A.2.1 The Seven Layers of the OSI Reference Model

Figure A.2 below shows the layers of the OSI Reference Model:

1. The physical layer is concerned with the physical wiring used to connect different systems together on the network. Examples include the serial and parallel cables mentioned earlier, Ethernet and Token Ring cabling, telephone cables, and even the specific connectors and jacks used by these cabling systems. Without strictly standardized definitions for the cabling and connectors, vendors might not implement them in such a way as that they would function with other vendor's implementations, which in turn would make it impossible for any communications to occur whatsoever. Each of these wiring systems therefore follows very strict standards, ensuring that the systems will at least be able to communicate without having to worry about the underlying cabling.
   
   
2. The data-link layer is used to define how information is transmitted across the physical layer, and is responsible for making sure that the physical layer is functioning properly. Some networks - such as the public telephone system, AM/FM radio and television - use analog sine-waves to transmit information, while most computer networks use digital "square" pulses to achieve this objective. If there are any problems with transmitting the information on the physical cabling (perhaps due to a damaged wire or circuit), then this layer must deal with those errors, either attempting to retransmit the information or reporting the failure to the network layer.
   
   
3. The network layer is used to identify the addresses of systems on the network, and for the actual transmission of data between the systems. The network layer must be aware of the physical nature of the network, and package the information in such a way that the data-link layer can deliver it to the physical layer. For example, if a telephone line is the physical layer, then the network layer must package the information in such a way that the data-link layer can transmit it over an analog circuit. Likewise, if the physical layer is a digital Ethernet LAN, then the network layer must encapsulate the information into digital signals appropriate for Ethernet, and then pass it to the data-link layer for transmission.
   
   On many networks, the network layer does not provide any integrity checking. It simply provides the packaging and delivery services, assuming that if the data-link layer is not reporting any errors then the network is operational. Broadcast television and radio work in this manner, assuming that if they can transmit a signal, then it can also be received. Many digital networking technologies also take this approach, leaving it up the higher level protocols to provide delivery tracking and reliability guarantees.
   
   
4. The transport layer provides the reliability services lacking from the network layer, although only for basic transmission services, and not for any application- or service-specific functions. The transport layer is responsible for verifying that the network layer is operating efficiently, and if not, then to either request a retransmission or to return an error to the layer above it. Since higher-level services have to go through the transport layer, all transport services are guaranteed when this layer is designed into the network software and used. Not all systems mandate that the transport layer provide reliability; indeed many networks provide unreliable transport layers for non-essential services such as broadcast messages.
   
   
5. The session layer is responsible for establishing "connections" between systems, applications or users. The session layer may receive this request from any higher layer, and then will negotiate a connection using the lower layers. Once a connection is established, the session layer simply provides an interface to the network for the higher layers to communicate with. Once the higher layers are finished, the session layer is responsible for destroying the connection as well.
   
   
6. The presentation layer provides a consistent set of interfaces for applications and services to utilize when establishing connections through the session layer. Although these interfaces could also exist at the session layer, that would burden it unnecessarily. It is better to have the session layer only manage sessions and not worry about verifying data or providing other extended services. An example of a service provided by the presentation layer is data- compression, allowing applications to take advantage of the performance gains that compression provides without forcing the applications to develop these services themselves.
   
   
7. Finally, the application layer provides the network's interface to end-user applications and services such as printing or file-sharing. This layer also provides some management services to ensure that the interfaces are being addressed and used correctly.
   

The OSI Reference model is extremely useful as a tool for discussing various network services. For example, if we were to look at a simple network service such as printing a document from a word processor to a locally-attached printer, we could use the OSI Reference Model to determine how this simple task was being achieved. We could also use the model to determine how printing over a Novell network was done, or how printing over a TCP/IP network was accomplished. Because all three of these examples use the same model, they can all be compared to each other even though they all use extremely different technologies to achieve the same objective.

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