In my last Industry Insight, I discussed how 802.11ax will provide efficient use of the RF spectrum to increase network capacity. In this article, I take a look at spatial reuse, support of 2.4GHz and other new ‘toys’ in 802.11ax.
At the outset, I talked about technology changes and use cases that are adding an increased number of devices consuming more content every year. For a network architect, this is the classic definition of a ‘high-density’ network.
The traditional way to design this network is to use 20MHz wide channels in the 5GHz band, and to limit the number of overlapping wireless cells to prevent excessive co-channel and adjacent channel interference. That works, up to a point.
All 802.11 technologies are ‘listen before talk’. This means they will measure the signal strength of other 802.11 frames on the wireless medium, and defer transmission if the signal strength is above a certain threshold. On 5GHz networks, the threshold is -82dBm RSSI. This process is called Clear Channel Assessment, and it does exactly as its name promises: it determines if the channel is clear for transmission.
As the network density increases, there is an increased chance that any two devices cannot hear each other, but each can hear a third device. If the two devices determine that the medium is ‘clear’, their transmissions may collide and an error may occur. These two devices are called ‘hidden nodes’, since they cannot hear each other and defer transmission.
In 802.11 networks, this hidden node issue is managed by using CTS (clear to send) and RTS (request to send) packets to ensure that only one device has the transmission rights for a given time slot. RTS/CTS works well at reducing medium access contention, but slows down the network throughput and capacity by adding extra management overhead to the network. Hardly a good solution to support increased numbers of wireless devices.
In 802.11ax, we now have a mechanism to mark the preamble of the wireless frame with a unique ‘colour’, called BSS Colouring, based on the BSS ID of the wireless cell. Using this technique, multiple networks can overlap.
With 802.11ac, the 2.4GHz band seemed like a long-forgotten sock lost in the wash. We kind of know it’s there, but we don’t really want to spend the time to try and use it.
A single BSS is essentially one 802.11 access point and its connected wireless clients. Each wireless client transmitting a wireless frame will include the unique identifier in the wireless preamble. The receiving device, whether an access point or another 802.11ax client, will compare the bits received in the preamble against its own BSS colour bits. If the bits match, then the packet belongs to the same BSS and medium access proceeds according to established rules. If the bits do not match, the transmitting device can ignore the frame, discard it, and medium access is not affected.
Consider how this technology can dramatically increase throughput in a high-density network like an education campus. When paired with load-balanced band steering, wireless clients can be spread amongst multiple overlapping access points for ultra-high-density along with high throughput.
With 802.11ac, the 2.4GHz band seemed like a long-forgotten sock lost in the wash. We kind of know it’s there, but we don’t really want to spend the time to try and use it. 802.11ax will include support for the 2.4GHz band and allow legacy industrial, medical and Internet of things (IOT) devices to continue operation, while also extending OFDMA advantages to newer 2.4GHz devices.
Other new toys in 802.11ax
The new standard will offer 1024 QAM to increase the number of bits transmitted in each symbol. 1024 QAM will provide a 25% speed improvement over 802.11ac. However, 1024 QAM will not work with small RU of 2, 4 or 8MHz. 1024 QAM can only be used with a full 20MHz channel. Additionally, the higher bit density will require greater single to noise ratio, between 35dB and 40dB, to achieve the higher speeds.
If you assume a noise floor of -90dB, that means the clients will have to receive the access point signal at a level of -55dB or lower. Designing a network at -50dB RSSI will require more access points – fortunately 802.11ax includes BSS Colouring to facilitate higher density and overlapping.
With unabated growth in smartphones and IOT devices, 802.11ax will include features designed to preserve battery life. The Target Wait Time (TWT) mechanism controls when and for how long a client device can be in sleep mode before it wakes up to receive packets from the access point. Combined with uplink transmission scheduling, TWT enables longer sleep times and coordinated and deterministic communications to reduce network errors and preserve battery life.
In essence, 802.11ax focuses on metrics that will improve the client experience and increase cell throughput. Network capacity is expected to increase by a factor of 4x vs existing 802.11ac networks.
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