The OSI reference model provides a model for computer networking. The OSI Reference Model was defined by the International Standards Organisation (ISO) and consists of seven layers. Each layer has a task to perform. The layers are: Application, Presentation, Session, Transport, Network, Data Link, and Physical.

The layers from Application through Network generally are implemented with both hardware and software. The responsibilities of each layer are outlined in TABLE 1.

ief definition of the OSI model and then compares it to the Department of Defence four-layer data communications model that originally was used to classify protocols within the TCP/IP protocol suite.

Layer	Description
Layer 7: Application Layer	The application layer consists of application programs and the user interface. It is through features in this layer that all exchange of meaningful information occurs between users.
Layer 6: Presentation Layer	The Presentation layer defines the representation of data, so data is exchanged in an understandable format.
Layer 5: Session Layer	The Session layer sets up and terminates communications on the network. It also manages the dialogue between users and systems. It is at this level that user and machine names are interpreted.
Layer 4: Transport Layer	The Transport layer controls the quality and reliability of the data transmission. Packets are sequenced and acknowledged at this layer. An example of a layer 4 protocol is TCP.
Layer 3: Network Layer	The Network layer routes data through the network. It allows any properly configured pair of nodes on an internetwork to communicate. The Network later calculates routes and controls congestion. An example of a layer 3 protocol is NetWare's IPX.
Layer 2: Data Link Layer	The Data Link layer packages and unpackages data for transmission across a single link. It deals with data corruption (through checksumming) and coordination of shared media. An example of a layer 2 protocol is Ethernet.
Layer 1: Physical Layer	The Physical layer establishes the physical connection between a computer and the network. It also controls the transmission of information and specifies the mechanical and electrical characteristics of the protocol in terms of connector size, pin assignments, and voltage levels. An example of a layer 1 protocol is RS-232.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE 1: RESPONSIBILITIES OF THE 7 OSI REFERNCE LAYERS.

The concept behind this model of network computer communication is that, at the application layer, an application will want to send some data to another application residing on another machine on the network. For example, a workstation mail program wishing to send e-mail to another user via a mail server. The workstation mail application has take the text of the message to be sent and package it in some way with an address if its destination. This information must somehow be encoded into electrical signals representing binary 1s and 0s that can be transmitted over a network cable. In this example, the application at later 7 will define what text gas to descend through the layers of this model, through software layers, through the network card and cable hardware, across a network cable of some kind, and ascend through the corresponding layers on the destination machine, to the destination application.

As the message descends through the layers, it looks less like human language and more like the 1s, and 0s that a computer understands. The terminology used by the ISO to describe this process is as precise and as abstract as one would expect of a large international committee.

There are two main operations that ensue between layers that should be noted:
1. Each layer will only 'talk' to its corresponding later on another machine, and
2. Each layer encapsulates information from the layer above as the message descends through the layers for transmission. The message that layer on the sending machine will send to its corresponding later on the receiving machine is termed a Protocol Data Unit (PDU). The message that passes between layers on the same machine is scales a Service Data Unit (SDU).

To make it clear about which type of data unit we are talking, each PDU, and SDU has a prefix attached. See FIG 1 below:

 

 

FIG 1: Units of communication described in the OSI data communication models

Now, we consider the Transport layer sending a Transport PDU from the sending to the receiving machine. To get the TPDU from the sending to the receiving machine, the sending machine's Transport layer sends a Network SDU to its Network Layer. The Network layer passes an LSDU to the Data Link layer, which passes a PhSDU to the Physical layer for transmission on to the network cable.
At the receiving machine, the process is reversed, until the Network layer sends a Network SDU to the Transport layer, at which point we can say that the receiving machine's Transport layer has received a TPDU from the sending machine.