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The OSI Model

December 24, 2007

Okay, so I lied… Here’s yet another topic: The seven-layer OSI model.

 The OSI, or Open Systems Interconnect, model consists of seven distinct layers and was developed by a standards organization called ISO, which is derived from the Greek word for equal.

Each layer, described below, represents a particular aspect of network functionality:

Layer 1: Physical Layer – This layer is responsible for defining the network standards relating to electrical signals, connectors and media types and the way that data is placed on the network media.

Layer 2: Data Link Layer – This layer is responsible for gathering together and completing all of the elements that make up a data packet and putting the whole thing together so that it can be passed to a Physical layer device and onto the network. This layer assembles outgoing packets and generates the CRC. For incoming packets, it checks the data validity by comparing its locally generated CRC value with that sent in the packet. This layer also determines whether it’s possible or permissible at any instant to try and send data to the network.

Layer 3: Network Layer – This layer understands addressing – how to find the ultimate destination address for a data packet – and routing, to make sure the packet ends up in the right place.

Layer 4: Transport Layer – This layer breaks the data packet into smaller, manageable chunks that will fit inside two or more packets. This is known as fragmentation. The Transport layer is also responsible for confirming whether transmitted packets have reached their destination intact and retransmitting them if they haven’t. For incoming packets, the Transport layers reassembles the fragmented data (called defragmentation), ensuring that received packets are processed in the correct order. The layer also manages the flow of the data, so that packets are sent at a pace that is suitable for receiving device and general network conditions.

Layer 5: Session Layer – This layer sets up, manages and terminates the data connections (called sessions) between networked devices.

Layer 6: Presentation Layer – This layer is responsible for managing and translating information by catering to differences in the ways some systems store and manage their data. Protocols in this layer are responsible for data encryption.

Layer 7: Application Layer – This layer represents the network-related program code and functions running on a computer system. Some application layer functions do exist as user-executable programs – for example, some file transfer and e-mail applications reside entirely on this layer.

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