the actual "bit path" between two directly-connected
communicating "entities". Hardware, in other words.
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, as well as
several others which were nothing like IP.
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.
Lecture 25: The OSIRM
in Brief, also Revision and Exam Preview