This is the first of two blogs where I discuss future channel reuse design concerns in the exciting world of 6 GHz Wi-Fi. The picture below is a preview of where I am headed with this discussion. But first I need to lay some groundwork with a discussion of medium contention.
Because of the half-duplex nature of the radio frequency (RF) medium, it is necessary to ensure that at any given time, only one Wi-Fi radio has control of the medium on a particular channel.
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a wireless medium access method that utilizes multiple checks and balances so Wi-Fi radios can effectively share the RF medium. One of these CSMA/CA checks is a physical carrier sense mechanism to determine if the RF medium is busy.
Physical carrier sense is performed constantly by all Wi-Fi radios that are not transmitting or receiving. When a radio performs a physical carrier sense, it listens to the channel to see whether any other RF transmissions are occupying it.
Physical carrier sense has two purposes:
Wi-Fi radios use a clear channel assessment (CCA) to appraise the RF medium to achieve these two physical carrier sense goals. The CCA involves listening for RF transmissions at the Physical layer. Wi-Fi radios use two separate CCA thresholds when listening to the RF medium. As shown in Figure 1, the signal detect (SD) threshold identifies any 802.11 preamble transmissions from another transmitting 802.11 radio. The preamble is a component of the Physical layer header of 802.11 frame transmissions. The preamble is used for synchronization between transmitting and receiving 802.11 radios. The SD threshold is sometimes referred to as the preamble carrier sense threshold. The signal detect (SD) threshold is statistically 4 dB signal-to-noise ratio (SNR) for most Wi-Fi radios to detect and decode an 802.11 preamble. In other words, a Wi-Fi radio can usually decode any incoming 802.11 preamble transmissions at a received signal of about 4 dB above the noise floor.
Figure 1 – CCA thresholds
The energy detect (ED) threshold is used to detect any other type of RF transmissions during the clear channel assessment (CCA). The 2.4 GHz and 5 GHz bands are license-free bands, and other non-802.11 RF transmissions may occupy a channel. As shown in Figure 1, the ED threshold is 20 dB higher than the signal detect threshold. For example, if the noise floor of channel 36 were at –95 dBm, the SD threshold for detecting 802.11 transmissions would be around –91 dBm, and the ED threshold for detecting other RF transmissions would be –71 dBm. If the noise floor of channel 40 were at –100 dBm, the SD threshold for detecting 802.11 transmissions would be around –96 dBm, and the ED threshold for detecting other RF transmissions would be –76 dBm.
The definition of both CCA thresholds is somewhat vague in the 802.11-2020 standard, which has often resulted in a misunderstanding of the actual threshold values. The interpretation of these thresholds by Wi-Fi manufacturers of 802.11 client and AP radios will usually differ. To complicate matters further, please remember that the receive sensitivity capabilities between radios can vary widely. Because of the difference in receive sensitivity, the perception of the noise floor can be quite different between Wi-Fi radios. Therefore, the two CCA thresholds may also vary due to differences in radio receive sensitivity.
This gets even more complex when discussing CCA thresholds when channel bonding is used. Channel bonding has first introduced with 802.11n (Wi-Fi 4). Two 20 MHz channels can be bonded together to double the frequency bandwidth to create a 40 MHz channel. Each 40 MHz channel consists of a primary and secondary 20 MHz channel. The bonded primary and secondary channels are used together only for any data frame transmissions between capable APs and clients. All 802.11 management and control frames are transmitted only on the primary channel for backward compatibility. Additionally, only the primary channel is used for data transmissions between an AP and legacy 802.11a/b/g clients. Although possible in 2.4 GHz, channel bonding has been used only in the 5 GHz frequency band in the enterprise.
Figure 2 – 802.11n primary/secondary CCA thresholds
As shown in Figure 2, the CCA thresholds for the primary channel remain consistent, however, for the secondary channel, there is no SD threshold, and the ED threshold is -62 dBm.
Back in the day, 802.11n APs were configurable for channel offsets for 40 MHz channels. A positive or negative offset indicates whether the secondary channel is above or below the primary channel. This configuration capability often led to nearby APs with a mismatched primary channel, as shown in Figure 3.
Figure 3 – Mismatched primary and secondary channels
This mismatch creates a big problem. Because there is no signal detect (SD) threshold on the secondary channel, there is no detection of the 802.11 preamble in the secondary channel.
In the example in Figure 3, any AP or client configured with channel 40 as the secondary will not hear any Wi-Fi device transmitting using channel 40 as the primary. Remember that the primary channel is always busier because that is where management and control traffic traverses.
The mismatched primary and secondary channels step on each other and result in collisions and data corruption. Throughput and performance are negatively impacted.
The technical term for this problem is sometimes referred to as primary/secondary overlapping basic service set (OBSS) interference.
Eventually, the secondary channel CCA thresholds were enhanced when 802.11ac (Wi-Fi 5) was introduced. As shown in Figure 4, the CCA thresholds for the primary channel remain consistent, however, for the secondary channel, a static SD threshold of -72 dBm is now defined, and the ED threshold remains at -62 dBm. These secondary thresholds are also used for 802.11ax (Wi-Fi 6). The secondary thresholds also apply to the multiple secondary channels used for 80 MHz and 160 MHz channels.
Figure 4 – 802.11ac/ax primary and secondary CCA thresholds
802.11ac also removed the channel offsets and instead referenced the center frequency of any bonded channel. However, WLAN vendors often do not reference the center frequency when configuring bonded channels on 802.11ac/ax access points. Instead, a 20 MHz channel number is selected, and that 20 MHz channel functions as the primary channel.
Once again, mismatches between primary and secondary channels occur depending on the channel selection. Although the SD threshold of -72 dBm is an improvement, it is often too high to detect the 802.11 preamble properly. So once again, the mismatched primary and secondary channels step on each other and result in collisions and data corruption. Primary/secondary OBSS can still negatively impact throughput and performance.
So how do you solve this problem? Bottom line, when configuring channel reuse patterns in the 5 GHz frequency band, ensure that you have a consistent selection of the primary channels.
Over time, most enterprise WLAN vendors realized these problems and guided administrators through configuration workflows to prevent a mismatch. But not always. Worse yet, even some vendors’ radio resource management (RRM) protocols have been known to create mismatched primary/secondary channels.
So what does all this have to do with 6 GHz and Wi-Fi 6E? The 6 GHz frequency band is the future for Wi-Fi for the next ten years and beyond. Can primary/secondary OBSS interference problems still occur in 6 GHz? My follow-up blog will discuss several scenarios and possible solutions when designing 6 GHz channel reuse patterns.
