The OSI 7 Layer Model

The Need For Standard Protocols

A protocol is an agreed way of doing something.

In our networks we have software and hardware bought from many different companies, yet it all works together. This is because all of the products use the same protocols - they stick to the agreed way of doing things.

In PC networking, one protocol is most common: the OSI Model.

The OSI Model

In 1984 the International Standards Organization published a set of networking protocols called the Open Systems Interconnection model. It is a set of rules about how networks should operate, and most PC networking hardware and software keeps to these rules.

The 7 Layer Model

The OSI Model, splits networking into 7 layers.

This is sometimes called a protocol stack - there is a stack of 7 protocols. Built-in to the network operating system are programs for each layer. Each layer talks only to the layer above and below. The bottom layer actually uses the NIC and cable to communicate with other computers.

Layer 1 - The Physical Layer

The OSI model for layer 1 says:

How bits (0s and 1s) are to be represented using electricity or light, e.g.:

what voltage is a ‘1’?

how long should a bit be?

                    What connector should be used, and what each pin should be used

                                for?

Network adapters made to the OSI standard will be able to send bits to each other without misunderstanding.

Layer 2 - The Data Link Layer

The job of this layer is to control the physical layer, so that it appears error-free to the layers above.

It takes data passed from above and breaks it into frames, typically a few hundred bytes each. It gives each frame a sequence number and adds special patterns of bits to each frame to mark its beginning and end. It also adds a special set of bits called a CRC - cyclic redundancy check - which can be used by the receiver to check that the data has not been damaged.

The Data Link layer of the receiver takes the bits from its physical layer and puts them back together into frames. Next it checks the CRC to make sure that the bits did not get damaged in transit. If everything is OK, it then sends an acknowledgment back to the sender, confirming receipt. If the sender does not get this acknowledgment it resends the frame.

The data link layer also handles the problems of both ends simultaneously sending and receiving.

Layer 3 - The Network Layer

This layer is concerned with routing data within complicated networks and between connected networks. On a simple network, this layer may be empty.

Layer 4 - The Transport Layer

This layer provides the interface to the lower levels for the levels above. Many services within a computer might need to use the network at the same time. For example I may be downloading a web page, while searching for a file on drive H:, while reading my e-mail. The Network Layer, enables all of these sessions to share a single network connection.

It also takes large pieces of data and breaks them down into smaller pieces before passing them down to the Network Layer. Along with the Network Layer at the other end, it provides a reliable connection for multiple sessions. We can be sure that any data passed from above to the Transport layer, will arrive in the same form, at the Transport Layer of the receiving computer.

Layer 5 - The Session Layer

The Session Layer enables users at different machines to set up sessions. A session is an activity such as logging on to an Internet server, or sending a file from one machine to another.

This layer also inserts checkpoints into the data to be sent (and removes them at the other end). In this way, if an extremely time consuming activity, like a two hour file download, fails part way through, it need only be restarted from the last checkpoint successfully received.

Layer 6 - The Presentation Layer

This is the first layer to be concerned with the ‘meaning’ of the data transported.

On a PC, the letter ‘a’ has ASCII code 01100001. On a large mainframe computer ‘a’ will have a different code. When both are talking on a network, the Presentation Layer of the sender takes characters in ASCII and converts them into a new code defined in the OSI standard. The characters are then sent across the network in this OSI format. When they get to the Presentation layer of the receiver, they are converted from the OSI format to the character code of the receiving machine. Similar codes are defined for numbers, dates, etc. In this way different types of computer can successfully transfer text and numeric data.

This layer may also compress the data to reduce the number of bits to be transmitted. When it is important to keep the data secret, encryption will also take place at this level.

Finally, the network redirector is implemented at this level. This is a program that looks at user’s commands and decides whether to keep them within the user’s PC or to send them across the network, for example, dir c: is processed within the local PC, but dir h: must be processed by a remote machine.

Level 7 - The Application Layer

This provides the interface between user programs and the network. For example, e-mail systems are programmed to talk to the network through the application layer. Multi-user databases are implemented using services provided by the application layer.

Back to top

| list of lessons | Course Plan | Instructor's Info | Links | UAE | HCT|Guestbook|View Guestbook | Books | Search|