Q&A Session Wireless and Mobile Networks

Copyright 2000-2021 Intelligent Networking Laboratory

Sungkyunkwan University

Q&A Session

Wireless and Mobile Networks

Mobile Computing

Sungkyunkwan University

Prepared by L. Mwasinga, T-D. Nguyen and H. Choo

[email protected]

2021-Fall Mobile Computing Intelligent Networking Laboratory 2/14

Wireless and Mobile NetworksQuestion 1

In IEEE 802.11, a station with ________ mobility is either stationary (not

moving) or moving only inside a Basic Service Set (BSS)

a) No-transition

b) BSS-transition

c) CSS-transition

d) ESS-transition

2021-Fall Mobile Computing Intelligent Networking Laboratory 3/14

Wireless and Mobile NetworksQuestion 2

Express your understanding about the Hidden Terminal Problem (HTP).

How can we resolve the HTP? Explain your selection(s):

a) Increase transmission power of the terminals (Case 1)

b) Move node A to the right (Case 1)

c) Move node A to in between node B and the obstacle (Case 2)

Case 1 Case 2

Obstacle

2021-Fall Mobile Computing Intelligent Networking Laboratory 4/14

Wireless and Mobile NetworksQuestion 3

What do you know about CSMA? (TRUE/FALSE)

a) Hosts must contend to access the medium

b) CSMA/CD can solve hidden terminal problem in wired networks, in

wireless networks, we must use CSMA/CA

c) The Interframe Space (IFS) is used in CSMA/CA

d) Positive acknowledgement is not necessary in CSMA/CA

2021-Fall Mobile Computing Intelligent Networking Laboratory 5/14

Wireless and Mobile NetworksQuestion 4

Following is a case that two hosts 𝐵 and 𝐶 want to send some data

frames. Herein, do you think CSMA/CD or CSMA/CA is used?

Explain your answer

CSMA/CD: Carrier Sensing Multiple Access/ Collision Detection

CSMA/CA: Carrier Sensing Multiple Access/ Collision Avoidance

2021-Fall Mobile Computing Intelligent Networking Laboratory 6/14

Wireless and Mobile NetworksQuestion 5

RTS/CTS and Exposed Terminal Problem (ETP) (TRUE/FALSE)

a) During the contention window, a node knows that the channel is busy

when it sends RTS and receives CTS

b) The use of RTS/CTS causes ETP

c) ETP increases the network throughput

RTS: Request to Send

CTS: Clear to Send

2021-Fall Mobile Computing Intelligent Networking Laboratory 7/14

Wireless and Mobile NetworksQuestion 6

What do you know about the contention window:

a) In CSMA/CA, why do you need a contention window after a DIFS?

b) What happens to the timer when the station finds the channel busy?

c) At what size of a contention window, we can guarantee no collision?

DIFS: DCF Inter Frame Space

2021-Fall Mobile Computing Intelligent Networking Laboratory 8/14

Wireless and Mobile NetworksQuestion 7

Show your understanding about the Network Allocation Vector (NAV)

a) Is it correct that NAV is the time the station should remain quiet?

b) Is it true that other stations start their NAV when a station accesses the

system and sends a CTS frame?

c) What happens after a NAV timer fires?

2021-Fall Mobile Computing Intelligent Networking Laboratory 9/14

Wireless and Mobile NetworksQuestion 8

Do a quick google search and fill in the empty cells:

Bluetooth Classic Bluetooth Low Energy

Frequency Band 2.4GHz

Number of channels 40 channels with 2MHz spacing

Topology P2P, Star, Mesh

Power consumption Low

Max Tx Range 100m

Max Transmission

Rate1-3Mb/s

Optimized for Continuous data stream

1

2

3

4

5

6

7

2021-Fall Mobile Computing Intelligent Networking Laboratory 10/14

Wireless and Mobile NetworksQuestion 9

Check to see if the following sets of chips can belong to an

orthogonal system:

a) [+1, +1, -1] and [-1, +1, +1]

b) [+1, +1, +1, +1] and [+1, -1, +1, -1]

c) [+1, +1] and [+1, -1]

2021-Fall Mobile Computing Intelligent Networking Laboratory 11/14

Wireless and Mobile NetworksQuestion 10

The rule for encoding the CDMA sequence is like this:

Why don’t we encode bit 0 as 0 instead of -1?

2021-Fall Mobile Computing Intelligent Networking Laboratory 12/14

Wireless and Mobile NetworksQuestion 11

Which of the following statements is/are the reason(s) that

CSMA/CD algorithm does not work in wireless LAN?

a) Wireless hosts cannot send and receive at the same time (do

not work in duplex mode)

b) Due to extended distance between stations, a station may not

hear a collision on the other end

c) A station may not be aware of another station’s transmission due

to obstacles or range problems

2021-Fall Mobile Computing Intelligent Networking Laboratory 13/14

Wireless and Mobile NetworksQuestion 12

What do you understand about the following general approaches in

systems?

a) Reactive

b) Proactive

c) Predictive

2021-Fall Mobile Computing Intelligent Networking Laboratory 14/14

Wireless and Mobile NetworksQuestion 13

Answering True/False about system organization approaches:

a) Decisions in centralized systems can be made at different nodes, and

then be aggregated to determine final system behavior

b) Decentralized systems are designed to solve global big tasks

c) Block-chain can be a distributed system but not a decentralized system

d) The internet can be considered as a distributed system and

decentralized system

Hint: refer to the following

▪ https://www.scaleyourapp.com/difference-between-centralized-decentralized-distributed-systems-explained/

▪ https://www.geeksforgeeks.org/comparison-centralized-decentralized-and-distributed-systems/

▪ https://whatis.techtarget.com/definition/distributed-search

Copyright 2000-2021 Intelligent Networking Laboratory

Sungkyunkwan University

Thanks to contributors

Mr. Lusungu J. Mwasinga

Dr. Tien-Dzung Nguyen

Prof. Hyunseung Choo

Mr. Haziq Hamzah