Aug 1997 AN215: Advanced Networking (Question 4) Suggested Solutions

4.	(a) What is the difference between baud rate and bits per second? Under what circumstances will they both be the same?		[4]
	Baud rate is the signalling rate---the number of changes per second on the line.		[1]
	The total data capacity is given in bits per second, which will usually be higher because each signalling element may carry more than one bit of data.		[1]
	There we have the relationship
		*bit rate = baud rate bits per signal element.**	[1]
	Bit rate and baud rate will be the same when each signal element carries one bit.		[1]

	(b) Explain the concept of modulation, including the different modulation techniques available for digital signals. You should describe (if applicable) how each can be used to achieve a variation between baud rate and bits per second.		[5]
	*Modulation is a signal process which shifts the signal from one frequency range to another (by using it to change a carrier* frequency in some way),**		[1]
	for better transmission.		[1]
	In amplitude modulation/amplitude shift keying, the input signal is used to vary the amplitude (wave height) of the carrier signal. Several bits can be carried in one carried one baud by using various different amplitude levels.		[1]
	*In frequent modulation/frequency shift keying,* the frequency of the carrier is varied, and several different frequencies may be used to denote several different bit values.**		[1]
	*In phase modulation/phase keying* the phase of the carrier is varied. The extent of the phase shift (0°, 90°, 180°, 270°) may be used to denote several different bit values.**		[1]

	(c) What factors limit the transmission rates possible over twisted pair cables?		[4]
	noise,		[0.5]
	transmission delay distortion,		[0.5]
	attenuation.		[0.5]
	Plus up to [2.5] for explanation.

	(d) What is pass band? As a result of the factors in part (c) (and others), what is the available pass band for a normal telephone line? Assuming that in ideal conditions, two samples are possible per cycle, what is the theoretical maximum baud rate for a telephone line?		[4]
	Pass band is the range of frequencies which can be successfully transmitted.		[2]
	Telephone line has pass band of 300--3300Hz.		[1]
	Consequently, max baud rate is 6600 per second.		[1]

	(e) The V.32 modem standard actually uses 2400 baud transmission, and achieves 9600 bits per second. How many distinct modulation patterns per baud must V.32 modems generate? Explain your answer.		[3]
	9600/2400 = 4 Therefore must achieve four bits per baud.		[1]
	2⁴ = 16, so we need sixteen different patters in each baud.		[2]