Computer Networks
Tutorial #11
- 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 few different data rates, say 2400 bps,
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 new OnRamp ISDN service) data call
between Bendigo and Melbourne (rate: NDD3) costs
(day rate) $2.73 for 10 minutes, and $5.34 for 20 minutes.
Calculate again the cost
to transfer 1 Mbyte between Bendigo and Melbourne (using the same
assumptions), except this time
using ISDN OnRamp. 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 $1.35 for 10 minutes, $2.58 for 20 minutes).
- In the lecture, it was noted that dial-in routers used by ISPs usually are
configured so that customer's systems appear to be connected to the
ISP's own LAN. How is this done?
- Attempt to calculate the minimum price for a data service whereby an
office in Castlemaine sends a daily transaction record of approximately
100 Kbytes 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 $4.00 per
hour for casual dial-in access.
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 OnRamp ISDN (Duo - the basic product), installation
is $295, with a monthly rental of $60. Call charges are given in
Q.2.
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Phil Scott