UNIVERSITY OF EAST ANGLIA School of Computing Sciences Main Series Examination 2013-14 COMPUTER NETWORKS CMPSMD22 Time allowed: 3 hours Answer Question 1 and THREE questions from questions 2 to 6. Notes are not permitted in this examination. Do not turn over until you are told to do so by the Invigilator. CMPSMD22 Module Contact: Dr. Ben Milner Copyright of the University of East Anglia Version 2
Page 2 1. (a) Two problems are encountered with 802.11 wireless networks that need to be considered when designing a collision avoidance scheme, namely the hidden station problem and the exposed station problem. Describe how the 802.11 standard overcomes these problems. (b) When signals are sent down an electrical cable they become weaker as distance increases. (i) Describe two methods that can restore the power of the signal and comment on their advantages and disadvantages. [4 marks] (ii) The signal received at the end of a cable has a power of 200mW. Background noise power on the channel is 20mW. Compute the signal-to-noise ratio (SNR) in decibels (dbs) at the receiver. (c) An interleaver, π is defined by the following function π(id + j) = jd + (d 1 i) Using the function, calculate the output sequence of packets for a 2x2 interleaver. (d) A communication channel has a bit rate of 120 kbit/s and a round trip delay time of 10ms. (i) Compute the duration of 1 bit in seconds. (ii) A stream of packets is sent over the channel where each packet contains 15 bytes. Calculate the length of time to load a single packet onto the network (iii) Calculate the optimal number of packets that should be transmitted before receiving an acknowledgement packet to maximize the efficiency of the connection. (e) The IPv4 classful addressing system is divided into five classes (A, B, C, D and E). Describe the structure of an IPv4 class C network address and, in doing so, explain which parts of the address are CMPSMD22 Version 2
Page 3 used for routing by IP and which only have local significance to the source/destination Local Area Network (LAN). (f) Traceroute is a common program for identifying the route packets take in an IP network. Describe how the program works and what protocol is used. (g) The Session Layer of the OSI 7 layer reference model uses synchronisation points to avoid having to retransmit an entire message in the event of a disruption to the network. Explain how this works. (h) Describe three parameters that can be used to specify the quality of service (QoS) of a network. CMPSMD22 Version 2 TURN OVER
Page 4 2. (a) (i) State the name of each layer in the OSI seven layer reference model and describe briefly the functionality of each layer. [7 marks] (ii) Discuss the principles and benefits of layered network architectures and explain how they reduce design complexity. Your explanation should include the role of layers, interfaces, protocols, encapsulation and peer processes. [7 marks] (b) 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 10101 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? [4 marks] CMPSMD22 Version 2
Page 5 3. (a) A 5 minute audio recording has been sampled at 16,000 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 12kbps. Calculate the amount of data storage now required to store the compressed audio. (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. (v) Discuss methods that could be employed to reduce further the time taken to download the audio recording. [4 marks] (b) There are a wide range of devices and approaches for securing and monitoring a network which include: (i) Firewalls (ii) IDS (iii) IPS (iv) ACLs For each of these, describe its function and discuss the strengths and weaknesses of its approach. [12 marks] CMPSMD22 Version 2 TURN OVER
Page 6 4. (a) Assume that N hosts attempt to transmit 10Mbit files simultaneously across a wired network. Explain how the operation of the IEEE 802.3 network protocol performs when N is 1, 2, and 20. (b) Two Ethernet stations (A and B) are situated at either end of a 2km 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 will the collision occur. (ii) How long will it be before each station detects a collision. [3 marks] [3 marks] (c) The IP protocol suite comprises a range of protocols which includes: (i) ICMP (ii) TCP (iii) PIM (iv) UDP Describe the functionality of each of these protocols. [12 marks] CMPSMD22 Version 2
Page 7 5. (a) (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 operate and show, using diagrams, how each method encodes the binary sequence 011001. (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 011001 is encoded using QPSK. (b) IPv6 addressing was proposed as the ultimate solution to IPv4 address depletion. Describe IPv6 logical addressing, focussing on the improvements made from IPv4 and the six main goals of the new IPv6 system. [8 marks] (c) The IP Address Resolution Protocol (ARP) provides a dynamic mapping between a logical and physical address. Describe, with the aid of diagrams, the ARP request and response procedures. [4 marks] CMPSMD22 Version 2 TURN OVER
Page 8 6. (a) An effective method of backwards error correction is the cyclic redundancy check (CRC). This detects the occurrence of bit errors by dividing a binary message by a generator polynomial to determine a remainder. (i) Given a generator polynomial of 1001 and a message sequence of 10111, show that the remainder is 101. (ii) Explain what is now transmitted across the network and determine the transmitted data using the values from above. [3 marks] (iii) Explain how the receiver uses the received information to detect the presence of bit errors that may have occurred in transmission. [3 marks] (b) Distance vector routing can be used in a packet-switched network to compute path costs to nodes. One serious drawback of this algorithm is known as the count to infinity problem. In particular, a network using distance vector routing responds rapidly to good news but slowly to bad news. Describe the problem and illustrate your explanation using a diagram showing how information held at each node of a simple network is updated in response to a node failing. [12 marks] END OF PAPER CMPSMD22 Version 2