EHT MAC Operation and Key Features
This chapter is from the book
This chapter is from the book
This chapter is from the book
extremely high throughput (EHT) basic service set (BSS): [EHT BSS] A BSS in which the transmitted Beacon frame includes an EHT Operation element.
P802.11be, Draft 5.0, Clause 3
This chapter describes MAC layer operation and other MAC feature enhancements for 802.11be, also called Extremely High Throughout (EHT). These enhancements complement those described in Chapter 6, “EHT MAC Enhancements for Multi-Link Operations,” which dealt with multi-link operation (MLO). First, we will review basic operation of EHT APs and EHT non-AP STAs within an EHT BSS. Next, we will explore some of the key MAC features and related procedures defined in 802.11be, including enhanced stream classification service (SCS) with QoS characteristics, restricted target wake time (R-TWT), triggered TXOP sharing, and Emergency Preparedness Communications Service (EPCS) priority access. The last section of this chapter describes the Wi-Fi security aspects unique to Wi-Fi 7.
EHT BSS Operation
A STA (an AP STA or a non-AP STA) that supports (mandatory and possibly optional) features defined in the 802.11be amendment is an EHT STA. Each affiliated AP of an AP MLD is an EHT AP, and each non-AP STA affiliated with a non-AP MLD is an EHT non-AP STA. For BSS operation, the AP first needs to start the BSS and start advertising the BSS’s operation parameters in beacon and probe responses. For EHT, the Station Management Entity (SME) manages starting each of the affiliated (EHT) APs of the AP MLD, through MLME primitives defined to start an AP (MLME-START.request). Calling the MLME primitive for an AP initiates the process of creating an EHT BSS for that AP. The MLME primitive provides all the operation parameters needed to start the BSS. Once the EHT BSS is created, the MAC layer notifies the SME about the result by sending an MLME-START.confirm primitive to the SME. The SME then initiates the MLME-START.request primitive for each affiliated AP of the AP MLD, which tells it to start the EHT BSS for that AP. Once started, an EHT BSS can provide connectivity for EHT non-AP STAs; it also provides connectivity for HE, VHT, and HT non-AP STAs based on the operating radio band of that EHT BSS. After the affiliated APs BSS are started, the non-AP MLD can then associate with the AP MLD using the ML setup after scanning and discovery.
Each generation of Wi-Fi defines a set of operation parameters specific to that generation in an operation element defined for that generation. An EHT AP indicates EHT-specific operation parameters for its EHT BSS in an EHT Operation element. The EHT Operation element of an AP is advertised in the Beacon and Probe Response frames and provided in the (Re)Association Response frames. In addition to the EHT Operation element, the operation of an EHT BSS is controlled by a combination of HT, VHT, and/or HE Operation elements, depending on the operating radio of the EHT AP:
If operating in 2.4 GHz, the EHT BSS operation is controlled by the EHT Operation element, the HT Operation element, and the HE Operation element.
If operating in 5 GHz, the EHT BSS operation is controlled by the HT Operation element, the VHT Operation element, the HE Operation element, and the EHT Operation element.
If operating in 6 GHz, the EHT BSS operation is controlled by the HE Operation element and the EHT Operation element.
Basic BSS Operation
An EHT AP advertises, in the EHT Operation element, the set of basic operational parameters that are required to be supported by any EHT non-AP STA for operation in that EHT BSS. Figure 7-1 shows the main set of BSS operation parameters included in the EHT Operation element.1 First, the AP advertises the Basic EHT-MCS and NSS Set field, which specifies the basic <EHT-MCS, NSS> tuples that must be supported by all EHT STAs in that BSS for both transmission and reception. Note that the 802.11be amendment defines two new EHT MCSs: MCS 12 and MCS 13 for 4K-QAM (as explained in Chapter 5, “EHT Physical Layer Enhancements”).
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FIGURE 7-1 EHT Operation Element Content
The maximum number of spatial streams (NSS) that must be supported for reception and transmission are signaled for EHT-MCS 0–7, EHT-MCS 8–9, EHT-MCS 10–11, and EHT-MCS 12–13 as shown in Figure 7-1. For example, the Rx Max NSS That Supports EHT-MCS 12–13 field signals the maximum number of spatial streams supported for MCS 12 and MCS 13 for reception, and the Tx Max NSS That Supports EHT-MCS 12–13 field signals the maximum number of spatial streams supported for MCS 12 and MCS 13 for transmission. The basic <EHT-MCS, NSS> sets must be supported for all of the mandatory bandwidths for EHT; those bandwidths include 20, 40, and 80 MHz for an EHT-STA that is not a 20 MHz-only STA. An EHT STA must not join an EHT BSS if it cannot support all of the basic <EHT-MCS, NSS> sets advertised in the EHT Operation element. So, a non-AP MLD should attempt to associate only on affiliated APs/links of an AP MLD for which it can support the basic <EHT-MCS, NSS> sets.
The EHT AP announces its BSS operating channel width in the Channel Bandwidth field in the EHT operation information if it is announcing a different bandwidth for the EHT STAs than the non-EHT STAs. If no separate EHT channel bandwidth is announced, then the operating bandwidth of EHT BSS is determined based on the HE, VHT, and/or HT operating bandwidth announced in the respective operation element, per the radio band of the EHT BSS. If an EHT channel bandwidth is announced, then the operating channel center frequency (CCF) is indicated through the CCFS0 and CCFS1 parameters.
