What does it do?
The OSI Reference model describes seven protocol layers, each with a unique function. In order to converse knowledgeably about network architectures and devices, it is important to be aware of this model. The seven layers and their functions are as follows:

Physical Layer -
This layer is concerned with the encoding and decoding of digital bits (1s and 0s) between network interfaces. It is typically a function of the interface card, rather than a software utility.

Data Link Layer -
The data link layer is concerned with the transmission of packets from one network interface card to another, based on the physical address of the interface cards. Typical data link protocols are Token Ring and Ethernet. These protocols are typically enabled by the device driver that comes with the network interface card. The device driver will be loaded in a specific order with the other protocol programs.

The data link layer is a point-to-point protocol, much like an airline flight. If you have a direct flight, one plane can get you to your final destination. However, if you have a connecting flight, the plane gets you to your connection point, and another will get you from there to your destination, but its up to you to make the connection yourself. Bridges operate at this layer.

Network Layer -
The network layer is concerned with the end-to-end delivery of messages. It operates on the basis of network addresses, that are global in nature. Using the airline example, the network layer makes sure that all the connecting flights are made, so that you will actually arrive in your final destination. Network layer protocols include the IPX portion of the Netware IPX/SPX protocol, and the IP portion of the TCP/IP protocol stack. Routers operate at this level.

Transport Layer -
The transport layer is concerned with issues such as the safe, intact arrival of messages. It makes the receiver aware that it is going to receive a message, insures that it does get it, and can control the flow of the message if the receiver is getting it too fast, or re-transmit portions that arrive garbled. In our airline analogy, suppose you are flying your children to Grandmas house unaccompanied. The data link layer planes will make their flights. A small fee will insure that network layer ground attendants get your kids from one flight to their connection. The transport layer will call Grandma to let her know they are coming and what their luggage looks like, and will expect a call from Grandma when she has them safe and sound. Typical transport layer protocols are the SPX portion of Netware SPX /IPX and the TCP portion of TCP/IP.

Session Layer -
The session layer is the layer that manages all the activities of the layers below it. It does this by establishing what is called a virtual connection. Essentially a virtual connection is established when a transmitting station exchanges messages with the receiving station, and tells it to set up and maintain a communications link. This is similar to what happens when you log into the network. Once you have logged in, a connection is maintained throughout the course of your user session until you log out, even though you may not be accessing the network continuously.

Presentation Layer -
The presentation layers function is to establish a common data format between communicating nodes. It is responsible for formatting the data in a way the receiving node can understand. It may also perform data translation between different data formats. Examples of data format differences include byte ordering (should it be read from left to right, or vice versa) and character set (ASCII characters or IBMs EBCDIC character set) as well as differences in numeric representation.

Application Layer-
The application layer provides the user-accessible services of the network. These services include such things as network file transfer and management, remote job initiation and control, virtual terminal sessions with attached hosts, electronic mail services, and network directory services.

How is it used?
The OSI reference model is a common point of reference for categorizing and describing network devices, protocols, and issues. Different network devices are designed to operate at certain protocol levels. Also, each network protocol can be mapped to the OSI reference model.

Where do I get more information?
Contact your local CBV Office.