The architecture of a network defines the protocols and components necessary to satisfy application requirements. One popular standard for illustrating the architecture is the seven-layer Open System Interconnect (OSI) Reference Model, developed by the International Standards Organization (ISO). OSI specifies a complete set of network functions, grouped into layers (see Figure 2-6), which reside within each network component. The OSI Reference Model is also a handy model for representing the various standards and interoperability of a wireless network.
The OSI layers provide the following network functionality:
Layer 7?Application layer: Establishes communications among users and provides basic communications services such as file transfer and e-mail. Examples of software that runs at this layer include Simple Mail Transfer Protocol (SMTP), HyperText Transfer Protocol (HTTP) and File Transfer Protocol (FTP).
Layer 6?Presentation layer: Negotiates data transfer syntax for the application layer and performs translations between different data formats, if necessary. For example, this layer can translate the coding that represents the data when communicating with a remote system made by a different vendor.
Layer 5?Session layer: Establishes, manages, and terminates sessions between applications. Wireless middleware and access controllers provide this form of connectivity over wireless networks. If the wireless network encounters interference, the session layer functions will suspend communications until the interference goes away.
Layer 4?Transport layer: Provides mechanisms for the establishment, maintenance, and orderly termination of virtual circuits, while shielding the higher layers from the network implementation details. In general, these circuits are connections made between network applications from one end of the communications circuit to another (such as between the web browser on a laptop to a web page on a server). Protocols such as Transmission Control Protocol (TCP) operate at this layer.
Layer 3?Network layer: Provides the routing of packets though a network from source to destination. This routing ensures that data packets are sent in a direction that leads to a particular destination. Protocols such as Internet Protocol (IP) operate at this layer.
Layer 2?Data link layer: Ensures medium access, as well as synchronization and error control between two entities. With wireless networks, this often involves coordination of access to the common air medium and recovery from errors that might occur in the data as it propagates from source to destination. Most wireless network types have a common method of performing data link layer functions independent of the actual means of transmission.
Layer 1?Physical layer: Provides the actual transmission of information through the medium. Physical layers include radio waves and infrared light.
The combined layers of a network architecture define the functionality of a wireless network, but wireless networks directly implement only the lower layers of the model. A wireless NIC, for example, implements the data link layer and physical layer functions. Other elements of the network (such as wireless middleware), however, offer functions that the session layer implements. In some cases, the addition of a wireless network might impact only the lower layers, but attention to higher layers is necessary to ensure that applications operate effectively in the presence of wireless network impairments.
Each layer of the OSI model supports the layers above it. In fact, the lower layers often appear transparent to the layers above. For example, TCP operating at the transport layer establishes connections with applications at a distant host computer, without awareness that lower layers are taking care of synchronization and signaling.
As shown in Figure 2-6, protocols at each layer communicate across the network to the respective peer layer. The actual transmission of data, however, occurs at the physical layer. As a result, the architecture allows for a layering process where a particular layer embeds its protocol information into frames that are placed within frames at lower layers. The frame that is sent by the physical layer actually contains frames from all higher layers. At the destination, each layer passes applicable frames to higher layers to facilitate the protocol between peer layers.