Wireless Transmission and Communication Challenges

Note: Word limit for each question part is 200 words excluding references, equations and diagrams.

a)A wireless transmission system needs to overcome several key issues (challenges) related to the behaviour of radio frequency (RF) propagation in wireless channels. One such challenge is multipath propagation.  Explain the effects of multipath on signals at a wireless receiver.   

b)In wireless communications, how does the power of a signal change with respect to distance? Critically analyse the relationship between power seen at the receiver and the distance ‘r’ between transmitter and receiver.  If the distance ‘r’ is increased by 4 times, how would it affect the signal power seen at the receiver? Show your workings.

c)Assume that an isotropic radiator transmitting at 100 mW is placed at a distance of 100 m from the receiver. If the receiver is using a parabolic dish of 0.25 m radius with an efficiency of 55%, what will be the power in watts seen by the receiver?  

d)Assume the following scenario: an Engineer John Doe from Newcastle City Council installed a new wireless LAN in the Eldon Square shopping Mall after performing a wireless site survey. After installation he measured RF signal strength and coverage and found it adequate. During the first two weeks of December 2019 (busy Christmas shopping season), bad weather forced many shoppers from the street into the mall such that it was crammed with people. Many complained about the poor quality of wireless network. During this time, the signal strength and quality of WLAN was less than desirable in many areas. What do you think is the reason(s) for poor wireless signals? Which RF signal property might have contributed to the poor signal reception?

Question 2

Note: Word limit for each question part is 200 words excluding references, equations and diagrams.

a)Suppose that a signal travels through a wireless transmission medium and its power is reduced to fourth (1/4). How much of the signal power is lost in dB? Show your workings in the answer.

b)IEEE 802.11n introduced few significant enhancements to the previous version (802.11g). Critically explain how these enhancements resulted in producing a wireless standard that was capable of achieving much higher data rates?

c)Analyse how VSWR could potentially damage the system

d)A WLAN Access Point configured at 2.4 GHz has two antennas that are separated by 4 inches. How many wavelengths separate the antennas? How this separation between the antennas would affect the reliability of transmission?

Question 3

Note: Word limit for each question part is 250 words excluding references, equations and diagrams.

a)Assume you are working as a Network Engineer for a Telecommunications service provider and your task is to estimate the feasibility of a 10 km link between two buildings with one access point and one client radio.  The access point is connected to an antenna with 10 dBi gain, with a transmitting power of 100 mW and a receive sensitivity of -84 dBm.  The client is connected to an antenna with 13 dBi gain, with a transmitting power of 16 dBm and a receive sensitivity of -82 dBm. The cables in both systems are short, with a loss of 2dB at each side at the 2.4 GHz frequency of operation. What is the link margin? By examining the link margin, comment on the reliability of the link.

b)What recommendation you would make to improve the link margin for the system deployed in part a above?

Question 4

Note: Word limit for each question part is 200 words excluding references, equations and diagrams.

a)Consider an IEEE802.11b based ad hoc wireless network that uses distributed coordinated function (DCF) as its MAC protocol. The network parameters are configured as follows: Raw data rate = 11 Mbps, Slot time = 20 us, SIFS = 10 us, DIFS = 50 us, CW_min (Slot time) = 31, CW_max (Slot time) = 1023. Assume that there are three wireless stations A, B and C are active on the network. Assume all stations are within each other's coverage range. If A and B both receive 1500 Byte data (payload) to transmit to C at the same time, A wins the contention and begins transmission. Using the virtual carrier sense performed by station B, how long it should remain quiet (inactive) before trying for transmission.

b)In a WLAN environment, will multiple SSIDs and BSSIDs affect the performance of the network? Justify your answer.

c)Briefly explain which factors are important for planning a Wireless LAN design for a campus network.

Question 5

Note: Word limit for part (a) is excluding references, equations and diagrams. Word limit for part(b) is 200 words excluding references, equations and diagrams.

a)Assume that Alice and Bob wish to exchange a secret message using the RC4 cipher. The secret message contains the time of meeting at a nearby cafe. Alice encodes the meeting time of 12:15 hrs using a secret key [4321]. Using a state vector of 8x3, find the value of the cipher text. Show your working of the iterations of RC4 algorithm.

Hint: You may ignore the “:” in the meeting time and consider it a string [1215]. The state vector consists of 8 elements and each element is represented by 3 bits.

b)In part (a) above, Alice achieved confidentiality by encrypting the message using the RC4 cipher. If both wish to further enhance the security of this communication such that it is safe from attacks that compromise integrity, authentication, and non-repudiation. Which security mechanism they must use? You may explain the details of the technique used with the help of a figure.  

Question 6

Note: Word limit for each question part is 250 words excluding references, equations and diagrams.

a)Wireless hijacking is a serious threat for the WLAN networks. Explain how a rogue user (adversary) could hijack an active wireless session. List the number of steps, providing brief explanation of each step. (Hint: you may use a simple diagram involving wireless client, Access Point, Adversary).    

b)Analyse what can happen when an intruder compromises the pre-shared key (PSK) or passphrase used during WPA/WPA2-Personal authentication?