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 wasnÕt it?

4. What is the purpose of the signal ground pin in RS232? Why is it important?

5. 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?

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

7. 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?

8. What are some of the claimed features of HDLC?

9. 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.