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

Computer Network Architectures, Reviewed

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?

• 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 still (?) 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, 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?

• 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 for their failure.

• 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: Basic Application Protocols

Covers material in

• Lecture 3 (Introduction to Internet Applications, Telnet),
• Lecture 4 (Electronic Mail: formats and protocols),
• Lecture 9 (Socket Programming Interface) and
• Lecture 10 (Domain Name System, DNS).

Question 2: HTTP and Related Technologies

Covers material in

• Lecture 5 (HTML and HTTP Basics),
• Lecture 6 (HyperText Transfer Protocol (HTTP) in detail),
• Lecture 7 (HTTP/1.1 Introduction),
• Lecture 08 (Web Commerce) and (maybe...)
• Lecture 24 (Data Formats and Encoding -- A Philosophy Lecture) (not really...).

Question 3: Network and Transport Protocols

Covers material in

• Lecture 11 (Reliable Transport -- TCP),
• Lecture 12 (IP Addressing) and
• Lecture 13 (IP and Internetworks).

Question 4: Network Technologies

Covers material in

• Lecture 14 (Multiaccess Networks),
• Lecture 15 (Point-to-Point Data Links) and
• Lecture 16 (Internet Topology and Structure).

Question 5: Security

Covers material in

• Lecture 17 (Introduction to Encryption: Single Key Systems),
• Lecture 18 (Public Key Encryption),
• Lecture 19 (Practical Encryption) and
• Lecture 20 (Network Security).

Question 6: Network Management

Covers material in

• Lecture 21 (Network Management #1),
• Lecture 22 (Network Management #2) and
• Lecture 23 (Network Management #3).

More Information

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:

• The exam is worth 60% of the final assessment for the subject^[1].

• 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 examiners...

• Revision:
  1. Old exam papers.

  2. Tutorial questions.

  3. Lecture notes.

^[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...

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

• 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.

INT31WE: Web Engineering

• This is a first-semester subject, previously offered in both semesters. Taken by Phil Scott
• Extends the Computer Networks stuff on the Technology of Electronic Commerce (mainly CGI programing).
• Extensive programming required: you will learn all about CGI programming in Perl and various related technologies.

INT32ENS: Encryption and Network Security

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

INT32WS: Web Services

• Offered in Semester 2, taken by Mal Sutherland.
• Extends the stuff mentioned in Lecture 24 of this subject, and follows on directly from Web Development. Offered at quite an advanced level.

INT32INW: Internetworking

• Offered in Semester 2, taken by Phil Rice.
• Extends the stuff mentioned in this subject and in Data Communications, both of which are pre-requisites. An advanced-level unit.

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!