Tuning your Wi-Fi by adjusting transfer rates

In most Wi-Fi systems you can disable the slowest transfer rates. This is typically done to improve efficiency since the transfers at slower rates eat up limited air time. This can backfire however with unexpected results.

In 802.11 standard defines basic rates and supported rates. The access points broadcast this information in every beacon. To associate with the network every device has to support all basic rates (i.e. they are requirements). Supported rates are optional. Supported rates that are common to both the device and the AP may be used. Typically there are one or a couple of basic rates and they are at the slow end of the scale. In most systems the administrator can configure the the rates by disabling unwanted ones.

Devices and APs will always use the highest rate the connection can carry. The rate is adjusted for each frame if the connection quality changes, for example the device moves. The lowest basic rate is used for all management frames (beacon, probe, probe request etc.) and for broadcasts and multicasts.

Available basic and supported rates:

802.11b (2.4 GHz only) 1, 2, 5.5 and 11 Mbps
802.11 OFDM 6, 9, 12 and 18 Mpbs
802.11 OFDM Extended 24, 36, 48 and 54 Mbps

Disabling lowest rates

It appears obvious that by disabling the lowest supported rates you can increase the throughput of the network. Slow transfers eat up a lot of the air time. Historically the difference  wasn’t that large, but today it can be 600 fold (1 Mbps vs. 600 Mbps). It also appears that roaming would improve since by disabling slow (i.e. bad) connections the devices will be forced to roam earlier.

Unfortunately it doesn’t quite work that way. The air time won’t increase as intended unless there is always a close AP nearby. If there isn’t a better AP the devices will stay associated with too high transfer rate that the connection can’t carry. This will result in transmission errors and retries, which will eat up the freed air time. The user at the end of the bad connection will experience high latencies and especially jitter (i.e. variation of latency).

If you think you have a dense network with APs close by you need to think of the edges, too. At the fringes the coverage will inevitably grow thinner. In the best case the fringe will be outside of the building where there are no users (in the upper floors at least). On ground level it is difficult to prevent devices from associating as soon as the network is detected.

Disabling the lower rates won’t help with roaming either. Most devices only track RSSI. As long as the signal is above the threshold the device won’t roam. The threshold doesn’t include rate information at all. The device will try to keep associated to the AP that appears to have signal strong enough – even if it can’t reliably exchange data. Only if the connection is completely lost will the device start looking for a new AP. This will appear as a connection problem to the user. In the worst case the “new” AP is the same since no other AP was stronger. The retries will continue.

Basic rates

In most networks there is only a single required basic rate like 1 or 6 Mbps. The idea is that the less requirements there are the more clients can connect. The devices will use the higher supported rates anyways, so what’s the point. However, all  unicast control frames (like acknowledgements) are sent using basic rates. Remember that in Wi-Fi all unicast frames are ack’ed.

If the basic rates are 6, 12 and 24 Mbps for example, then all control frames are sent using the highest basic rate that is lower than the current transfer rate. For example in a 150 Mbps connection the acks will be sent using 24 Mbps. In 18 Mpbs connection 12 Mbps would be used. While the acks are short there are a lot of them. It would be silly to send them at 1 or 6 Mbps. You shouldn’t add all possible rates as required basic rates since a device can’t connect if it doesn’t support all of them. 6, 12 and 24 Mbps are very commonly supported rates.

Disabling highest rates

Occasionally I have seen guest networks where the highest rates have been disabled. The thought must have been to limit the bandwidth the guest network can use. I practice it works just the other way around. The bandwidth isn’t limited, the data is just transferred more slowly. The net effect is that guest network will consume more air time and has a bigger impact on other users.

Recommendations

Old 802.11b-only devices are getting rare, yet most APs still support 802.11b just in case. If you can’t disable 802.11b altogether then you should at least disable speeds 1, 2, 5.5 and 11 Mbps in both basic and supported rates.

In 5GHz band the lowest rate is 6 Mbps. If you have a dense enough deployment you should consider setting the lowest rate to 12 Mbps. Most devices will support the change. Some devices appear to have problems if the lowest rate is 18 or 24 Mbps. Disabling 6 Mbps will hinder devices in areas with weak signal. If you have holes in the coverage or there are users outside the building, like on the rooftop or in the parking lot, disabling 6 Mbps may have a negative effect.

You should set 6, 12 and 24 Mbps as basic rates (or 12 and 24 Mpbs if you have disabled 6 Mpbs).

If you want to decrease the cell size to improve roaming you should decrease the transmit power of the APs. This will decrease the signal so the devices will roam earlier. This will also require dense AP placement so there will be a better AP avalable. In most Wi-Fi systems you can configure the minimum signal level required to associate with the network (Minimum RSSI or similar). However, setting min RSSI too high can backfire as well and cause same problems as disabling lower rates.

Do not disable higher rates. The faster the data is transferred the more air time there is to share. To limit guest network traffic there is typically a setting lige bandwidth shaping or bandwidth throttling.

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