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