Dec 1998 AN215: Advanced Networking (Question 3)

(a)	Give two advantages of using fibre optics over coaxial cable.	[2]
	Higher bandwidth (1 mark): Immune to interference (1 mark). Allow other appropriate answers.
(b)	Briefly describe any two types of noise that can occur during transmission.	[4]
	Any two of the following (allow credit for other valid points): Thermal Noise (1 mark). This arises from electron motion and is a function of temperature (1 mark). Inter-modulation Noise (1 mark). This occurs when two signals pass through a non-linear medium (1 mark). Crosstalk (1 mark) .This occurs when there is electrical coupling between cables carrying multiple signals (1 mark). Impulse Noise (1 mark). These are irregular pulses of high amplitude that seriously degrade the data (1 mark).
(c)	In addition to noise, there are two other impairments which can result in faults occurring during transmission. What are these two faults?	[2]
	Delay Distortion (1 mark). Attenuation (1 mark).
(d)	RS232-C is a physical layer standard with four components. One is the procedural specification. Explain this feature of RS232-C in detail, referring where necessary to the other parts of the specification.	[8]
	Award marks for points from the following. Allow up to (3 marks) for a well-labelled diagram showing the nine connections between devices. The procedural specification for RS-232C specifies the sequence of events that occur between DCE (modem) and DTE (PC) in order to transmit data (1 mark). Initialisation When the computer is powered up, it send a Data Terminal Ready signal to the modem (1 mark) Whenever the modem powers up, it sends a Data Set Ready signal to the PC (1 mark). Transmit Whenever the PC wants to transmit data, it sends Request To Send signal to the modem (1 mark). In response to modem transmits a carrier frequency on a telephone line and sends a Clear To Send signal to the PC to indicate that it is ready to receive data from the computer (1 mark). In response to DCE signal, the computer transmits data over the transmission line (1 mark). Receive Whenever the modem detects a carrier frequency on the transmission line, it means that the remote modem is transmitting. So, the modem sends a carrier detect signal to the computer (1 mark). In response the PC receives the incoming data over the Receive line (1 mark).
(e)	A modem is runnig at 9600 baud. It modulates signals using both amplitude modulation and frequency modulation. It uses two different amplitudes and four different frequencies. How many bits per second can it transmit? Show all working and assumptions that you make.	[4]
	Two amplitudes and four frequencies means eight distinct modulation patterns (1 mark). That is 2³ , so 3 bits may be represented (2 marks). 9600 x 3 = 28800 bits per second (1 mark)