Effect of Bandwidth and Guard Interval in WiFi 802.11ac

Open NetSim and Select Examples > Internetworks > Wi-Fi > Effect of bandwidth in WiFi 802.11ac then click on the tile in the middle panel to load the example as shown in below Figure

Figure4-20

The following network diagram illustrates what the NetSim UI displays when you open the example configuration file as shown below Figure.

Figure4-21

Network Settings

  1. Environment Grid length: 50m * 50m.
  2. Click on the Application icon present in the top ribbon/toolbar and set Transport Protocol to UDP
  3. Channel Characteristics: NO PATHLOSS in wireless link properties.
  4. Set Bit Error rate and Propagation delay to zero under wired link properties
  5. Set 802.11ac standard and Bandwidth to 20MHz under Wireless Interface->Physical Layer properties of the access point and wireless node.
  6. Set DCF as the medium access layer protocol under Wireless Interface-> datalink layer properties of access point and wireless node
  7. Enable packet trace and plots.
  8. Set generation rate as 100 Mbps under Application properties (Packet Size = 1460 Bytes, Interarrival time = 116 microseconds). Generation rate can be calculated by using the formula below: $$Generation \ rate (Mbps) = Packet \ Size(Bytes) \ast \frac{8}{Inter \ arrival \ time } \mu s$$ $$= 1460 (Bytes)*8/116 (\mu s) \backsim 100 Mbps$$
  9. Run simulation for 10s and see Application Throughput in the Results Window.
  10. Go back to the scenario and increase the Bandwidth 20 to 40, 80, 160 respectively and check the throughput in the results window

Analytical Model

The average time to transmit a packet comprises of

  • DIFS
  • Backoff duration
  • Data packet transmission time
  • SIFS
  • MAC ACK transmission time

The timing diagram is as shown below Figure.

Figure4-22

The Average throughput can be calculated by using the formula below $$Average \ Throughput \ (Mbps) = \frac{Application \ payload (Bytes)}{Average \ Times \ per \ packet (\mu s)}$$ Average time per packet ($\mu s$) $$= DIFS + Average \ Backoff \ time + Packet \ Transmission + SIFS + Ack \ Transmission \ Time$$ $$Packet \ Transmission \ Time (\mu s) = Preamble \ time + (MPDU \ Size / Data rate )$$ $$Average \ Backoff \ time (\mu s) = (CWmin/2) \ast SlotTime$$ $$Ack \ Transmission \ Time (\mu s) = preamble \ time + (Ack\ Packet \ size / Ack \ data \ rate)$$ $$DIFS(\mu s) = SIFS +2 \ast SlotTime$$ $$Average \ Backoff \ time (\mu s) = (CWmin/2)*SlotTime$$ where
$$Application Payload = 1460 \ Bytes$$ $$Average \ time \ per \ packet = 34 + 67.5+185.36+16+212.88 = 513.74 \mu s$$ $$SIFS = 16 \mu s$$ $$Slot time = 9 \mu s$$ $$CWmin = 15 slots \ for \ 802.11ac$$ $$DIFS = SIFS + 2 \ast SlotTime = 16 \mu s+ 2 \ast 9 \mu s$$\ $$Average \ BackOff \ time = 67.5 \mu s$$ $$Packet \ Transmission \ Time = 44 \mu s+(1532 \ast 8/86.7 \ Mbps ) = 185.36 \mu s$$ $$Preamble \ time = 44 \mu s for \ 802.11ac \ standard$$ $$MPDU \ Size = 1460 + 8 + 20 + 44 = 1532 \ Bytes$$ $$Ack Transmission Time = 44 \mu s + + (152 \ Bytes \ast 8/7/2 Mbps) = 212.8 \mu s$$ $$Average \ throughput = 1460 \ast 8/(513.74) = 22.7 Mbps$$

Similarly calculate throughput theoretically for other samples by changing bandwidth and compare with Simulation throughput. Users can get the data rate by using the formula given below $$Phy \ rate (802.11b) = Phy \ layer \ payload \ast 8/(Phy \ end \ time - phy \ arrival \ time -44 )$$

Results and Discussion#

Bandwidth (MHz) Analytical Estimate of Throughput(Mbps) Simulation Throughput (Mbps)
20 22.70 22.81
40 33.77 33.94
80 43.39 43.65
160 49.35 49.75

One can observe that there is an increase in throughput as we increase the bandwidth from 20MHz to 160MHz.

Effect of Guard Interval#

Open NetSim and click on Examples > Internetworks > Wi-Fi > Effect of Guard Interval in WiFi 802.11ac then click on the tile in the middle panel to load the example as shown in Figure

Figure4-23

The following network diagram illustrates what the NetSim UI displays when you open the example configuration file as shown Figure 4-24.

Figure4-24

Network Settings

  1. Environment Grid length: 50m * 50m.
  2. Click on the Application icon present in the top ribbon/toolbar and set Transport Protocol to UDP
  3. Channel Characteristics: NO PATHLOSS in wireless link properties.
  4. Set 802.11ac standard and Bandwidth to 20MHz under Wireless Interface->Physical Layer properties of the access point and wireless node
  5. Set DCF as the medium access layer protocol under Wireless Interface-> datalink layer properties of access point and wireless node.
  6. Set Bit Error rate and Propagation delay to zero under wired link properties.
  7. Enable plots.
  8. Set Guard interval to 400ns under Wireless Interface->Physical Layer properties of access point and wireless node.
  9. Set generation rate as 100 Mbps under Application properties (Packet Size = 1460 Bytes, Interarrival time = 116 microseconds). Generation rate can be calculated by using the formula below: $$Generation \ rate (Mbps) = Packet \ Size(Bytes) \ast \frac{8}{Inter \ arrival \ time } \mu s$$ $$= 1460 (Bytes)*8/116 (\mu s) \backsim 100 Mbps$$
  10. Run simulation for 10s and note down the throughput.
  11. Go back to the scenario and increase the Guard interval to 400 to 800 and check the throughput in the results window

Calculate throughput theoretically as explained above and compare with Simulation throughput.

Results and Discussion#

Guard Interval (ns) Theoretical Throughput(Mbps) Simulation Throughput(Mbps)
400 17.76 22.81
800 16.87 21.39