NM-1.04

Question 1

Explain what 802.11 is?

Answer 1

Family of evolving specifications for wireless local area networks develoepd and maintained by a group of IEEE.

Question 2

What are the limitations of IEEE.802

Answer 2

1. QoS

Throughput is lower

3. Coexistence

• Multiple standards
  -Overlapping frequency bands

4. Security; Easy to eavesdrop

Question 3

What are the goals of 802.11?

Answer 3

1. Global Operation
   - Coordination of international spectrum regulisation

Question 3

What is the difference between Infrared and Radio

Answer 3

Infrared:
1. Short-range point-to-point data transfer

2. Can use diffused reflected light, but generally requires LOS for good data rates
3. Tx: LED or lasers
4. Rx: photodiodes

Radio:
1. Most wireless networks use radio frequencies

Question 4

Describe the difference between Infastructure-based and Adhov

Answer 4

Infrastructure-based

1. Access point is responsible for forwarding messages, limited control and management functions

Access points acts as a bridge to other wirelines and wireless networks

2. Applications: office-wide WLANs, hotspots

Ad-hoc
1. All devices directly connect to each other.

Question 5

How many subcarriers does OFDM have?

Answer 5

52

Question 6

Explain why the typical maximum layer 3 data rate is lower

Answer 6

Due to different things such as shadowing and free space path loss

Question 7

Explain DSSS

Answer 7

Spread the signal that it’s noise like. 3G uses this and there’s issues when there’s a hundred users.

Question 8

Explain the pros and cons of IEEE 802.11a

Answer 8

Adv:

1. Fast max speed
2. Supports multiple users

Disadv:
1. Highest cost
2. Shortest range signal -> inside the body; 5GHz
3. Easily obstructed

Question 9

Explain the pros and cons of IEEE 802.11b

Question 10

Explain the pros and cons of IEEE 802.11g

Answer 10

2.4 GHz

Adv:
1. Longest range
2. Supports multiple users
3. Not easily obstructed

Disadv:
1. More costly than 802.11b
2. Other appliances may interfere in same frequency band

Question 11

Explain IEEE 802.11n

Answer 11

Higher data rate through MIMO

Operating frequency: 2.4GHz/5GHz

Spacing of antennas; wavelength/2 = c/2f
Which is why ‘mm’ is good; Can pack in more antennas

Question 12

Explain IEEE 802.11ad

Answer 12

1. Data throughput -> 7Gbps
2. Large bandwidth
3. Reduced interference

Range -> few metres

Question 13

Explain IEE 802.11ax

Answer 13

High-density public environments -> trains, stadiums

Beneficial for IoT

Designed for cellular data off-loading

Theoretical BW -> 14Gbps

Uses MU-MIMO w/ OFDMA -> don’t mix this up with duplexing

Question 14

For which of the following scenarios an ad-hox architecture is at least indicated?

1. Disaster aftermath
2. Airport hotspot
3. Vehicular networks

Answer 14

Airport hotspot -> you can wire cables to make this architecture but can’t do this for other 1 or 2

Question 15

In a machine type…

1. Four
2. One
3. Doesn’t matter

Answer 15

2. One

Question 16

What can the rank tell us about MIMO?

Answer 16

The higher the rank, better to go for MIMO spatial multiplexing

Lower the rank, better to go for spatial diversity

Question 17

Explain the difference between 802.11a, 802.11b, 802.11g

Answer 17

802.11a; short range signal and fastest

802.11b
802.11g; longest range, data rates higher than 802.11b, not easily obstructed

Question 18

In case we want to improve the spectral efficiency of a communication link, what should we do?

Answer 18

1. Increase the bandwidth, transmit at a higher data rate.
2. Increase the number of antennas
3. Channel coding scheme with high redundancy