- Why is a modem needed for data communications over the telephone system?
- The fastest external 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 sometimes needed where
RS232 interfaces are used to built point-to-point data links? 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
- The Department of Information Technology at Bendigo has a dial-in router
(for staff use only, sorry!) which is connected to subnet 20, ie
- When registered users dial in to this router, what would you expect
the network/subnet part of their home machines IP address to be?
- The dial-in router is connected to subnet 20 and has IP address
A separate router (
126.96.36.199) provides the link
to the university backbone from subnet 20. Draw a labelled sketch of this
portion of the network.
- (Optional research question -- you do not need to understand
the answer to this part!) Now, consider when a dial-in user connects to
the dial-in router. A route must somehow be established to their machine,
and announced to the backbone router. The two ways in which this can be
done are by the use of "proxy-ARP" or by the dial-in router "announcing
a host route". Attempt to describe how each of these might operate.
- In a dial-in situation, IP addresses are usually "dynamically-allocated",
and therefore different for each dial-in session. It's usually possible to
pay a somewhat higher rate but have a "static" IP address, which is the same
for every dial-in. Why do you think dial-in accounts with dynamically-allocated
addresses are cheaper?
- How are IP addresses normally allocated in the situation where a point-to-point
link is used to connect two routers together? Can you imagine a more address-space-efficient
way of allocating these addresses?
- What is the function of PPP in a data link using modems?
- Research Question: when a dial-in Internet user connects to an ISP using
PPP, their machine has to somehow discover its own IP address. How do this
think this happens? Harder research question: in the olden days, when SLIP
was commonly used for dial-in access, the protocol provided no support for
allocating an IP address to a remote user. How did the dial-in machine discover
its own IP address using SLIP?
Copyright © 2004 by Philip
Scott, La Trobe University.