Rate Adaptation in 802.11b

NetSim rate adaptation is explained in section 3.1.19 of this document.

Open NetSim, Select Examples > Internetworks > Wi-Fi > 802.11 Rate Adaptation then click on the tile in the middle panel to load the example as shown in below Figure.

Figure4-15

The following network diagram illustrates what the NetSim UI displays when you open the example configuration file for Rate Adaptation.

Figure4-16

Network Settings

  1. Environment Grid length: 500m * 500m
  2. Distance between AP and Wireless Node is 65.5m
  3. Enabled Packet Trace and plot option
  4. Set rate adaptation as Generic in datalink properties of access_point and wireless node
  5. Set DCF as the medium access layer protocol under datalink layer properties of access_point and wireless node.
  6. Click on the Application icon present in the top ribbon/toolbar and set Transport Protocol to UDP
  7. Set WLAN Standard $\rightarrow$ 802.11b
  8. Channel Characteristics $\rightarrow$ Path Loss only, Path Loss Model $\rightarrow$ Log Distance and Path loss Exponent $\rightarrow$ 3.25. (Wireless Link Properties)
  9. CBR application with 10Mbps generation rate (Set Packet Size: 1460 Bytes, Inter Arrival Time: 1168 micro sec)
  10. Simulate for 10 sec.

Results and Discussion#

Open Packet Trace and filter Packet Type to CBR, Transmitter_ID to Access Point 3 and then calculate Phy rate. Phy rate can be calculated using packet trace by using the formula shown below: $$Phy \ rate (802.11b) = \frac{Phy\ layer\ payload * 8}{(phy \ end \ time - phy \ arrival \ time - 192)}$$

192$\mu s$ is the approximate preamble time for 802.11b

Calculate PHY rate for all the data packets coming from Access Point to Wireless Node. For doing this please refer NetSim User Manual > Section 8.4.2 How to set filters to NetSim Trace file

Figure4-17

The ‘Generic’ rate adaptation algorithm is similar to the Auto Rate Fall Back (ARF) algorithm. In this algorithm:

  • Rate goes up one step for 20 consecutive packet successes.
  • Rate goes down one step for 4 continuous packet failures

In the above screenshot, the Phy rate reduces from 11Mbps to 5.5Mbps, since there are 4 consecutive data error packets. Then the rate increases from 5.5Mbps to 11Mbps one there is 20 consecutive successful data packet transmissions.