The OSI Reference Model

The purpose of the OSI reference model

The OSI reference model is the primary model for network communications. Although there are other models in existence, most network vendors, today, relate their products to the OSI reference model, especially when they want to educate users on the use of their products. They consider it the best tool available for teaching people about sending and receiving data on a network.

The OSI reference model allows you to view the network functions that occur at each layer. More importantly, the OSI reference model is a framework that you can use to understand how information travels throughout a network. In addition, you can use the OSI reference model to visualize how information, or data packets, travels from application programs (e.g. spreadsheets, documents, etc.), through a network medium (e.g. wires, etc.), to another application program that is located in another computer on a network, even if the sender and receiver have different types of network media.

In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. This separation of networking functions is called layering. Dividing the network into these seven layers provides the following advantages:

• It breaks network communication into smaller, simpler parts.
• It standardizes network components to allow multiple-vendor development and support.
• It allows different types of network hardware and software to communicate with each other.
• It prevents changes in one layer from affecting the other layers, so that they can develop more quickly.
• It breaks network communication into smaller parts to make learning it easier to understand.

The OSI Reference Model

The seven layers of the OSI reference model

The problem of moving information between computers is divided into seven smaller and more manageable problems in the OSI reference model. Each of the seven smaller problems is represented by its own layer in the model. The seven layers of the OSI reference model are:

Layer 7: The application layer
Layer 6: The presentation layer
Layer 5: The session layer
Layer 4: The transport layer
Layer 3: The network layer
Layer 2: The data link layer
Layer 1: The physical layer

During the course of this semester, you will start your studies with Layer 1 and work your way through the OSI model, layer by layer. By working through the layers of the OSI reference model, you will understand how data packets travel through a network and what devices operate at each layer as data packets travel through them. As a result, you will understand how to troubleshoot network problems as they may occur during data packet flow. For more information about the OSI model, visit the following site:

The OSI Reference Model

The functions of each layer

Each individual OSI layer has a set of functions that it must perform in order for data packets to travel from a source to a destination on a network. Below is a brief description of each layer in the OSI reference model as shown in the Figure.

Layer 7: The Application Layer
The application layer is the OSI layer that is closest to the user; it provides network services to the user's applications. It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model. Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs. The application layer establishes the availability of intended communication partners, synchronizes and establishes agreement on procedures for error recovery and control of data integrity. If you want to remember Layer 7 in as few words as possible, think of browsers.

Layer 6: The Presentation Layer
The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system. If necessary, the presentation layer translates between multiple data formats by using a common format. If you want to think of Layer 6 in as few words as possible, think of a common data format.

Layer 5: The Session Layer
As its name implies, the session layer establishes, manages, and terminates sessions between two communicating hosts. The session layer provides its services to the presentation layer. It also synchronizes dialogue between the two hosts' presentation layers and manages their data exchange. In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service, and exception reporting of session layer, presentation layer, and application layer problems. If you want to remember Layer 5 in as few words as possible, think of dialogues and conversations.

Layer 4: The Transport Layer
The transport layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system. The boundary between the transport layer and the session layer can be thought of as the boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data transport issues.

The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details. Specifically, issues such as how reliable transport between two hosts is accomplished is the concern of the transport layer. In providing communication service, the transport layer establishes, maintains, and properly terminates virtual circuits. In providing reliable service, transport error detection-and-recovery and information flow control are used. If you want to remember Layer 4 in as few words as possible, think of quality of service, and reliability.

Layer 3: The Network Layer
The network layer is a complex layer that provides connectivity and path selection between two host systems that may be located on geographically separated networks. If you want to remember Layer 3 in as few words as possible, think of path selection, routing, and addressing.

Layer 2: The Data Link Layer
The data link layer provides reliable transit of data across a physical link. In so doing, the data link layer is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control. If you want to remember Layer 2 in as few words as possible, think of frames and media access control.

Layer 1: The Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications. If you want to remember Layer 1 in as few words as possible, think of signals and media.

The OSI Reference Model

Encapsulation

You know that all communications on a network originate at a source, and are sent to a destination, and that the information that is sent on a network is referred to as data or data packets. If one computer (host A) wants to send data to another computer (host B), the data must first be packaged by a process called encapsulation.

Encapsulation wraps data with the necessary protocol information before network transit. Therefore, as the data packet moves down through the layers of the OSI model, it receives headers, trailers, and other information.

To see how encapsulation occurs, lets examine the manner in which data travels through the layers as illustrated in the Figure . Once the data is sent from the source, as depicted in the Figure, it travels through the application layer down through the other layers. As you can see, the packaging and flow of the data that is exchanged goes through changes as the networks perform their services for end-users. As illustrated in the Figures, networks must perform the following five conversion steps in order to encapsulate data:

Figure :

1. Build the data.

As a user sends an e-mail message, its alphanumeric characters are converted to data that can travel across the internetwork.

2. Package the data for end-to-end transport.

The data is packaged for internetwork transport. By using segments, the transport function ensures that the message hosts at both ends of the e-mail system can reliably communicate.

3. Append (add) the network address to the header.

The data is put into a packet or datagram that contains a network header with source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.

4. Append (add) the local address to the data link header.

Each network device must put the packet into a frame. The frame allows connection to the next directly-connected network device on the link. Each device in the chosen network path requires framing in order for it to connect to the next device.

5. Convert to bits for transmission.

The frame must be converted into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire). A clocking function enables the devices to distinguish these bits as they travel across the medium. The medium on the physical internetwork can vary along the path used. For example, the e-mail message can originate on a LAN, cross a campus backbone, and go out a WAN link until it reaches its destination on another remote LAN. Headers and trailers are added as data moves down through the layers of the OSI model.

The OSI Reference Model

Names for data at each layer of the OSI model

In order for data packets to travel from the source to the destination, each layer of the OSI model at the source must communicate with its peer layer at the destination. This form of communication is referred to as Peer-to-Peer Communications. During this process, each layer's protocol exchanges information, called protocol data units (PDUs), between peer layers. Each layer of communication, on the source computer, communicates with a layer-specific PDU, and with its peer layer on the destination computer as illustrated in the Figure.

Data packets on a network originate at a source and then travel to a destination. Each layer depends on the service function of the OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field; then it adds whatever headers and trailers the layer needs to perform its function. Next, as the data moves down through the layers of the OSI model, additional headers and trailers are added. After Layers 7, 6, and 5 have added their information, Layer 4 adds more information. This grouping of data, the Layer 4 PDU, is called a segment.

The network layer, for example, provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer has the task of moving the data through the internetwork. It accomplishes this task by encapsulating the data and attaching a header creating a packet (the Layer 3 PDU). The header contains information required to complete the transfer, such as source and destination logical addresses.

The data link layer provides a service to the network layer. It encapsulates the network layer information in a frame (the Layer 2 PDU); the frame header contains information (e.g. physical addresses) required to complete the data link functions. The data link layer provides a service to the network layer by encapsulating the network layer information in a frame.

The physical layer also provides a service to the data link layer. The physical layer encodes the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.