Uplink OFDMA is not PCF
In the previous blog, we discussed DL-OFDMA and the 802.11 frame exchanges between an 802.11ax AP and multiple 802.11ax clients for multi-user downlink transmissions. In future blogs, we will discuss the necessary UL-OFDMA frame exchanges to achieve synchronized multi-user uplink transmissions. However, there is already some confusion between UL-OFDMA and some legacy RF medium access methods.
In the original 802.11 standard, the IEEE defined an optional medium access method known as Point Coordination Function (PCF). This access method was a form of polling. The AP performed the function of the point coordinator (PC). With PCF mode, the AP could poll clients for uplink transmissions during a contention-free period (CFP) of time while the AP controlled the medium. During the contention-free period, the AP polled only clients in PCF mode about their intention to send data. This was a method of prioritizing clients for single-user (SU) uplink communications.
When the IEEE ratified the 802.11e-2005 amendment to address quality-of-service (QoS) in Wi-Fi, an additional optional medium access method known as HCF Controlled Channel Access (HCCA) was defined. HCCA was wireless medium access method that used a QoS-aware centralized coordinator known as a hybrid coordinator (HC), which operated differently from the point coordinator in a PCF network. The AP functioned as the HC and had a higher priority of access to the wireless medium. Using this higher priority level, it could allocate TXOPs to itself and other stations to provide a limited-duration controlled access phase (CAP), providing contention-free transfer of QoS data. This was a method of prioritizing clients for single-user (SU) uplink transmissions for QoS data frames.
The PCF and HCCA medium access methods were never adopted by WLAN vendors and are considered obsolete. Although the IEEE defined a total of eleven data frames to use for either PCF or HCCA operation, these frames only exist on paper and are not used by Wi-Fi radios. As a matter of fact, the IEEE now considers PCF to be obsolete and will most likely remove all references to PCF in later revisions of the 802.11 standard.
As shown in Figure 1, 802.11ax introduces mechanisms where the AP can control the medium for uplink transmissions using UL-OFDMA. However, you should understand that UL-OFDMA has nothing to do with PCF or HCCA; the methods are very different. With UL-OFDMA, the standard rules of medium contention still apply and an 802.11ax AP must first contend for the medium and win a TXOP. Once the 802.11ax AP wins a TXOP, it can then coordinate uplink transmissions from 802.11ax clients. Similar to HCCA, an 802.11ax AP using UL-OFDMA can poll clients about buffered data and about the QoS category of the client data using trigger frames. However, polling it not always required because Wi-Fi 6 clients can also implicitly deliver information about their buffered data without the need for polling.
Another big difference is that the information derived from the Wi-Fi 6 clients is used to assist the AP in allocating resource units (RUs) for synchronized multi-user (MU) uplink transmissions. Additionally, UL-OFDMA uses trigger frames to instruct clients about other uplink transmission requirements such as number of spatial streams, MCS rates and client transmission power. Stay tuned for the next couple of 802.11ax blogs dedicated to UL-OFDMA operations.
Portions of this blog have been excerpted from the 5thedition of Sybex Publishing’s Certified Wireless Network Administrator (CWNA) Study Guide: http://a.co/bXX3i9F