OSI Model

Appendix. OSI Model

The Organisation Internationale de Normalisation (International Standards Organization, or ISO), which has been developing international standards since 1946, introduced the Open Systems Interconnection (OSI) model in an effort to define a conceptual model for accomplishing communication and interoperability between systems, regardless of the specific hardware or networking characteristics of the systems.

Adopted in 1984, the OSI Basic Reference Model defines a network architecture consisting of seven layers in the communications process, as shown in Figure A-1. In very rough terms, the top layer of the OSI model (layer 7) deals with the end user interface. The next three layers (layers 6, 5, and 4) define the characteristics of the systems at the two ends of the communication. The bottom three layers (layers 3, 2, and 1) define the network facilities necessary to transfer a message.

Figure A-1. Open Systems Interconnection (OSI) model

At the highest layer (the application layer), an application program in one computer communicates with an application in another computer. For example, you might send an electronic mail message to a user or to another computer. The mail program on the sending and receiving computers each resides in its own application layer.

Within the transmitting computer, the information to be transferred is passed down from one layer to the next layer, with each layer performing the work needed to prepare the data for physical transmission. Each side of the transmission performs an equivalent function. For example, a layer on one side adds routing information; the other side removes it.

Each step in the process is a type of transformation because eventually the information is transferred from one computer to the next in a very different formfor example, as radio waves, telephone line signals, or pulses of light in an optical fiber. At the other end of the communications link, the transformations have to be reversed so the other side of the communication can understand the information.

Table A-1 shows the functions particularly those that are security-relatedperformed by each layer of the OSI model.

Table A-1. OSI model

Layer	Function
7 (Application)	Defines how a user accesses the network. Provides the end-user interface and servicesfor example, the specifics of electronic mail and file transfer.
6 (Presentation)	Prepares information for applications, often by data formattingfor example, performing blocking, data compression, and code conversion. Ensures that information is delivered in a form that the recipient can understand. May perform end-to-end encryption.
5 (Session)	Establishes a communications session between the two sides of the communication, and synchronizes the communication. Selects the necessary network services. The session layer typically deals with starting up new tasks, if necessary, and with security (for example, authentication of nodes).
4 (Transport)	Provides transparent, reliable data transmission and end-to-end transmission control, including correcting errors and reestablishing communication after a network failure. This layer is responsible for general network management functions and resource optimization.
3 (Network)	Adds routing information, and selects the appropriate facilities for transmitting the message. Controls the destination addressing and physical routing and flow of data. In packet switching networks, breaks messages into packets at the sending end and reassembles the packets into messages at the receiving end. This layer handles routing through gateways to other networks. The network layer is often X.25, CCITT's protocol for packet-switching networks.
2 (Data Link)	Formats the messages for transmission. Handles point-to-point synchronization and error control for information transmitted over the physical link. May perform link encryption.
1 (Physical)	Establishes the physical, mechanical, and electrical connection (e.g., the actual cable connection) and transmits the actual bits.

If you are involved at all with networking, you have likely already memorized this table. Certainly, you cannot expect to converse with a network engineer without hearing reference to "Layer 3 addressing," or "Application Layer issues." Those new to the model can keep the layers straight using some simple mnemonics, in which the first letter of each word corresponds in order to the layers of the OSI model:

Top (Layer 7) to bottom (Layer 1):

• All People Seem To Need Data Processing.

• All Programmers Seem To Need Double Pepperoni.

Bottom (Layer 1) to top (Layer 7)

• Please Do Not Throw Sausage Pizza Away.

• Please Do Not Tell Sales People Anything.