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OSI Layer Definitions
- Physical
- the actual "bit path" between two directly-connected
communicating "entities". Hardware.
- Data Link
- Framing and media access control protocols. It's quite useful
to have this separate layer.
- Network
- Equivalent to IP, the Internet Protocol. The OSI model actually
defined a very similar protocol to IP, called CLNS.
- Transport
- Pretty much exactly equivalent to the transport layer in the
Internet. The OSI protocol TP4 was functionally equivalent to
TCP.
- Session
- no real equivalent in the Internet architecture, this layer was
supposed to schedule connections, etc, subject to cost or other
constraints.
- Presentation
- the functions of this layer are integrated into the application
layer in the Internet. Mainly provides data formatting services.
ASN.1 was part of this layer.
- Application
- Very similar in function to the Internet application "layer",
although the OSI application protocols were, in general, vastly
more complex.
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