Notes and comments on the grading of the exam 3.9.2003

Please note that these are not 'model answers', but grading principles
and corrections to common misunderstandings.


1 Terminology

(0,5p for each correct explanatin, 0,5 for relation/difference)

Circuit and packet switched
- Both are means to tranfer data through a network. Circuit switched
  reserves a line or other resources and all data is sent along this
  path, while packet switched means individual data packets are
  transmitted based on the address information included in each
  packet.

WWW and Internet (4 students, that is almost 20% got this wrong...)  
- The Internet is the 'network of networks', a worldwide network
  connecting e.g. LANs in different parts of the world together. On
  top of this network different services are offered, one of them
  beeing WWW (hypertext-documents transfered using the http protocol)

Vector and pixel graphics 
- In vector grafics the image is defined as a set of mathematical
  curves and surfaces, while in pixel graphics the image is a matrix
  of 'dots' (pixels) each with a specified colour. Note that both can 
  be used in 2D and 3D graphics.

Strong and weak encryption
- Strong encryption can not be broken with current knowledge /
  computing capasity in any reasonable time. (Often based on some hard
  to solve mathematical problem). Weak encryption uses algorithms OR
  key lengths that can be broken with current computing resources within 
  a reasonable time.


2. Wireless transmission

a) 0,5+0,5p for each correct example + reasonable (error free) 
   justification.

b) 3 * 1p for describing a likely/real error and suggesting a
   solution.  I was looking for errors+solutions at the signal or
   transmission level, but most of the ansvers were of the type "if
   you are far away from the sender you don't get enough signal,
   solution: move closer to the sender".  This is not a really a
   solution, --> only 0,5p for identifying a problem.


3. Statements

0,5p for right answer + 0,5 for explanation. If the explanation is
completely worng --> 0p

a) Encryption does _not_ ensure data integrity. Usually modifying an
encrypted message results in a mess and you can see that the message
has been tampered with, but depending on the used algorithm it may be
possible to modify an encrypted message in a realtively controlled
way. Signatures and message authentication codes (MAC) ensure integrity.

b) A security policy defines high level security objectives, but does
not include the technical implementation. Key length can be considered
a technical implementation or - in the case of minimum key length - a
definition of a more general target security level. Both yes and no
were accepted if the explanation was valid.

c) In general, DoS attacks cannot be prevented with cryptography. Any
communication request, secure or not, requires resources at the
server. (*)

d) PSTN = Public Switched Telephone Network, the old ordinary
telephone network, was desinged for speech tranmission.

e) SMS uses the signalling channel, so messages can be received during
a phone call.

f) Multiplexing (e.g. TDM) does in a way split the data stream into 
'packets' but the switchin is still the same circuit switching it 
originally was. All 'packets' still go along the same line


(*) It is possible to construct a protocol where the client does
a lot of computation and the server only little, turning the attack
back at the attacker. This still doesn't prevent a ditributed attck.



4 Multimedia

a) Multimedia i) has several media, ii) is time dependent (syncronisation)
and iii) interactive.

b) Points to consider
- AD transform, sampling and quantisation
- compression of the raw digital signal
- taking advantage of signal properties - before or after sampling/quant. 
  (the properties are of course different but the idea is the same)
- taking advantage of limitations in human hearing / seeing
- how compressable is image / sound, redundancy in signal and needed 
  quality 

Grading: 1p for each of the above up tp 4p. If only sound OR image 
considered 0,5p

Note: a photograph (or the light passing the lens in your digital
camera) is originally analog, it has to be sampled and quantisised
just as an analog sound signal. Several students assumed that images
are originally digital.


5 New students guide to internet

Grading: raw point given as follows (8p), final points based on these and 
the essay in general (raw points + 0-1p).

Discuss four channels, for example the ones used on the course: email,
www, news and irc. You get 0,5p for each of the following issues and up
to 2p / channel.

- technical description (used protocols or description of how the
  message is transfered)
- what kind of communication is the channel suited for?
- privacy, publicity, other points worth noting
- netiquet, how to behave online
- well strucutred writing, proper for the target audience
- other relevant issues

Common reasons for losing points:
- discussing applications (browser, pine, ...) instead of communication 
  channels
- stating that technology will not be discussed (it was recuired in 
  the assignment!)
- trying to be funny, having an attitude not acceptable in a guide