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Many different types of computers are used today, varying in operating systems, CPUs, network interfaces, and many other qualities. These differences make the problem of communication between diverse computer systems important. In 1977, the International Organization for Standardization (ISO) created a subcommittee to develop data communication standards to promote multivendor interoperability. The result is the Open Systems Interconnection (OSI) model.
The OSI model doesn’t specify any communication standards or protocols; instead, it provides guidelines that communication tasks follow.
NOTE: It’s important to understand that the OSI model is simply a model—a framework—that specifies the functions to be performed. It doesn’t detail how these functions are performed. ISO, however, does certify specific protocols that meet OSI standards for parts of the OSI model. For example, the CCITT X.25 protocol is accepted by ISO as an implementation that provides most of the services of the Network layer of the OSI model.
To simplify matters, the ISO subcommittees took the divide-and-conquer approach. By dividing the complex communication process into smaller subtasks, the problem becomes more manageable, and each subtask can be optimized individually. The OSI model is divided into seven layers:
TIP: One easy way to remember the order of the layers (from the top down) is by making a sentence from the first letters of the layer names: All People Seem To Need Data Processing.
Each layer is assigned a specific set of functions. Each layer uses the services of the layer beneath it and provides services to the layer above it. For example, the Network layer uses services from the Data Link layer and provides network-related services to the Transport layer. Table 23.2 explains the services offered at each layer.
NOTE: The concept of a layer making use of services and providing services to its adjacent layers is simple. Consider how a company operates: the secretary provides secretarial services to the president (the next layer up) to write a memo. The secretary uses the services of a messenger (the next layer down) to deliver the message. By separating these services, the secretary (application) doesn’t have to know how the message is actually carried to its recipient. The secretary merely has to ask the messenger (network) to deliver it. Just as many secretaries can send memos in this way by using a standard messenger service, a layered network can send packets by handing them to the network layer for delivery.
| Layer | Description |
|---|---|
| Physical (Layer 1) | This layer provides the physical connection between a computer system and the network. It specifies connector and pin assignments, voltage levels, and so on. |
| Data Link (Layer 2) | This layer “packages” and “unpackages” data for transmission. It forms the information into frames. A frame represents the exact structure of the data physically transmitted across the wire or other medium. |
| Network (Layer 3) | This layer provides routing of data through the network. |
| Transport (Layer 4) | This layer provides sequencing and acknowledgment of transmission. |
| Session (Layer 5) | This layer establishes and terminates communication links. |
| Presentation (Layer 6) | This layer does data conversion and ensures that data is exchanged in a universal format. |
| Application (Layer 7) | This layer provides an interface to the application that a user executes: a “gateway” between user applications and the network communication process. |
NOTE: Don’t confuse the Application layer with application programs you execute on the computer. Remember that the Application layer is part of the OSI model that doesn’t specify how the interface between a user and the communication pathway happens; an application program is a specific implementation of this interface. A real application typically performs Application, Session, and Presentation layer services and leaves Transport, Network, Data Link, and Physical layer services to the network operating system.
Each layer communicates with its peer in other computers. For example, layer 3 in one system communicates with layer 3 in another computer system.
When information is passed from one layer down to the next, a header is added to the data to indicate where the information is coming from and going to. The header-plus-data block of information from one layer becomes the data for the next. For example, when layer 4 passes data to layer 3, it adds its own header. When layer 3 passes the information to layer 2, it considers the header-plus-data from layer 4 as data and adds its own header before passing that combination down.
In each layer, the information units are given different names (see Table 23.3). Therefore, by knowing the terms used to reference the data, you know which layer of the model is being discussed.
| OSI Layer | Information Unit Name |
|---|---|
| Application | Message |
| Transport | Segment |
| Network | Datagram |
| Data Link | Frame (also called packet) |
| Physical | Bit |
Before the advent of the OSI model, the U.S. Department of Defense defined its own networking model, known as the DOD model. The DOD model is closely related to the TCP/IP suite of protocols, as explained in the following section.
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