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The Holmes Page The ISO Open Systems Interconnection Model

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The International Standards Organization Open Systems Interconnection (ISO/OSI) model describes the communication process as a hierarchy of layers, each dependant on the layer directly beneath it. 
Each layer has a defined interface with the layer above and the layer below. This interface is made flexible so that designers can implement various communication protocols and still follow the standard. All of the major network operating systems conform to the ISO/OSI model. 
Layer 1: Physical

This layer defines the physical connection between the computer and the network, including the mechanical aspects of the connection (cables and connectors) and the electrical aspects (voltage, current levels, and the techniques used to modulate the signal). This layer also defines the network's topology. 
Layer 2: Data Link

This layer defines the protocol that computers must follow to access the network for transmitting and receiving messages. These messages are sent onto the network as specially formatted discrete frames of information rather than being continuously broadcast. If data input to this layer is large enough, the data-link layer will break it up into several frames. This layer also specifies handling of frame-receipt acknowledge (if required). 

The first two layers together are called the Hardware Layer.

Layer 3: Network

This layer defines how packets -- communications composed of a defined format of data frames -- are routed and relayed between networks. It also regulates packet flow and defines how status messages are sent to computers on the network. 
Layer 4: Transport

The transport layer defines how you address the physical locations/devices on the network, how connection between nodes can be made and un-made, what the protocol is for guaranteed message delivery, and how to handle the inter-network routing of messages. 
Layer 5: Session

This layer functions as the conceptual interface to the transport layer for applications. For example, it is this layer that lets you refer to devices by name rather than by their network address. This lets you write software that will run on any given installation of a given kind of network. 

Layers three, four, and five are frequently described as the network's subnet level. NETBIOS and LU 6.2 (the basis for IBM's Advanced Program-to-Program Communications software) on IBM's Token-Ring Network, MAP, and TOP are all examples of subnet protocols. Although inconvenient, it is possible to support more than one protocol in a single subnet. (A device called an internet can join networks with different hardware levels but identical subnet and upper levels. If subnets are also different, the connection is then called a gateway.)

Layer 6: Presentation

This layer defines how applications can enter the network, and it translates the format and syntax of the data they produce and consume for it's transmission on the network. 
Layer 7: Application

This uppermost layer simply defines the network applications that support file serving. Conceptually, this is where electronic mail and other network utility software exists.

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