Extending Network Capacity in Enterprise WLANs with 802.11ax

The new standard includes a feature enabling more simultaneous transmissions, known as ‘spatial reuse’ or ‘BSS coloring.’

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When building networks in the ‘real world’ like city centers, stadiums, apartment buildings, and even office buildings, we frequently come across situations where many access points, installed independently or managed as one network, create overlapping coverage areas. When these access points choose to use the same channel, the performance of all users in such an area is reduced, as the Wi-Fi algorithm used to avoid collisions on the air is quite conservative.

One focus of the next Wi-Fi standard, 802.11ax is to improve the performance of ‘real-world’ networks. To this end, the new standard includes a feature enabling more simultaneous transmissions. This feature is known as ‘spatial reuse’ or ‘BSS coloring.’

BSS coloring identifies when two or more nearby (but not adjacent) access points or devices are able to safely transmit at the same time, without causing mutual interference. These transmissions would have been disallowed by the channel access protocols used prior to 802.11ax, but are now allowed.

The ‘Old’ Power-Detect Rules in 802.11

Since the very beginning with 802.11b, Wi-Fi has used CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to control access to the air. CSMA/CA is a set of rules, implemented identically on all devices to ensure they all have equal access. It requires that when a device (client or access point) hears a Wi-Fi frame on the air in its RF channel above the defined ‘power-detect’ threshold, it must wait for that frame to finish before transmitting its own frame.

The algorithm is quite complex, but at its core is the power-detect threshold, part of the CCA (Clear-Channel Assessment) algorithm. It is designed to ensure that two nearby devices do not transmit frames simultaneously, as that would result in interference, and one or both of the frames would be received in error.

But the relatively low power-detect threshold defined up to 802.11ax, while effective in preventing frame collisions on the air, reduces network performance by more than it needs to. Consider an office building, perhaps 60 - 80 meters square. An enterprise-class WLAN might use 16 access points to cover a floor, in order to service a dense client population at high data-rates. But, with a limited number of available 40 or 80 MHz channels, some of the access points would need to be on the same channel. Given the conservative power-detect threshold, any access points (or their clients) on the same channel would be audible to the others. In such a channel, only one client or access point could transmit at a time, even if they were in opposite corners of the building.

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This was a safe arrangement, because it is important to avoid excessive collisions and retries. But it limited performance to such an extent that, beyond a certain point, adding access points would no longer increase network capacity; and this was frustrating because we know it is possible to do better.

Better Spatial Reuse with BSS Coloring

Traditional Wi-Fi uses a single CCA power-detect value based on the power level of a packet on the air to determine if it is ‘clear.’ 802.11ax allows for two values, one for access points and their clients that are nearby (currently -82 dBm) and others for more distant cells (probably -62 dBm), the idea being that distant cells will not be affected by interference from a local transmission.

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The definition of a nearby access point is one with a different ‘color’ tag in its and its clients’ transmissions. The tag is added to the 802.11ax preamble: 6 bits are allocated for 63 possible values. When a network is colored correctly, cells at different ends of a building – which would previously have deferred to each other’s CCA, so only one could transmit at any time – will be able to transmit simultaneously, without incurring higher retransmissions and error rates.

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The change in power-detect threshold is linked to transmit power control, ensuring that the power used is just sufficient to reach the intended receiver, and no more. And it may be linked to RTS-CTS (Request-to-Send, Clear-to-Send) to clear the channel for the transmission: but these are details, the significant improvement is due to dynamic power-detect thresholds.

Opportunities for AP Control Software

The new feature offers yet another opportunity for enterprise access point vendors to devise new algorithms for WLAN control, improving network capacity under different conditions. The key question with BSS coloring is how to allocate colors across a WLAN, a decision at the discretion of the access point: access points can even change the color plan dynamically if they detect neighboring or overlapping networks have changed.

Along with Multi-user MIMO, and OFDMA (discussed in previous blogs) the access point now has extensive control over network behavior, for both downlink and uplink transmissions.

We expect BSS coloring to offer a significant performance improvement in dense Wi-Fi areas, including enterprise deployments.

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