Lecture 25: The OSIRM in Brief, also Revision and Exam Preview


Computer Network Architectures, Reviewed

In Lecture #2, we saw a layered network architecture model which provided the conceptual framework for the first part of the subject.

We will briefly extend this model today by looking at the Open Systems Interconnection Reference Model (OSIRM, or just "OSI").

What is OSI? ...and why do I need to know about it?


The Classic OSI Reference Model Diagram

The Open Systems Interconnection Reference Model (OSIRM) specifies a 7 layer architecture for Computer Networking.

Diagramatically:

Classic OSI
    7-layer model
The conceptual intention here is that each the software which implements each layer communicates with its Peer Layer software, using services provided by the lower layers.

The software and protocols which implement a layer are called entities in OSIRM.

The diagram is sometimes called (rather disrespectfully) the 7-layer-cake model.


OSI Layer Definitions

Physical
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.


Why Did OSI Fail?

The development of OSI was a huge project, spanning close to 20 years. Yet it failed, and the Internet is now dominant. Why?


The Exam Plan

The exam for 2004 follows the same structure as the 2003 exam paper, except for ONE MAJOR CHANGE: this year you only have to attempt five(5) out of the six questions. That is, you get to ignore one question on the paper. Which one you ignore is your personal choice.
Question 1: Basic Application Protocols
Covers material in

Question 2: HTTP and Related Technologies
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Question 3: Network and Transport Protocols
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Question 4: Network Technologies
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Question 5: Security
Covers material in

Question 6: Network Management
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More Information

The following lectures are not explicitly examined:

Lecture 1 (Introduction To INT21CN Computer Networks)
Lecture 2 (Network Architectures) and
Lecture 25: The OSIRM in Brief, Revision and Exam Preview (ie, this lecture)

Also, note:

[1] This is true for students taking the subject at the Bendigo campus. Presentations of the subject at other campuses may have different weightings.


If You Enjoyed It...

Finally, the advertising.

Computer Networks is, more than anything, an overview subject. If you enjoyed this subject and you think you'd like to know more, you should investigate the following LTU, Bendigo IT subjects:

INT22DC: Data Communications

INT31WE: Web Engineering

INT32ENS: Encryption and Network Security

INT32WS: Web Services

INT32INW: Internetworking

Now the REAL advertising... This probably seems like "just another subject" while you're at Uni, and it is. But the career opportunities in this area are huge. Do not underestimate the value of what you've learnt this semester -- it could be worth heaps to you!

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Copyright 2004 by Philip Scott, La Trobe University.
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