Dec 1998 CS202: Computer Architecture and Data Communication (Question 4)

(a)	Data transmission may be serial or parallel, and simplex, half-duplex, or full-duplex. Explain three of these terms, and illustrate each with an example.	[6]
	Any three form:
	Simplex: unidirectional and no feedback from receiver (1 mark). Example: telemetry (1 mark).
	Half-duplex: bi-directional, and either transmit, or receive (1 mark). Example: walkie-talkie (1 mark).
	Full-duplex: bi-directional, and both transmit and receive simultaneously (1 mark). Example: telephone (1 mark)
	Serial transmission: Bits of data are transmitted one after another, sequentially (1 mark). Example: modem (1 mark).
	Parallel transmission: Several bits of data are transmitted simultaneously. One character is transmitted at a time (1 mark). Example: local parallel printer connection (1 mark).
(b)	What is a multiplexer? Explain the main purpose of using it during the transmission of data.	[3]
	A multiplexer is a device that converts several low-speed signals from different device (1 mark). Its main purpose is to transmit combined signals over a high-speed line (1 mark). This is because the use of a single high-speed line is cheaper than transmitting over several low-speed lines (1 mark)
(c)	Explain the following terms:	[6]
	(i) Frequency-division Multiplexing (FDM)
	A limited bandwidth channel is divided into narrower bands (subchannels) (1 mark). Each band carries a separate transmission at a lower frequency (1 mark). Narrow bands of unused frequency called guard bands separate these bands (1 mark). Award marks for other relevant points, max 3 marks.
	(ii) Statistical time-division Multiplexing (STDM)
	Like Time-division multiplexing but time slots are allocated dynamically on demand (1 mark). Thus it takes advantage of the fact that the attached devices are not all transmitting all of the time (1 mark). Therefore it can typically support more devices than TDM (1 mark). It is necessary to buffer input, however, since the peak load may exceed the channel capacity. (1 mark). Award marks for other relevant points, max 3 marks.
(d)	For each of the following, suggest a suitable multiplexing technique, giving reasons for the answer.	[5]
	(i) to enable several telephone connectinos to share one trunk route;
	Use FDM (1 mark). All may require truly simultaneous access (e.g. if using modems), but the bandwidth requirements for each is not very great (1 mark). Also allow credit for TDM, with explanation. Award marks for relevant explanation. Award up to 3 marks in total.
	(ii) to enable several interactive terminals to be connected to a remote mainframe.
	Use STDM (1 mark). Use of each terminal is not continuous, but varies with typing speed, etc. (1 mark). Small amounts of buffering are quite acceptable when dealing with peak loads (1 mark) Award marks for relevant explanations. Award up to (3) marks in total, subject to the maximum of (5) for this whole part.