辅导案例-EL822

  • August 18, 2020

UNIVERSITY OF KENT EL822 – R / 2020 FACULTY OF SCIENCES LEVEL 7 EXAMINATION SCHOOL OF ENGINEERING AND DIGITAL ARTS DATA NETWORKS AND THE INTERNET (3 hours) Instructions There are FIVE questions, divided into THREE sections. Candidates must answer FOUR questions – at least ONE from each section. turn over Q1 (i) In your own words, describe how the use of slotted operation improves the performance of contention access protocols compared to the purely random access defined in ALOHA. Discuss in your own words how adding a “carrier-sense” function, as in Carrier-Sense Multiple Access (CSMA) protocols, further improves performance. [4 marks] (ii) Following from (i), state in your own words how the addition of a collision detection (CD) function improves the throughput performance of CSMA-based protocols in networks such as Ethernets. Describe in your own words why the a-parameter must be less than 0.5 for correct collision detection operation. [4 marks] (iii) An Ethernet Local Area Network (LAN) has a data transmission rate of 100 Mb s-1 and uses a frame structure with the following fields: preamble (7 octets); start delimiter (1 octet); source and destination addresses (6 octets each); logical link control (LLC) field length indication (2 octets); frame check sequence (4 octets). The total worst case end-to-end delay is 1.65 s. Calculate the padding (PAD) field length necessary to ensure correct CD operation if there are 13 octets in the LLC field. [5 marks] (iv) Describe, in your own words, the strategies by which the a-parameter requirement is met in Gigabit Ethernets. [3 marks] (v) In your own words, describe the need for and the use of a spanning- tree protocol in bridged and switched LANs. [4 marks] 2 EL822 – R / 2020 Q2 (i) In your own words, first describe the operation of the CSMA-type protocol used in IEEE standard wireless LANs, in general terms. Then, in your own words, describe the virtual channel sensing collision avoidance mechanism that is used. [6 marks] (ii) An IEEE wireless LAN uses a 30-byte header and a 4-byte error- check in a trailer. It operates at 54 Mb s-1. Assuming the SIFS is 10 µs, and the DIFS is 40 µs, what is the minimum time taken for the completion of the transmission and acknowledgment of a payload of 750 bytes? What happens to this minimum time if the payload has to be broken up into 2 fragments? [6 marks] (iii) In your own words, briefly describe the types of devices and topologies used for wireless interconnection in ZigBee networks. [3 marks] (iv) In your own words, state how ZigBee networks provide advantages compared to Bluetooth in terms of numbers of devices, power consumption and range. [3 marks] (v) What is the main difference between 6LowPAN and ZigBee networks? [2 marks] (Note: SIFS: Short Inter Frame Space, DIFS: DCF Inter Frame Space, DCF: Distributed Coordination Function) 3 EL822 – R / 2020 turn over Section B Q3 (i) In your own words, discuss the following functions of the Internet Control Message Protocol (ICMP), stating the message types used in: (a) Error reporting (b) Performance measuring. [4 marks] (ii) Show the application of the shortest-path first algorithm in router R5 of the example network in Figure. 3.1, using the cost values indicated for the links. Note that you must show the steps involved in the algorithm, not simply the final result for the routing table in R5. The cost metric being used is “inverse-bandwidth”; explain in your own words how the routing protocol is advantageous compared to one simply using a “number of hops” metric. [7 marks] Figure 3.1 (iii) In your own words, discuss how Network Address Translation (NAT) has alleviated the problem of the lack of IPv4 addresses. State the problems that exist with the use of NAT. [6 marks] (iv) Discuss in your own words how router operation is streamlined with the introduction of the IPv6 (version 6) standard. [3 marks] 4 EL822 – R / 2020 Q4 (i) In your own words, describe the “three-way handshake”, including its use of SYN and ACK flag bits, in the connection establishment procedure of the Transmission Control Protocol (TCP). [3 marks] (ii) Briefly describe in your own words the two TCP schemes that can be used with wireless local area networks (WLANs). [4 marks] (iii) Referring to the use of both recursive and iterative approaches, briefly describe the operation of resolvers for the Domain Name System (DNS) in your own words. [3 marks] (iv) (a) In your own words, describe in general terms, the operation of substitution, transposition and product ciphers. [4 marks] (b) Why is the implementation of such ciphers referred to as “electronic codebook mode”, and what is the principal drawback? How does cipher block chaining overcome this, and what is its principal resulting drawback? [4 marks] (vi) State the functions provided in the two main protocols in IPsec, Authenticated Header (AH) and Encapsulated Security Payload (ESP) in your own words. [2 marks] 5 EL822 – R / 2020 turn over Section C Q5 (i) For a system in which there are a large number of independent sources, randomly generating packets, the probability of x packet arrivals in any given time can be related to the mean number µ expected in that time, by the Poisson distribution ( ) e ! x P x x   . Show that for such a system, the inter-arrival time distribution is a negative exponential given by   e tp t   where  is the mean packet arrival rate. [7 marks] (ii) Show that the state probabilities np for a M/M/1 infinite queue are given by  1 nnp    where  is the traffic intensity and n is the state. State all assumptions made. [8 marks] Hence, show that the mean queue length  E n is given by   ( ) 1 E n     [3 marks] (iii) Assuming random packet arrivals at a mean rate of 25000 s-1 and a mean packet duration of 20 s, calculate the mean queue length for a single-server system with an infinite queue. [2 marks] End of Examination Paper 6 EL822 – R / 2020

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