Previous exam topics

Question 1

What is meant by refractive index of an optical medium

Answer 1

A property of an optical medium relating the speed of light in the medium to c, the speed in a vacuum. The speed of light in the medium is given by c/n, where n is the refractive index.

Question 2

What is the speed of light in glass that has a refractive index of 1.57

Answer 2

v = c/n

c will be a random number/ n the refractive index

Question 3

Why is refractive index relevant to the construction of optical fibres

Answer 3

Because the light does not travel at the speed of light in fibre communication so it is ued to calculate speed

Question 4

What are the 2 modulation schemes

Answer 4

4-QAM, and 16 QAM

Question 5

How many bits per symbol does each modulation scheme give

Answer 5

4 QAM is 2

16 QAM is 4

Question 6

Compare the modulation schemes in terms of susceptibility to error

Answer 6

16 is more susceptible to error as symboles are closer together and can create noise

Question 7

What is meant by antenna gain?

Answer 7

A measure that compares the performance of a directional antenna with that of a reference antenna, such as an isotropic antenna or half-wavelength dipole. It is the ratio of the power sent by the directional antenna in its preferred direction to the power sent by the reference antenna.

Question 8

The gain of a particular antenna is found to be 7dB. What is this gain expressed as a ratio?

Answer 8

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Question 9

6 visibility catergories, a - f. How many bits per symbol would be required if a fixed length code were used?

Answer 9

3
2^3 = 8
2^2 = 4

Question 10

What is meant by source extension?

Answer 10

The effective source when source symbols are grouped together.

Question 11

What is meant by core network?

Answer 11

A high-speed communications trunk network, served by access networks. Core networks are found in the public switched telecommunications network (PSTN), as well as competing networks, such as those owned by mobile communications operators.

Question 12

What is meant by access network?

Answer 12

The communications network that links end-users’ equipment to the core network. The access network is analogous to the minor roads that give access to motorways and other trunk routes.

Question 13

MPEG, summarise I-frames, P-frames and B-frames

Answer 13

Intraframe
prediction frame
bi directional frame

Question 14

Briefly explain how FDD Frequency Division Duplexing separates uplink from downlink traffic.

Answer 14

The use of separate frequency bands for downlink and uplink communications, enabling them to take place simultaneously.

Question 15

In WCDMA, the channelisation code 1 -1 1 -1 is used for a 1, what code would be used for a 0?
Write the encoded version of 1 0.

Answer 15

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Question 16

In relation to security, what is meant by integrity?

Answer 16

ensuring that a message is not vulnerable to change by a third party while in transit.

Question 17

A common way of checking integrity involves use of a hash, explain how a hash ensures integrity.

Answer 17

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Question 18

What are the main benefits of using fibre rather that copper

Answer 18

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Question 19

Describe the physical difference between MM fibre and SM fibre

Answer 19

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Question 20

How do you calculate signal power

Answer 20

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Question 21

How do you calculate noise power

Answer 21

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Question 22

How do you calculate the signal to noise ratio

Answer 22

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Question 23

How do you calculate the maximum channel capacity

Answer 23

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Question 24

How to you work out how many check digits are required for a CRC

Answer 24

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Question 25

How do you work out the error detecting capabilities

Answer 25

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Question 26

Whats the difference between lossy and lossless compression. Name one example of each type of compression

Answer 26

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Question 27

Why might 16 QAM be used in preference to 64 QAM

Answer 27

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Question 28

How are symmetric keys used for confidentiality

Answer 28

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Question 29

How are asymmetric keys used for confidentiality.

Answer 29

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