Computer Networks
Tutorial #12
- Why is a modem needed for data communications over the telephone
system?
- 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.
- Why the disparity?
- Why does the serial port operate at a weird speed like
115200bps?
- 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).
- 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)?
- 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?
- What is the function of PPP in a data link using modems?
- 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)?
- What are the characteristics of a "Basic Rate" ISDN service?
- 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).
NB: Phone call charges used in the tutorial are not necessarily current.
See Prac #12 for the practical exercises
accompanying this tutorial.
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Copyright © 2000 by
Philip Scott,
La Trobe University.