Copyright of the University of East Anglia Version 1

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1 UNIVERSITY OF EAST ANGLIA School of Computing Sciences Main Series Examination NETWORKS CMP-5037B/CMP-6009B Time allowed: 2 hours Answer any THREE questions. Notes are not permitted in this examination. Do not turn over until you are told to do so by the Invigilator. CMP-5037B/CMP-6009B Module Contact: Dr. Ben Milner Copyright of the University of East Anglia Version 1

2 Page 2 1. (a) A 2 minute audio recording has been sampled at 44,100 samples per second and quantised using 2 bytes to represent each sample. The audio is also recorded in stereo. Calculate: (i) The total amount of data storage required to represent the audio track. (ii) The throughput capacity of the network needed to allow the audio to be streamed in real time. (iii) Audio compression has now been applied to the data which has reduced its bit rate to just 10kbps. Calculate the amount of data storage now required to store the compressed audio. [4 marks] (iv) The compressed audio is now to be downloaded over a mobile connection that has a 5kbps payload capacity. Calculate the time taken for the audio to be received and state how many times slower than real-time this is. [4 marks] (v) Discuss methods that could be employed to reduce further the time taken to download the audio recording. [6 marks] (b) (i) Keying methods such as amplitude shift keying (ASK), frequency shift keying (FSK) or phase shift keying (PSK) allow digital data to be transmitted over an analogue channel. Explain how each of these three keying methods operates and show, using diagrams, how each method encodes the binary sequence (ii) An extension of PSK is n-psk. Explain how the more general n-psk method operates and discuss the advantages and disadvantages as the value of n increases. Show how the binary sequence is encoded using QPSK. CMP-5037B/CMP-6009B Version 1

3 Page 3 2. (a) Assume that N hosts attempt to transmit 10Mbit files simultaneously across a wired network. Explain how the operation of the IEEE network protocol performs when N is 1, 2, and 20. (b) Two Ethernet stations (A and B) are situated at either end of a 1km cable. The propagation speed is 250,000km/s. Assuming that both stations begin transmitting at exactly the same time, with the use of a space-time diagram, show: (i) Where on the cable the collision will occur. (ii) How long it will be before each station detects a collision. [5 marks] [5 marks] (c) Firewalls are commonly used to implement security control in a network. (i) Explain the design and operating processes of a firewall. (ii) Firewalls differ from Intrusion Prevention Systems. Describe the differences and identify the uses for each in a network. CMP-5037B/CMP-6009B Version 1 TURN OVER

4 Page 4 3. (a) Near Field Communication (NFC) systems use modified Miller coding, otherwise known as delay encoding, for low-speed data transmission from active to passive devices. (i) Demonstrate how the string would be encoded using Modified Miller coding. (ii) Why is the Modified Miller code useful when transmitting from an active to a passive device in particular? (b) Explain the differences between packet switched networks and circuit switched networks, and give an example of each type of network. (c) Explain the differences between connection-orientated and connectionless network protocols. Give an example of each type of protocol and explain the type of data that each is best suited to. CMP-5037B/CMP-6009B Version 1

5 Page 5 4. (a) A 7-bit codeword is shown below. This comprises 4 data bits (D0, D1, D2 and D3) and 3 parity bits (P0, P1 and P2) which are used for error correction. P0 P1 D0 P2 D1 D2 D The illustration shows that data bits D0=1, D1=0, D2=0 and D3=1. (i) Using odd parity, compute the values of the 3 parity bits (P0, P1 and P2) from the 4 data bits (D0, D1, D2, D3). Show all your working. (ii) The codeword has now been transmitted over an unreliable channel and the following 7-bit codeword received, as shown below. This shows that data bit D3 is in error. P0 P1 D0 P2 D1 D2 D Recompute the 3 parity bits from the received data and show how they are used to locate and correct the erroneous bit in the received codeword. (b) In classful addressing, IPv4 addresses can be divided into five classes: A, B, C, D and E. Identify the main disadvantages with using such a system. [8 marks] (c) Consider the subnet shown in Figure 1. Distance vector routing is used, and the following vectors have just come to router B: from A: (0, 3, 4, 6, 2, 3); from C: (4, 5, 0, 6, 2, 3); and from F: (3, 8, 3, 3, 1, 0). The measured delays to A, C, and F, are 4, 5 and 8, respectively. What is B s new routing table? Give both the outgoing line to use and the expected delay. [12 marks] CMP-5037B/CMP-6009B Version 1 TURN OVER

6 Page 6 Figure 1. A subnet 5. (a) (i) Name each layer in the OSI seven layer reference model and describe briefly its functionality. [14 marks] (ii) The TCP/IP model is an alternative layered network architecture. Name each layer in the TCP/IP model and explain how the functionality of the OSI seven layer reference model is distributed in the layers of the TCP/IP model. [6 marks] (b) Describe briefly the structure of an OSI reference model layer 2 bridge and a layer 3 IP router, highlighting their differences. (c) A communication channel has a bit rate of 1000 kbit/s. (i) Compute the duration of 1 bit in seconds. (ii) Packets containing 20 bytes are transmitted over the channel. Calculate the length of time required to load a packet onto the network. (iii) The round trip delay of the channel is 5ms. Calculate the optimal number of packets that need to be transmitted in succession, before receiving an acknowledgement packet, so as to maximize the efficiency of the connection. [4 marks] END OF PAPER CMP-5037B/CMP-6009B Version 1