Computer Networks

Tutorial #13

1. 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 for a couple of different data rates, say 14400 bps and 33600 bps. What if the connection was between Bendigo and Castlemaine ($1.50 per 10 minutes)? What about Bendigo to Perth ($5.12 per 10 minutes)?

2. 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 1 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).

3. Attempt to calculate the minimum price for a data service whereby an office in Castlemaine sends a daily transaction record of approximately 1 Mbyte to a Bendigo head office at the end of each working day. Include the cost of all equipment and services (modems, installation, etc) You can realistically assume there are 250 working days in a year. You should consider PSTN and ISDN[1] initially. Is using the Internet (using email to transfer the data) a viable alternative, given that a local call is required at each end, and that ISPs typically charge $3.00 per hour for casual dial-in access.

4. One for the accountants in the class: discuss the issues involved in making the decision of the previous question - for example, it's not obvious to what extent installation and annual rentals can be charged to the data communications function (presumably the phone service would also be used for other purposes at other times of the day). Then there's the problem of the various call-charge plans which are available for OnRamp2. And you could probably negotiate a better per-hour connection plan with your ISP.