WIRELESS INFRASTRUCTURE

  • 08/01/2016
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4 WiFi Band Steering Myths

Find out common misconceptions about the band steering technique used in dual-band WLANs.

Band steering is a technique used in dual-band WiFi deployments to encourage dual-band client devices, such as most modern smartphones, tablets, laptops, and PCs, to use the less-congested and higher capacity 5 GHz band. This is usually advantageous because 5 GHz tends to be better for WiFi performance as it has more and wider channels available and tends to have less overall interference from both neighboring WiFi and non-WiFi sources.

As with many modern WiFi features, however, band steering is often overhyped. Consequently, it's important to understand the conditions under which band steering can and should be used, and where it may cause more harm than good. Here are four common misconceptions about band steering and some advice on best practices.

1: Band steering is standard

Band steering has never actually been part of the IEEE 802.11 specification. In the early days of 802.11n, engineers at Aruba Networks came up with the idea that it would be good to steer dual-band clients to the 5 GHz band, and most other vendors ultimately copied the idea in various forms. As a result, there is no “standard” for band steering, and each vendor implements band steering differently, with some vendors doing it better than others.

While no vendor actually publishes how its band steering algorithm works in detail, the generic method is to identify dual-band client devices from their probe requests and then preferentially respond to them only on the 5 GHz band, so that clients do not see the 2.4 GHz network and connect to the 5 GHz network. Since client devices use the same MAC address on both the 2.4 GHz and 5 GHz bands, it's fairly easy for an access point to identify dual-band capable client devices.

2: Band steering is activated automatically

When using band steering, all of the parameters for the SSID must match on both bands, including the SSID name, security type and settings, VLAN assignments, and so forth. For some vendors, band steering is automatically enabled if you define the SSID and indicate it should be used on both bands. For other vendors, band steering needs to be explicitly enabled.  

For those vendors that require band steering to be explicitly enabled, you can actually have performance issues if you make your SSIDs match on both bands but do not enable band steering. In this case, the client device sees the 2.4 GHz SSID and the 5 GHz SSID as two separate access points, and can actually “roam” between the 2.4 GHz SSID and the 5 GHz SSID on the same access point, causing unnecessary roaming events and disruptions in service.

Best practice: When using band steering, your SSIDs and all of their parameters must match identically. When not using band steering, your SSIDs must be differentiated, for example, “My WiFi 2.4G” and “My WiFi 5G.”

3. Band steering should always be used

In order to function properly, band steering generally assumes that the coverage areas on both the 2.4 GHz bands and 5 GHz bands are the same, or at least roughly equivalent. However, band steering will prove problematic if coverage on 5 GHz is significantly weaker and has coverage holes, as compared to coverage for 2.4 GHz. While dual-band access points have been around for five to six years, many modern networks are still designed from a 2.4 GHz mentality, and do not account for the fact that 5 GHz signals have significantly higher attenuation than a 2.4 GHz signal from the same access point at the same power level. 

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WiFi
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(Image: iSergey/iStockphoto)

Proper 5 GHz coverage designs usually require comparatively more access points and a 6 dB power offset between transmit power at 5 GHz vs. power at 2.4 GHz, so as to roughly equalize the coverage area on each band. Coverage equivalence will highly depend, however, on the building structure, layout, and materials. The key message is that it's all too easy to deploy a WiFi network that has too much 2.4 GHz coverage and too little 5 GHz coverage simultaneously.

Best practice: Design your networks for simultaneous 5 GHz and 2.4 GHz coverage. For existing deployments where this may not be feasible and your coverage is quite different on both bands, avoid using band steering.

4. Band steering is just a checkbox

Band steering is more than just an either/or proposition; the functionality varies widely across vendors. Some vendors currently offer multiple variations and tunable parameters for band steering, so that you can adjust the band steering behavior to suit your network needs.  

Some AP vendors offer multiple modes of band steering when it's explicitly enabled. While different vendors use different terminology, these modes generally can be categorized as follows:

  • Force 5 GHz connections:  With force 5 GHz, a dual-band client device will only be allowed to connect to the network on the 5 GHz band, and any requests to connect on the 2.4 GHz band will be ignored. This mode works quite well when the signal strength is good on the 5 GHz band, but will prove problematic if there are weak coverage areas on 5 GHz because the network will not allow the client device to “fall back” to the 2.4 GHz network.
  • Prefer 5 GHz connections:  This mode involves specifying a threshold received signal strength indicator (RSSI) such that so long as the client device has an RSSI value above the threshold, the access point will preferentially encourage the client device to connect on the 5 GHz band. When the 5 GHz RSSI dips below the threshold, the client device will be allowed on the 2.4 GHz band. The challenge here is selecting an appropriate RSSI threshold. Set the threshold too high, and devices that really should be connecting on the 5 GHz band are actually connecting at 2.4 GHz. Set the threshold too low, and devices may be getting relatively poor performance on the 5 GHz band when they would get better performance on the 2.4 GHz band.
  • Load-balance connections:  In environments with a high density of client devices, it may be advantageous to balance connections between the 2.4 GHz and 5 GHz bands.  As an example, take a classroom with 30 student iPads. In a normal load-balancing scenario, all 30 devices would be connected on the 5 GHz radio, leaving the 2.4 GHz radio (and the band) idle.  One would get better airtime utilization from the AP by purposely shifting some proportion of the clients to the 2.4 GHz band. Such a mode usually allows the specification of a load-balancing threshold and/or percentage of clients to allow on the 5 GHz band vs. the 2.4 GHz band. 

Best practice: If there are tunable options for band steering available, then the safest option is to select prefer 5 GHz with a relatively low threshold, such as -80 dBm. This will make client devices connect normally on the 5 GHz band, but will allow them to connect on 2.4 GHz in areas where 5 GHz coverage is weak but 2.4 GHz coverage is still available. 

In environments with a high density of 5 GHz clients and few single-band 2.4 GHz clients, taking advantage of load-balancing features makes sense to utilize the 2.4 GHz airtime for some of the client devices, as the 2.4 GHz band would otherwise be unused while the 5 GHz band is crowded.


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