On-Line Examination COMP30091

One hour

UNIVERSITY OF MANCHESTER SCHOOL OF COMPUTER SCIENCE

Digital Wireless Communications and Networks

Thursday 17th January 2008

Time: 14:00 15:00

Please answer any TWO Questions from the THREE questions provided

This paper will be taken on-line and this is the paper format which will be available as a back-up.

The use of electronic calculators is permitted provided they

are not programmable and do not store text [PTO]

Page 2 of 5 COMP30091

For full marks your answers should be concise as well as accurate. In order to achieve full marks you should show your working for any calculations.

This paper version is for reference only as the exam will be taken online. 1. 1.1) a) As employed in telephone and computer networks, what is meant by

circuit switching and packet switching networks? What is the most important difference between these two techniques?

b) Why do traditional telecommunications networks use circuit switching?

c) Why are telephony providers such as British Telecom in the UK

changing to packet switching for exchange to exchange connections? d) What is the difference between Quality of Service (QoS) in licensed and

license exempt wireless spectrum? (5 marks) 1.2) What are the causes of delay between users speaking and their mobile telephones

transmitting the data in: a) a 2nd generation mobile telephony system using Global System for Mobile

communication (GSM)? b) a 3rd generation mobile telephony system using the Universal Mobile

Telecommunications System (UMTS)? (4 marks)

Page 3 of 5 COMP30091

1.3) A GSM telephone using the General Packet Radio Service (GPRS) to send a Hypertext Transfer Protocol (HTTP) request generates 512 bits of data. The data rate using Coding Scheme 1 (CS-1) is 9050bps and only 1 slot is used per GSM Time Division Multiple Access (TDMA) frame for this GPRS data. A frame has the structure:

GSM TDMA frame

5 7 8 1 2 4 63

4.615 ms

GSM time-slot (normal burst)

a) How many slots containing our GPRS data are sent per second (you can

round your answer to one decimal place)? (2 marks)

b) How many bits of user data are transmitted in 1 second (again you can round your answer to the nearest integer value)? (2 marks)

c) Comment on your answer to 1.3.b above in relation to the suggested data

rate being 9050bps. If there is a difference, suggest reasons for the difference. (2 marks)

1.4) Given that a single Global System for Mobile communication (GSM) frequency

channel can service 8 simultaneous voice users, how many simultaneous voice users can be serviced by a single Universal Mobile Telecommunications System (UMTS) channel? Is UMTS more or less spectrum efficient than GSM? Why? List any technical assumptions you have made in answering this question.(5 marks)

546.5 s 577 s

guard guard S user data tail tail user data S Training space space

3 bits 57 bits 26 bits 57 bits1 1 3

Where: guard space is to prevent overlap with other transmissions etc.

S i t l bit f th d t b i d t t

Page 4 of 5 COMP30091

2. 2.1) Briefly explain how spread spectrum techniques work. What are their advantages and disadvantages? What are the two main ways to achieve spread spectrum?

(5 marks)

2.2) A Direct Sequence Spread Spectrum (DSSS) chip sequence is generated using the Barker code 11101.

a) What is the spreading factor of this code? (2 marks) b) Using one or more diagrams show how the data sequence 01 will be

spread by the Barker code 11101. (3 marks) c) Assuming there are no errors, explain how the receiver recovers the

original data from the coded chip sequence it receives. (3 marks) d) Using the received chip sequence 1100100110 show how the system can

recover the original data from this received chip sequence. (3 marks)

2.3) A digital television (TV) broadcast channel requires approximately 6Mbps of

network bandwidth in order to successfully decode the pictures. An IEEE 802.11g Access Point (AP) is currently delivering 3 different digital TV broadcasts to three wireless enabled laptop computers at a notional data rate of 54Mbps using TCP/IP.

a) Why might the AP fail to deliver the broadcasts successfully? b) How could the system be changed to improve the chances of success? (4 marks)

Page 5 of 5 COMP30091

3. 3.1) Answer each of the following short or multiple choice questions. a) How long does a radio wave take to travel 30km? (speed of light c = 3 x 108 m/s) (1 mark) b) In the time it takes light to travel 30km, how much data would a WiMax

(IEEE 802.16) system operating at 54Mbps transmit? (1 mark) c) This question is a multiple choice question and therefore cannot be

published. d) If a receiver hears:

A transmitter at 100dBW and

Background noise at 120dBW calculate the signal to noise and interference ratio (SNIR) in dBs of the

signal from the transmitter as heard by the receiver. (1 mark) e) What is the total power heard at the receiver in dBW? (1 mark) 3.2) Modern hybrid mobile telephones supporting transmission by GSM, Bluetooth

and WiFi (IEEE 802.11) are able to provide free telephone calls using Voice over IP (VoIP) as well as charged calls over GSM. Describe two different mechanisms one using a WiFi gateway (Access Point) and one using an ad hoc network to provide free calls.

What routing methods are required to route calls using ad hoc connections?

Using a small example network, explain how routing in an ad hoc network of telephones might be achieved.

What (should) happen when mobile telephones change location? Is this different

to how GSM already handles mobility? Does this raise other issues and if so give examples? (14 marks)

END OF EXAMINATION