Model+OSI

__ OSI MODEL __ The OSI reference model (Open System Interconnection, Open Systems Interconnect ) was developed by theISO(International StandardOrganisation,International Organization for Standardization ) in 1984 asa guide for defininga set ofopen protocols.Its purposeis to provide acommon basis for thecoordinated developmentof standards forinterconnectionof systems,while allowingexisting standardssetin the perspective ofglobalreference model. It also aims toidentify areasthat requirethe developmentand improvement ofstandards andconsistencyof all standardswithin acommon framework

The OSI modelhas seven layersor levels:




 * 1.Physical**

The physical layer communicates directly with the media, is responsible for sending and receiving bits. This layer decides what voltage and what describes a 0 a 1; microseconds a bit lasts, whether transmission can be performed simultaneously in both directions, how to set the initial connection and how it is broken, and so on.


 * 2.Data link**

The data link layer provides the communication node to node in the same local area network. This should provide an address mechanism that allows messages to deliver the correct nodes and must translate messages from the upper layers in bits that can transmit the physical layer.

When the link layer receives a message is formatted to transform it into a data frame sections of a data frame are called fields. The fields of a frame type are the following:

• Start indicator.

• Source Address.

• destination address.

• Information control.

• Data.

• Error Handling.


 * 3.Network**

The network layer is responsible for controlling the operation of the subnet, providing a mechanism that directs messages from one network to another.

To deliver messages to a network, each subnet must be uniquely identified by a subnet address. When receiving a message from the upper layers, the network layer adds a header to the message includes the addresses of source and destination subnet. This data set added to the network layer is called a package. The information in the subnet address is used to deliver the message to the correct subnet, then the data link layer may use the address of the node to deliver the message.

The process of getting the packets to the correct subnet routing is called, and the devices that route the packets are called routers (or routers).


 * 4.** **Transport**

The basic function of the transport layer is to accept data from the session layer, split into smaller units if necessary, pass it on to the network layer and ensure that all units arrive correctly at the other end.

The transport layer also ensures flow control, ie, regulates the flow of information so that a node does not get saturated fast one slow.


 * 5.** **Session**

The session layer allows users on different machines to establish sessions between them. A session allows ordinary data transport, as does the transport layer, but also provides enhanced services useful in some applications.

One of the services of the session layer is to manage dialogue control. The way of dialogue can be of three types:

• Simplex.

• half duplex.

• Duplex.


 * 6.** **Presentation**

The role of the presentation layer is to present data to the application layer, dealing with translating data with a character encoding to another address if necessary (because the start and end nodes use a different encoding such as ASCII and Unicode), ditto with the representation of numerical data.

Other functions that correspond to the presentation layer is the encryption / decryption and compression / decompression of data.


 * 7.** **Application**

The application layer provides services used by applications, such as:

• Email,

• access to remote files

• remote task execution,

• network management.

As the only layer that provides services directly to application processes, the application layer provides all OSI services directly used by application processes.