Computer Networks

Tutorial #12

1. Why is a modem needed for data communications over the telephone system?

2. The fastest modems which can be purchased at present operate at 56Kbps, yet the serial ports on most home computers are set up to operate at 115Kbps or faster.
   1. Why the disparity?
   2. Why does the serial port operate at a weird speed like 115200bps?

3. What is a null modem and why is it often needed where RS232 interfaces are in use? Describe briefly the connections required in a minimum RS232 null modem (ie: one which uses only pins 2, 3 & 7).

4. What is the link efficiency (or utilisation) in an asynchronous system which sends 8 bits of data with one start bit and one stop bit? What if the data was only 7 bits, as in ancient ASCII data links? How many 8-bit bytes per second can be transmitted using a 28.8kbps modem (ignoring the possibility of compression)?

5. When a "home user" dials into an Internet Service Provider, how are their IP address allocated? How is this different to the situation where a point-to-point link is used to connect two routers together?

6. What is the function of PPP in a data link using modems?

7. Calculate how much it would cost to transfer 1 Mbyte of data between Bendigo and Melbourne using the PSTN, with a long distance (STD) call. Assume that you've got all of the hardware needed at each end of the link (ie 2 phone services, 2 modems), and that the only charge is the STD phone call. Assume also that the call is done at "Day" rate (ie, during working hours), for which the charge is (was, last time I looked!) $3.50 for each 10 minutes between Bendigo and Melbourne. We'll further assume -- even though its clearly silly to do so -- that there are no overheads (ie, each byte transferred consists of 8 bits and there are no synchronisation, protocol or other overheads). Work your calculations based on a data rate of, say 33600 bps, which is all you're likely to get over this distance. What if the connection was between Bendigo and Castlemaine ($1.50 per 10 minutes)? What about Bendigo to Perth ($5.12 per 10 minutes)?

8. What are the characteristics of a "Basic Rate" ISDN service?

9. An ISDN (using Telstra's OnRamp2 ISDN service) data call between Bendigo and Melbourne (rate: NDD3) costs (day rate) 15 cents flagfall plus 0.45 cents per second (you get no time allowance for the flagfall in NDD2 and NDD3 data calls). Calculate again the cost to transfer one Mbyte between Bendigo and Melbourne (using the same assumptions), except this time using ISDN OnRamp2. Assume, as before, that all of the necessary phone services are already connected and paid for for the purposes of this question. Recalculate for Bendigo to Castlemaine (NDD2 day rate, 15 cents flagfall and 0.2333 cents per second).