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802.11ax introduces "massively parallel" technologies, especially OFDMA, multi-user MIMO and "BSS coloring". All of these require access points to make control decisions that have a significant impact on network performance, which is a new focus of Wi-Fi, which will bring access point networks closer to cellular base station infrastructure in terms of function.

What is OFDMA?

OFDMA (orthogonal Frequency Division multiple access) allows a single transmission to AP or a single transmission from AP to include data to multiple clients: for downlink, the access point simultaneously uses subcarrier groups within the RF channel, whereas previously it will have the ability to send data in a continuous series of packets to obtain the entire RF channel bandwidth. In the uplink direction, several clients send at the same time, and their signals reach access points on different subcarriers, so they can receive them in parallel. Reducing header and contention overhead can improve efficiency, and small packet throughput is significantly better, but the real magic of OFDMA lies in its strong control over a single transport.

Wi-Fi transmission is a complex interaction among many parameters, including transmission bit rate, transmission power, reception sensitivity and noise level, channel bandwidth and so on. OFDMA provides each transport and each customer with the opportunity to optimize these parameters and then group them in different combinations for best efficiency and performance.

First, the access point can choose which packets to transmit immediately, which are not time-sensitive, and can be sent later (this is only needed when network traffic is close to capacity). Using 802.11ax, it can view its packet buffer and select a packet group for each transmission-for example, it may choose to use many small subchannels to group many short packets into a single transmission, while longer packets can get the opportunity to transmit themselves, taking up the entire channel bandwidth.

The access point also selects the bit rate used for each transmission for each client. It can use a high bit rate in the shortest transmission time, or slightly reduce the rate to reduce the possibility of errors and retransmissions, which may be a more efficient option in some cases. For example, it might think that 256-QAM with an error rate of 2% is better than 1024-QAM, with an error rate of 15%.

In addition to errors and retransmission rates, OFDMA allows access points to control latency and jitter. Prior to 802.11ax, clients that generated small packets at frequent intervals had to compete for the transmission opportunity of each packet, and if other clients in the network sent long packets, they had to delay the transmission, resulting in packet buffering and jitter. With 802.11ax, the access point can allocate frequent short transmission opportunities, so it can send and receive packets without buffering them. For example, a traditional IP voice connection produces a packet of ~ 160 bytes every 20 milliseconds, so the access point can ensure that it arranges periodic transmission opportunities for this traffic.

In 802.11ax multiuser mode, AP controls uplink transmission

In 802.11ax OFDMA, access points allocate client traffic to subchannels, not only for downlinks but also for uplinks.

The new "trigger frame" mechanism allows access points to poll clients to discover the traffic they want to send on the uplink. When it collects the trigger frame response, it designs a plan and sends it to the client in another trigger frame. Then, the client builds the frame according to its instructions, sets the data rate, transmits the level and subchannel, and sends the data frame back to the access point.

Multi-user MIMO improves Spectrum efficiency

Another multi-user mechanism in 802.11ax is multi-user MIMO. It uses the same trigger frame control protocol as OFDMA and improves 802.11ac.

Multiuser MIMO itself is a complex protocol that requires probing packets to determine multipath conditions and packet MIMO clients, all of which are under the control of the access point.

At any time, the access point can choose to use traditional single-user transmission or multiple users using OFDMA or MIMO. This opens up a new dimension of traffic management.

For example, delay and jitter can be strictly controlled. Previously, customers had to compete for transport opportunities with relatively unknown and uncontrollable delays that might be limited by the WMM (Wireless Multimedia) QoS priority mechanism but not precisely defined. Using 802.11ax multiuser control, frequent and predictable transmission slots can be provided for delay-sensitive traffic to accurately control jitter and delay. Because OFDMA provides access point control modulation rate and transmit power on uplink and downlink, it has sufficient control to define QoS parameters for each traffic flow.

In fact, 802.11ax access points can enforce a structure similar to TDM (time Division Multiplexing) in communication with clients, which can control the timing and parameters of each transmission, uplink and downlink. When the traffic pattern is known and changes slowly, this is the most efficient way to achieve spectral efficiency: the amount of data carried in a given RF channel width.

MU-MIMO is more efficient for large packets such as large file transfers, while OFDMA is a better choice for shorter packets such as IM or e-mail. Deciding which mode to use-single user, OFDMA or MU-MIMO-depends on the access point.

Use BSS shading to better reuse space

Another area where access point control becomes more powerful in 802.11ax is the participation of many centrally managed or separate access points in "spatial reuse", often referred to as "BSS coloring". We will delve into this important feature in future blogs.

As a result, 802.11ax introduces several new features in which access points can control network behavior, and access point vendors have the opportunity to increase complexity and improve performance. Multi-user flow control in uplinks and downlinks, as well as new OFDMA features and enhancements to multi-user MIMO, provide more opportunities for deterministic QoS, while spatial reuse (a potentially powerful feature) will depend on its performance on how access points determine how color tags are applied.

With the introduction of 802.11ax, access point vendors are expected to celebrate the power of their scheduling algorithms and flow control capabilities.

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