Computer Networks (CITS3230) - Tutorial 3

1. Consider the operation of an automatic teller machine (ATM) or an electronic fund transfer at point of sale (EFTPOS) terminal at a supermarket. Based solely on your observations and knowledge of how we interact with ATMs, do you think that ATMs form a virtual circuit or use datagrams to communicate with their central bank?

   Do you think your ATM card's personal identification number (PIN) is stored on the card itself?

   How did you reach your conclusions?

2. When using a datagram-based (connectionless) Network Layer, as opposed to a virtual circuit service, each packet carries the network addresses of both source and destination. As telephone, ISP, and pay-TV companies in Australian move headlong into the digital delivery of their services, what "new" consumer services will these companies be able to provide?

   Will these be good or bad innovations?

3. A group of 2ⁿ-1 IMPs are connected in a balanced binary tree, with an IMP at each node. IMP i communicates with IMP j by sending a message to the root of the tree which then sends the message to IMP j. Give an expression for the mean number of hops per message in terms of n (assume that all IMP pairs are equally possible).
4. Another protocol problem: "The Greek Village Problem" - no sexism implied!

   In a certain ancient Greek village there are a large number of (pairs of) married men and women. Each man is married to only one woman and each woman to only one man (this was ancient Greece).

   All the Greek women love to gossip and will tell everything to any other woman, but will only convey gossip to their husbands, and to no other men. All the Greek men are very macho and although they absorb all gossip told to them by their wives, they do not pass any gossip on to any other men or women, nor ask any "gossipy" questions of their own wives.

   Unfaithfulness is rife in this village. Naturally, no woman will tell her husband that she is being unfaithful as, by an old Greek tradition, upon learning that his wife is unfaithful, a Greek man may kill his wife at sunset that very day. So no man hears that his wife is being unfaithful.

   So the unfaithfulness runs rampant, until one day the Greek God of Chastity decides to put a stop to it. The Greek God of Chastity decides to implant one piece of knowledge in the heads of all Greek men - "There is at least one unfaithful woman in the village".

   What happens? Is the Greek God of Chastity appeased? And, again, what has this problem got to do with networks?

Chris McDonald

April 2008.

From rec.humor.funny Sat Nov 16 14:14:02 1991
From: rcoats@hayes.UUCP
Subject: If Computer Nerds Had Controlled TV Programming

Gilligan's Island

The Professor finds some rare binary coconuts that can be used to upgrade the ROM in his bamboo clone modem to support X.25. After hours of effort, he manages to establish a connection via his papaya-based VT 100 terminal and satellite uplink with Telnet, but falls asleep from exhaustion. Gilligan walks up and types "+++" then "ATH" and drops the line. The Skipper thumps him with a rock and finally loses it, his seafaring years catch up with him and he starts making lewd remarks to Mary Ann about playing Leisure Suit Larry IV together. Meanwhile, Mr. Howell finally figures out how to work the ISDN set that was accidently dropped by a passing JAL jet and has a heart attack when he hears via Dow Jones News Retrieval that the stock market has soared above 3000 points. Ginger calls CompuServe and E-mails some software developers in an effort to convince them that she should would be a good model for MacStripPoker 2.0. Eventually, all connectivity on the island is lost, when Gilligan discovers an actor in a bad gorilla costume has shorted pins 2&3 on the serial cable, and everybody on the island has just been echoing characters to each other.