WLAN System Capacity

1
WLAN System Capacity

• Zahid Iqbal

2
WLAN Technologiess

• IEEE802.11a
• IEEE802.11b
• IEEE802.11g

3
1EEE802.11a

• Operates on 5GHz band
• Uses a Radio Transmission called Orthogonal
  Frequency Division Multiplexing (OFDM)
• Data Transmission rates from 6Mbps to 54Mbps
• Use Carrier Sense Multiple Access with collision
  avoidance (CSMA/CA) for access method

4
Channels and Capacity
300MHz
5.75-5.85GHz
5.25-5.35GHz
5.15-5.25GHz
Low band with maximum power output of 50mW
Middle band with maximum power output of 250mW
High band with maximum power output of 1W
5
Channels and Capacity

• 802.11a standard has 12 non-overlapping channels
  spread across 3 bands
• 8 of the channels are restricted to indoor use
• 4 channels can be used indoor or outdoors

6
Channels and Capacity

• Maximum Wireless Capacity (Mbps) Rate of Non
  Overlapping Channel
• Maximum 802.11a Capacity 5412
• 
  648Mbps

7
IEEE 802.11b

• Operates on 2.4GHz (ISM) band
• Uses a Radio Transmission called Direct Sequence
  Spread Spectrum (DSSS)
• Data Transmission rates from 1Mbps to 11Mbps
• Use Carrier Sense Multiple Access with collision
  avoidance (CSMA/CA) for access method

8
Channels and Capacity

• Total 11 channels are available in the US
• 3 non-overlapping channels for use (Channela 1,
  6,11)
• Max 802.11b Capacity 113
• 33Mbps

9
IEEE 802.11g

• Operates on 2.4GHz (ISM) band
• Uses a Radio Transmission called Orthogonal
  Frequency Division Multiplexing (OFDM)
• Data Transmission rates upto 54Mbps

10
IEEE 802.11g

• Provides backword compatability with 802.11b
• Use Carrier Sense Multiple Access with collision
  avoidance (CSMA/CA) for access method

11
Channel and Capacity

• Total 11 channels are available in the US
• 3 non-overlapping channels for use (Channela 1,
  6,11)
• Max 802.11g Capacity 543
• 
  162Mbps

12

• Capacity and Performance of 802.11

13
Coverage and Throughput Performance

• Free space path loss

14
Coverage and Throughput Performance

• Free space path loss of 2.4GHz and 5GHz with
  distance 10m

15
Coverage and Throughput Performance
16
Coverage and Throughput Performance

• The multipath effect is modeled as a Rayleigh
  fading channel with a 50 ns delay spread.
• The shadow fading can be modeled as a log-normal
  distributed random variable with an 8 dB standard
  deviation.

17
Coverage and Throughput Performance

• For indoor radio environments, a two-slope path
  loss model with a breakpoint is used. The break
  point is related to the first Fresnel zone
  clearance, antenna height, and the transmitted
  frequency.

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Coverage and Throughput Performance
802.11a 802.11b 802.11g
Data Rate (Mbps) Distance (Feet) Data rate (Mbps) Distance (Feet) Data rate(Mbps) Distance (feet)
54 24 11 115 54 42
48 35 5.5 180 48 47
36 83 2 225 36 65
24 88 24 85
18 133 18 107
12 170 12 135
6 225 6 180
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Coverage and Throughput Performance
20
Coverage and Throughput Performance
For a cell radius of 65 feet, more than four
802.11b cells would have to be overlaid on top of
each other to achieve the same average throughput
of a single 802.11a cell.
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Impact of co-channel Interfenece on System
Capacity

• 802.11b/g has 3 non-overlapping channels

11
6
1
22
Impact of co-channel Interfenece on System
Capacity
23
Impact of co-channel Interfenece on System
Capacity

• Mechanism to model the effect of co-channel
  interference
• Clear Channel Assesment Method
• Hidden Cells

24
Impact of co-channel Interfenece on System
Capacity

• Co-channel interference immunity (CCI)
• CCA Threshold
• Practical CCA threshold

25
Impact of co-channel Interfenece on System
Capacity
26
Conclusion

• 802.11a can provide coverage comparable to
  802.11b upto 225 feet in a typical office
  environment
• The data rate of 802.11a is approximately 2 to 5
  times better than 802.11b within the coverage
  distance.

27
Conclusion

• 802.11g can provide coverage comparable to
  802.11b as well.
• The data rate of 802.11g in a single cell is
  higher than 802.11a because of the better
  frequency dependant path loss.

28
References

• James C. Chen, PhD., Jeffrey M. Gilbert, PhD,
  Measured Performance of 5-GHz 802.11a Wireless
  LAN Systems
• Proxim Corporation, A Detailed examination of
  the environment and protocol parameters that
  affect 802.11g network performance