Computer Networks
Tutorial #13
- 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)?
- 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).
- 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.
- 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.
Note: exact answers are not required in this tute
- we're looking for "back of the envelope" calculations
to get some idea of which option is cheapest.
[1] For OnRamp2 ISDN (Standard Plan), installation
is $295, with a monthly rental of $60. Call charges are given in
Q.2.
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Phil Scott