does not occur, since this would lead to bottlenecks.

The avoidance of such bottlenecks is a function of the Network Layer. Incidentally, this problem of Networking is hardly likely to arise in a broadcast network.

The Network Layer is also entrusted with solving the problems of differences in protocols and addressing schemes between different sub-networks, which may occur if these sub-networks have been added to the main network at different times. The Network Layer also maintains the necessary statistics, including for billing purposes, should such billing be required.

4. Transport Layer:

The Transport Layer lies above the communications subnet (the Physical Layer, the Data Link Layer and the Network Layer are considered to be within the communications subnet, since they perform functions related to communications).

This layer is considered to be the bottommost layer of the host network and its standard functions are to create network connections for each transport connection needed by the layer above it, that is, the Session Layer.

It receives data from the Session Layer above it, breaks it down into smaller pieces (if required), and passes it down to the layer below it, the Network Layer. In the protocol architecture, the transport layer protocols sit above the Network Layer which provides network related services and below the application layer and other upper layer protocols.

Naturally, it is free to make multiple network connections if required by the exigencies of the demand from the Session Layer; for example, if the data is large and the Session Layer needs high throughput. On the other hand, it may multiplex several Transport Layer connections into one Network connection, if the exigency requires that.

The Transport Layer determines the type of service that is provided to the user (actually to the Session Layer, the Session Layer and the Layers above can practically be treated as the user).

The Transport Layer creates connections from the origin right up to the destination; this differentiates it (along with the layer above it) from the three layers below it, that is, the Physical Layer, the Data Link Layer and the Network Layer.

Since some machines on the network could be multi-programmed, the Transport Layer gives a header to each message to distinguish between the messages and send each message to the appropriate network. Finally, it participates with other layers in controlling the flow of information.

5. Session Layer:

The Session Layer manages such things as synchronization and token management. If two hosts are in session between one another, the transmission of data between them must be properly controlled. Therefore, it is necessary to make sure that the two hosts do not try to use the same service from the network, at the same time.

This is controlled by the Session Layer. Similarly, suppose a large file transfer is being done between two hosts. If the file transfer crashes at any time before the end of transfer, normally the entire transfer would have to be done again.

This can be particularly time-consuming if the entire file transfer is likely to take a long time, say hours. In such cases, intermediate checkpoints being inserted would prove very useful. One of the functions performed by the Session Layer is to insert checkpoints in such data streams, so that in case the file transfer is aborted in between, the data transfer between files can be resumed from the last checkpoint recorded.

6. Presentation Layer:

The main functions performed by this layer are mainly concerned with the data-presentation service. The concern of the Presentation Layer, therefore, is not just with the reliability of the bits transferred, but also with the syntax and semantics of the data transferred. Different hosts on the network may use different codes for, say, coding characters. It is the function of the Presentation Layer to reconcile these differences.

The reader may note that the data being transferred could consist of such items as names, dates, currency symbols, floating point numbers, integers and so on, each of which may cause this problem of reconciliation of differences in transfer of data. All these differences will be reconciled by the Presentation Layer.

This will be done by the Presentation Layer by converting each such data item from the mode of internal representation by the particular machine to the mode of representation that is standard in the network and back to the mode of internal machine representation, on receipt of the data.

7. Application Layer:

An interesting way to reconcile such things as different and incompatible terminal types in a network is to define a Network Virtual Terminal. This is an abstract concept developed to take care of terminal incompatibility. One of the protocols used by the Application Layer is to do this.

In Network Virtual Terminal, the function of the software is to map all the functions of the virtual terminal on to the existing real terminal. An example could be the use of a text editor. In different terminals, for example, there could be different methods of creating blank lines or placing the cursor at an appropriate position on the screen. This software must make this function for the text editor.

This software resides in the Application Layer. Similarly, functions such as E-mail, file transfers, directory look-ups are examples of facilities required by the user. The Application Layer handles all the incompatibilities involved in functions of this nature. The Network Virtual Terminal, incidentally, is not the only method of handling incompatibilities mentioned above; other methods also exist.