Dynamic Rate Switching

As client station radios move away from an access point, they will shift down to lower bandwidth capabilities using a process known as dynamic rate switching (DRS). Access points can support multiple data rates depending on the spread spectrum technology used by the AP’s radio card.

For example, an 802.11b radio supports data rates of 11, 5.5, 2, and 1 Mbps. Data rate transmissions between the access point and the client stations will shift down or up depending on the quality of the signal between the two radio cards, as pictured in Figure below.

There is a correlation between signal quality and distance from the AP. As a result, transmissions between two 802.11b radio cards may be at 11 Mbps at 30 feet but 2 Mbps at 150 feet.

Dynamic rate switching (DRS) is also referred to as dynamic rate shifting, adaptive rate selection, and automatic rate selection. All these terms refer to a method of speed fallback on a wireless LAN client as signal quality from the access point The objective of DRS is upshifting and downshifting for rate optimization and improved performance.

Effectively, the lower date rates will have larger concentric zones of coverage than the higher data rates, as pictured in Figure below.

The algorithms used for dynamic rate switching are proprietary and are defined by radio card manufacturers. Most vendors base DRS on receive signal strength indicator (RSSI) thresholds, packet error rate, and retransmissions. nd retransmissions.

RSSI metrics are usually based on signal strength and signal quality. In other words, a station might shift up or down between data rates based on both received signal strength in dBm and possibly on a signal-to-noise ratio (SNR) value.

Because vendors implement DRS differently, you may have two different vendor client cards at the same location while one is communicating at 5.5 Mbps and the other is communicating at 1 Mbps.

For example, one vendor might shift down from data rate 11 Mbps to 5 Mbps at –70 dBm while another vendor might shift between the same two rates at –75 dBm. Keep in mind that DRS works with all 802.11 PHYs.

For example, the same shifting of rates will also occur with ERPOFDM radios shifting between 54, 48, 36, 24, 18, 12, 9, and 6 Mbps data rates. As a result, there is a correlation between signal quality and distance from the AP.

It is often a recommend practice to turn off the two lowest data rates of 1 and 2 Mbps when designing an 802.11b/g network. The two reasons that a WLAN network administrator might want to consider disabling the two lowest rates on an 802.11b/g access point are medium contention and the hidden nIn Figure below, you will see that there are multiple client stations in the 1 Mbps zone and only one lone client in the 11 Mbps zone. Remember that wireless is a half-duplex medium and only one radio card can transmit on the medium at a time.

By forcing the higher data rates, it is easier to force more distributed capacity over the access points. This is not typically necessary when planning solely for coverage. solely for coverage.

All radio cards access the medium in a pseudo-random fashion as defined by CSMA/CA. A radio transmitting a 1,500-byte data frame at 11 Mbps might occupy the medium for 100 microseconds.

Another radio transmitting at 1 Mbps per second will take 1,100 microseconds to deliver that same 1,500 bytes. Radio cards transmitting at slower data rates will occupy the medium much longer, while faster radios have to wait.

If multiple radio cards get on the outer cell edges and transmit at slower rates consistently, the perceived throughput for the cards transmitting at higher rates is much slower due to waiting for slower transmissions to finish.

For this reason, too many radios on outer 1 and 2 Mbps cells can adversely affect throughput. Another reason to consider turning off the lower data rates is the hidden node problem.