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OSI Reference Model (Open Systems Interconnect)

The protocols defined in each layer of the model are responsible for:

Messages are transmitted from a computer through level 7 down to the Physical Level 1 for transmission to the receiving computer.
There are seven layers with Layer 1 being the lowest.

LEVEL 1 (Physical Layer) This layer defines the physical characteristics of the interface, such as mechanical components, connectors, and electrical conditions such as voltage levels.  Data communication links such as RS232, RS449.  LAN systems such as Ethernet, token-ring and FDDI (Fibre distributed Data Interface) [Top]

LEVEL 2 (Data Link Layer)  This layer defined the rules for sending and receiving information across a physical connection between two devices. Its main purpose is to divide the data given to it by the upper layer (Network Layer) into frames, which are then transmitted via the physical layer one frame at a time.[Top]

LEVEL 3 (Network Layer) This layer provides internet services. This ensures that packets of information reach their destination when travelling across multiple point-to-point links.  If a message is transmitted to an address location on the same LAN then data-link protocols are used.  But if the message is toi an address on another LAN then network protocols are used. (such as the TCP/IP set of protocols) [Top]

LEVEL 4 (Transport Layer)   This layer sets up connection protocols between source and destination and transmits data in a stream of packets. Each packet is numbered to ensure correct delivery. The packets may be transmitted across a mesh network via a variety of routers but will be re-assembled at the receiving end into correct order. [Top]

LEVEL 5 (Session Layer) This layer coordinates the exchange of information between systems by using conversational techniques and dialogues.  The latter can indicate where to restart communication if a temporary break in transmission occurs.[Top]

LEVEL 6 (Presentation Level) This layer establishes the protocols for presenting data.Data for printing or display are formatted at this layer.Data encryption and the translation of other character sets are handled.[Top]

LEVEL 7 (Application Layer) This layer is where applications that deal with file transfers, terminal sessions, and message exchange (eg e-mail)[Top]


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