Lecture 25: The OSIRM in Brief, also Revision

Computer Networks Overview

• The first half is a (roughly) "top-down" exposition of the layered protocol components which make up the Internet.

• The second is a "grab bag" of various topics: network management, encryption/security and technology of electronic commerce

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 OSI Reference Model (OSIRM).

^[1] Actually, the split was closer to 2/3 for the first part and 1/3 for the second.

What is OSI?

• Because it's an important part of the history of computer networks. It's also something you're likely to get asked about in job interviews, which is where it really matters...

• It's a genuine International Standard (ISO/IEC IS 7498:1993) of the International Organisation for Standardisation and International Electrotechnical Committee(ISO/IEC)
  • Mechanism: working draft --> committee draft --> draft IS --> IS, with ballots at each stage.

• Was adopted (in part) as the definitive standard for network architectures by most western governments: GOSIP
  • National policy to mandate an OSI subset (or profile) for government systems, discontinued in 1994/5.

• Huge and complicated, never fully implemented anywhere (see GOSIP, as above) although some portions are commercially available.

• Has had some influence on the Internet community: some upper layer OSI protocols (eg X.500, X.509, ASN.1) have been adopted in the Internet.

The Classic OSI Reference Model Diagram

Diagramatically:

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.

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.

Why Did OSI Fail?

• Technically, the OSI protocols were often criticised for their complexity and lack of "elegance". In fact, the protocols were known to be quite difficult to implement. This alone is not a sufficient reason.

• The TCP/IP protocol suite was supplied at no cost with BSD Unix, the operating system of choice in academia and research throughout the 1980s. By the end of the decade it was widely used.

• The OSI protocols suffered from a "specify first, implement later" design philosophy. At a time when the designers were not really sure of the best way to do things, this led to long delays before any implementation could be undertaken. Classically, the same problems have caused the failure of many large "Information Systems", and for similar reasons.

• The Internet protocols were developed using an "engineering" approach: "broad consensus and running code". Implementation experience continually fed back into design.

• etc...

The Exam Plan

Question 1: Application Protocols

This question is worth 30 marks (out of 120 total) and covers material in lecture 4, lecture 5, lecture 6, lecture 7 and lecture 8.

Question 2: Network and Transport Protocols

This question (and all subsequent) is worth 18 marks and covers material in lecture 3, lecture 10 and lecture 11.

Question 3: Network Technologies

Covers material in lecture 12, lecture 13 and lecture 14. 18 marks.

Question 4: Network Management

Covers material in lecture 15, lecture 16 and lecture 17. 18 marks.

Question 5: Security

Covers material in lecture 18, lecture 19 and lecture 20. 18 marks.

Question 6: Electronic Commerce

Covers material in lecture 21, lecture 22, lecture 23 and lecture 24. 18 marks.

More Information

Lecture 1: Computer Networks Overview

Lecture 2: A Network Architecture Example

Lecture 9: The Programming Interface (all that sockets stuff, etc)

Lecture 25: The OSIRM in Brief, and Revision (ie, this lecture)

• The exam is worth 60% of the final assessment for the subject.

• Note, also, that each question has many parts, and in some cases, sub parts.
  • Each of the parts/subparts is small, and carries between 2 and 6 marks. As usual, it's an exam which tests breadth of knowledge more than depth. On the other hand, it's hard to really stuff it up.

  • It is in your interest to try and keep all the answers to each question together in your answer booklet. Please. If only for the sanity of the examiner...

• Revision:
  1. Old exam papers. Most important.

  2. Tutorial questions.

  3. Lecture notes.

If You Enjoyed It...

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:

INT20DC: Data Communications

• Offered in semester 2, taken by Phil Rice
• Covers the "bottom half" of the protocol stack in far more detail.
• Overlaps slightly with Computer Networks.

INT30WEN: Web Engineering

• Offered in Semester 2, taken by yours truly, Phil Scott
• Extends the Computer Networks section on the Technology of Electronic Commerce.
• Extensive programming required: you will learn all about CGI programming in Perl and various related technologies.

INT30ENS: Encryption and Network Security

• Offered in Semester 2, taken by someone from the Department of Mathematics (maybe Christopher Lenard?), and Phil Scott.
• Extends the Computer Networks sections on encryption and security of Internet-connected computer systems.

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!