Understanding Rate Adaptation in Wifi

Rate Selection or Adaptation in Wireless Networks

Depending on the Wlan standard, a wireless device can support multiple data rates. For example, an 802.11b compliant device supports 11, 5.5, 2, & 1Mbps speeds. A 802.11g capable device supports OFDM data rates of 6,9,12,18,24,36,48 and 54 Mbps and DSSS data rates of 11, 5.5, 2 and 1 Mbps.

The date rate specifies the rate at which data is transmitted on the physical layer. A higher data rate guarantees a higher throughput. A high data rate requires a high received signal strength for the receiver to decode the frame. If Received signal strength of the frames is less than a specific value for that data rate, then the frames are dropped at the receiver. In poor channel conditions, the RSSI is lesser due to interference .Low data rates require low SNR values ( or low signal strength ) to decode the frames. In poor channel conditions or noisy environments, it is better to send frames at low data rates, since frames or not dropped.

So in order to maximize throughput, the data transmission rate has to be dynamically adapted depending on the channel conditions. Also as the client moves farther away from the AP, the RSSI value decreases. Even in this case, the rate has to be switched to lower rate dynamically.

The standard has not specified any particular method that has to be adopted to change the rates dynamically. The implementation of rate adaptation or selection logic is left to the discretion of chip set vendors.

Rate adaptation algorithms may use the signal quality as a measure to adapt the rate. Signal quality may be measured using the Signal to Noise Ratio or by having a count of the number of frames that require re transmissions. SNR may be measured as a average of certain number of frames or the instantaneous SNR of the last received frame. Another way of measuring the signal quality is to check if a re transmitted frame has reached it retry limit count. Every frame transmitted in a wlan is associated with a retry count ( short retry count or long retry count) . If the frame’s retry count is reached and the frame has not been successfully delivered, then step down to the next lower data rate and try again. This logic may be repeated until the frame is delivered at a particular lower data rate and or the lowest possible data rate is reached.

The following are some rate adaptation algorithms:

1. Auto Rate Fallback (ARF)

2. Adaptive Auto Rate Fallback (AARF)

3. Collision Aware Rate Adaptation (CARA)

4. Robust Rate Adaptaion Algorithm ( RRAA)

5. Loss Differentiation Rate Adaptation ( LDRA)

6. Ministrel Rate Adaptation






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