OSI Reference Model

When developing a language there must be standards. These standards act as guidelines for all developing languages. The International Standards Organization (ISO) developed a seven layered model called the Open Systems Interconnection (OSI) reference model. This model acts as a guide for companies that wish to develop protocols.

The figure below shows the seven layers and how they are arranged.

Each layer is designed to be responsible for a different function. The following paragraphs will explain each layer’s functions.

Application Layer

The application layer is responsible for application type functions. These functions include file, print, messaging and database services. Actual applications, such as word processing, spreadsheet and database programs do not function here, but the call routines that these programs make do function at this layer. These call routines could make requests to save or retrieve a file from disk, print to a printer, query a database, or send and retrieve email messages.

Presentation Layer

The presentation layer is responsible for data encryption and decryption, data compression and decompression, and data translation. Encryption methods are generally used for security purposes, preventing individuals from accessing unauthorized data materials. Different companies will use their own designed security algorithms. A "C2" classification indicates that the security protocols meet with US government security requirements. Data compression algorithms are used to reduce the amount of space that is used by the data resulting in faster transmission. Just as one may compress coffee into "freezed-dried" coffee, which is the process of removing the water from the coffee leaving the coffee crystals and allowing more coffee to be put into a jar than could be if the coffee was left in it's liquid form. The consumer "uncompresses" the coffee by simply adding hot water to a small amount of the coffee crystals. Data translation techniques provide data conversion from one file type to another. Data conversion could be if a user has a WordPerfect file that is being converted into a Microsoft Word for Windows format or a PC file being converted for use on a Macintosh type computer. If direct conversion is not possible, then translation is made to a mutually agreeable format, such as an ASCII coded file.

Session Layer

The session layer is responsible for session establishment, maintenance and release of the established connection.

Transport Layer

The transport layer is responsible for data segmentation and reliable data delivery. Sometimes when data is passed from the transport layer to the network layer, the data passed may be too large for the network layer to handle. In such a case, the transport layer will segment or divide the data packet into two or more parts. When data segments are passed from the network layer to the transport layer, the transport layer will combine these segments into a single data packet. Another function of the transport layer is reliable data delivery. This ensures data reception. Basically, what you send is received and acknowledged by the recipient. This receipt is analogues to certified or registered U.S. Mail. For each letter you send certified, you get an immediate receipt from the recipient that they actually received the letter.

Network Layer

The network layer is responsible for route discovery and logical addressing. When packets go from system to system, a route must be established so that the packets can get from the source to the destination. Network layer protocols find these routes and maintain a table of possible routes for later usage. When you travel from one destination to another, there are usually several routes that one can take to reach the final destination. Just as one makes a decision as to which route to take, the networks must choose a route for the data to travel on to reach its final destination. Each disparate network must have a unique cabling address common to all systems on the network. This address is considered a logical address. This network addressing scheme for the Internet Protocol is discussed in IP Addressing.

Data Link Layer

The data link layer is responsible for data framing, error control and physical addressing. When data is received at this layer, the packet is "framed" for transmission. A parity bit or a Cyclic Redundancy Check (CRC) field can be added for data error checking. Each system, connected to the network has a unique physical address. This address is usually "burnt-in" into the computer’s network card. It is a physically established number and therefore considered the computer’s physical address. Again, each system must have a unique physical address. Sometimes, this physical address is referred to as the Media Access Control (MAC) address.

Physical Layer

The physical layer is responsible for electrical and mechanical specifications. Physical layer protocols will transmit or receive the data across the media.

When companies develop protocols, they will develop a protocol to function at one or more of these seven layers. A group of protocols that work in conjunction with one another is considered to be a protocol stack.

The OSI model was designed to be a generic layer of what protocols should do and how they interact with each other. The figure below shows data being passed from one system type to another system type.

On the UNIX side, as data is passed from the application layer down to the physical layer, each layer takes the information from the above layer and makes it it’s own data field. That layer will append a header field to this data and pass it down to the next lower layer. The physical layer on the UNIX side will send the entire bit pattern to the physical layer of the DOS system. In order to be translated, the DOS system’s physical layer will then send this information up to the application layer. Each layer will strip off the header information of the data it received, and then pass that field to the above layer. Each layer that strips off header information is left with the same data from its equivalent layer from the originating system. For example, the data that is received by the transport layer in the DOS system was the data transmitted by the transport layer in the UNIX system. Each system’s layer will understand the equivalent layer in the other system.

Many companies used the OSI model to develop their suite of protocols. IBM’s SNA networking architecture was developed prior to the OSI model and matches up to the OSI model very closely. In later chapters, it will be discussed how networking equipment also map out to the OSI reference model.

All People Seem To Need Data Processing is a common mnemonic device to help remember the seven layers of the OSI model. All represents the Application layer and Processing represents the Physical layer.