Copyright 2016© University of Southampton Page 1 of 5 UNIVERSITY OF SOUTHAMPTON ELEC3222W1 SEMESTER 1 EXAMINATIONS 2015/16 COMPUTER NETWORKS Duration 150 mins (2.5 hours) This paper contains 4 questions Answer ALL questions. Only University approved calculators may be used. A foreign language dictionary is permitted ONLY IF it is a paper version of a direct ‘Word to Word’ translation dictionary AND it contains no notes, additions or annotations. 5 page examination paper ELEC3222W1 Copyright 2016© University of Southampton Page 2 of 5 Answer ALL questions 1 (a) State one advantage and one disadvantage of protocol layering. [2 marks] (b) Using Ethernet/IEEE 802.3 as an example, explain the difference between a network hub, switch, and router. [3 marks] (c) Alice has many uncompressed digital photographs taken at a resolution of 1920 x 1080 pixels with 3 bytes/pixel. She needs to send some to Bob, and considers two options: 1) using a modem with a data- rate of 56 kbps, or 2) using a courier to deliver a large USB flash drive. The courier is available immediately, and travels at an average speed of 60km/hr. Assuming that Bobby lives 100 km from Alice, calculate how many photos Alice must send to Bobby in order for the courier to offer a higher data rate than the modem? [7 marks] (d) A network is formed from two battery-powered wireless nodes: A and B. Node A very infrequently transmits data to Node B. Describe and compare the impact that the MAC layer has on the delay/latency and the energy consumption of A and B when: (i) using the ALOHA MAC protocol (ii) using the B-MAC protocol [7 marks] (e) A LAN contains 20 stations, a fixed frame size of 100 bits, and has a data-rate of 1 kbps. Calculate the worst-case (longest) time that a station might have to wait before it can begin transmitting its frame when: (i) using the basic bit-map MAC protocol (ii) using the binary-countdown MAC protocol [6 marks] ELEC3222W1 TURN OVER Copyright 2016© University of Southampton Page 3 of 5 2 A bit string, 110111111012, is passed from the network layer to the data link layer for transmission. (a) The data link layer first applies a (15,11) Hamming code (using even parity). Calculate the bit string that is produced. [8 marks] (b) Next, the data link layer frames the data using bit stuffing (the flag byte is 011111102). Using your answer from (a), calculate the bit string that is produced. If you were unable to answer (a), assume that the original bit string was unchanged by Hamming coding. [5 marks] (c) On receiving the bit string from the data link layer, the physical layer applies Manchester coding. Using your answer from (b), sketch the waveform that is produced for the first 10 bits transmitted by the physical layer. If you were unable to answer (b), assume that the original bit string was unchanged by bit stuffing. [5 marks] (d) Briefly state the purpose of applying each of the above three steps (a)-(c), instead of just directly transmitting the bit string passed from the network layer? [3 marks] (e) Calculate the maximum overhead that the data link layer may add through the above steps (a)-(b), in the general case where any 11-bit string could be passed from the network layer. The overhead is equal to the number of bits that the data link layer passes to the physical layer minus the number of bits it had originally received from the network layer. [4 marks] ELEC3222W1 Copyright 2016© University of Southampton Page 4 of 5 3 (a) Explain why, In cryptography, it is necessary to have redundancy in messages? State a method of protecting against replay attacks and evaluate your solution. [7 marks] (b) Suppose that a system uses Public Key Infrastructure (PKI) based on a tree hierarchy of Certification Authorities (CAs). Alice wishes to communicate with Bob and gets a certificate from Bob signed by CA X. If Alice has never heard of X, explain what can she do to verify that she is talking to Bob? [7 marks] (c) Table 1 shows a mutual authentication process using a PKI directory server (PKIDS). Describe how it works and identify all the symbols used. At step 7, RB is encrypted. Is this necessary or would plaintext do? Explain your answer. [11 marks] Step Direction Contents 1 Alice → Bob Request for EB 2 PKIDS → Bob EB 3 Alice → Bob EB(A,RA) 4 Bob → PKIDS Request for EA 5 PKIDS → Bob EA 6 Bob → Alice EA(RA,RB,KS) 7 Alice → Bob KS(RA) Table 1 ELEC3222W1 END OF PAPER Copyright 2016 © University of Southampton Page 5 of 5 4 (a) Explain how Network Address Translation (NAT) and Classless Interdomain Routing (CIDR) work and what their purpose is. [9 marks] (b) An IPv4 router has the routing table as shown in Table 2. Describe what the router does with a packet addressed to each of the following destinations: 128.96.39.10, 128.96.40.12, 128.96.40.151, 193.4.153.17 and 192.4.153.90. [5 marks] Subnet number Subnet mask Next hop 128.96.39.0 255.255.255.128 IF0 128.96.39.128 255.255.255.128 R1 128.96.40.0 255.255.255.128 R2 128.4.153.0 255.255.255.128 R3 R4 Table 2 (c) A set of transport layer system calls and the messages that result when they are called is given in Table 3. Draw a diagram or a table showing the different state transitions involved when a client requests a file from a server. Distinguish between messages initiated by the client from those initiated by the server. [11 marks] System call TPDU sent Meaning LISTEN None Block until a process tries to connect CONNECT CON. REQ. Attempt to get a connection SEND DATA Send information RECEIVE None Block until a TPDU arrives DISCONNECT DCON. REQ. Wait until a DCON. REQ. arrives Table 3
