Computer Networks

Tutorial #9

1. Why is a modem needed for data communications over the telephone system?

2. 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).

3. Could the RS232 interface have been designed in such a way that null modems would not be needed? Why wasn't it?

4. The fastest modems which can be purchased at present operate at 56Kbps, although most people are still using 28.8Kbps or slower modems. Yet, the serial ports on most home computers are set up to operate at 115Kbps or faster.
   1. Why the disparity?
   2. Why does the serial port operate at a weird speed like 115200bps?

   3. 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? How many 8-bit bytes per second can be transmitted using a 28.8kbps modem (ignoring the possibility of compression)?

   4. Which is more efficient for large data transfers: synchronous or asynchronous communications?

   5. In the lecture, it was claimed that there were many more modern interfaces than RS232. Why do you think these have not come into common use?

   6. What are the characteristics of a "Basic Rate" ISDN service?

   Practical exercises:

   • Research Question: if you have a computer at home, how fast can its serial (RS232) port operate? Have you tried it to see if it works at that speed?

   • If you dial-in from your computer at home using a modem, what is the best "download speed" you ever see? If you use Netscape, it might report this (in bytes/second!) in the status line. Other software will report it at the end of a file download. How does it compare to the nominal modem speed, or serial port speed, of operation? What factors will affect this?

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   Phil Scott