Computing 205/406 & IT8
Tutorial #10
- Why is a modem needed for data communications over the telephone
system?
- 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).
- Could the RS232 interface have been designed in such a way that null
modems would not be needed? Why wasn�t it?
- What is the purpose of the signal ground pin in RS232? Why is it
important?
- 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 characters per second can each of these
transmit if the bit rate in use is 28.8kbps?
- Which is more efficient for large data transfers: synchronous or
asynchronous communications?
- 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?
- What are some of the claimed features of HDLC?
- What are the characteristics of an ISDN microlink 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?
- If you have access to a computer at home, use Kermit to transfer some
files from home to the Unix systems at the Uni, and vice versa. Look at
some of the options Kermit provides to increase the efficiency of
transmission. Compare the use of Kermit to other file transfer
protocols such as Zmodem in terms of ease of use and efficiency.
This tutorial is also available in PostScript format.
Phil Scott