The Disabled Subchannel Bitmap field is included when there are punctured channels. It provides a list of the subchannels (of 20 MHz width) that are punctured in the BSS operating bandwidth (see the “Preamble Puncturing” section later in this chapter). MCS 15 is a longer-range MCS mode that was introduced in 802.11be, but is not certified in Wi-Fi 7. The MCS 15 Disable field indicates whether the AP has disabled or enabled the reception of the EHT PPDU with EHT MCS 15 in both the Data field and the EHT-SIG field.
A closely related element is the EHT Capabilities element.2 This element defines the capabilities of an EHT STA rather than those of the EHT BSS. The EHT Capabilities element is transmitted by both the AP and the non-AP EHT STA. A non-AP STA declares that it is an EHT STA by transmitting the EHT Capabilities element in the (Re)Association Request frame. An AP includes the EHT Capabilities element in the Beacon, Probe Response, and (Re)Association Response frames. The EHT Capabilities element advertises the set of EHT capabilities supported by the EHT STA, including the EHT MAC capabilities, the EHT PHY capabilities (covered in Chapter 5), the Supported EHT MCS and NSS Set, and optionally the EHT PPE thresholds.
The set of supported EHT MAC features is identified in the EHT MAC Capabilities Information field by both the AP and the non-AP STA, as shown in Figure 7-2. Capabilities related to new EHT MAC features include SCS traffic description support, restricted TWT support, triggered TXOP sharing feature-related capabilities, and capabilities related to EPCS priority access. These MAC features are described later in this chapter. The maximum MPDU length that can be supported for 2.4 GHz is indicated by the Maximum MPDU Length field. For the 5 GHz and 6 GHz bands, the maximum MPDU length supported is the same as that indicated in the VHT Capabilities and HE Capabilities elements, respectively. Some other capabilities are signaled in fields shown under the miscellaneous category in Figure 7-2; the names of these fields are self-explanatory (OM = operating mode, TRS = triggered response scheduling, BQR = bandwidth query report).
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FIGURE 7-2 EHT MAC Capabilities
Besides the basic set of <EHT-MCS, NSS> tuples that must be supported by all the non-AP STAs connecting to an EHT BSS, an EHT STA can support other combinations of <EHT-MCS, NSS> at different bandwidths. This capability is indicated by the Supported EHT-MCS and NSS Set field in the EHT Capabilities element. As shown in Figure 7-3, the Supported EHT-MCS and NSS Set field indicates the supported combination of <EHT-MCS, NSS> at different PPDU bandwidths: 20 MHz only for the 20 MHz-only STA, and ≤ 80 MHz, 160 MHz, and 320 MHz for reception (Rx) and transmission (Tx). For each channel width, the EHT STA indicates support for a maximum NSS for different EHT MCSs for Rx and Tx in the corresponding EHT MCS Map field for that channel width. For example, for 80 MHz, the STA indicates support for its maximum NSS for MCS 12 and MCS 13 for reception in the Rx Max NSS That Supports EHT-MCS 12–13 field. Similarly, it indicates support for its maximum NSS for MCS 12 and MCS 13 for transmission in the Tx Max NSS That Supports EHT-MCS 12–13 field. Both are included in the EHT-MCS Map (BW <= 80 MHz) field. A 20 MHz-only STA supports the basic <EHT-MCS, NSS> set. The EHT PPE Thresholds field provides the nominal packet padding for an EHT PPDU (see Chapter 5 for details).
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FIGURE 7-3 Supported EHT-MCS and NSS Set in EHT Capabilities Element
In addition to signaling the operation parameters and capabilities, an EHT STA supports operating mode updates to dynamically update its operating mode parameters. To do so, it uses the Operating Mode Notification (OMN) or Operating Mode Indication (OMI) mechanism; these mechanisms are described in the next section, “Operating Mode Updates.”
An EHT STA determines the maximum receive NSS for an EHT-MCS that can be supported by a peer EHT STA (corresponding to different PPDU bandwidths) as the smaller of the values indicated in the Supported EHT-MCS and NSS Set and the maximum receive NSS indicated through the operating mode update procedure3 by that STA. An EHT STA uses the maximum Rx NSS determined for a peer STA and its own maximum Tx NSS to determine the maximum NSS to be used for transmitting EHT PPDUs to that STA. Similarly, an EHT STA determines the maximum transmit NSS for an EHT-MCS that can be supported by a peer STA as the smaller of the values indicated in the Supported EHT-MCS and NSS Set and the maximum transmit NSS (Tx NSTS) indicated through the OMI update (see the “Operating Mode Updates” section).
Operating Mode Updates
Sometimes, an AP or a non-AP STA may change its receive and/or transmit NSS and operating bandwidth as a means of saving power or improving performance. The AP may also update its BSS operating bandwidth for better performance. Such use cases are addressed by enabling dynamic updates to operating mode parameters.
The 802.11ac/VHT amendment introduced the operating mode notification (OMN) mechanism, whereby a STA (either an AP or non-AP STA) can notify another STA of changes in its operating mode (maximum NSS and channel width) by transmitting an Operating Mode Notification frame. The Operating Mode field included in the frame indicates the maximum number of spatial streams that the STA can receive (Rx NSS) and its supported channel widths (20, 40, 80, 160, or 80 + 80 MHz). An AP can notify another STA of changes in its maximum Rx NSS by sending either a broadcast or an individually addressed Operating Mode Notification frame, or alternatively by including an Operating Mode Notification element (which includes the Operating Mode field) in the Beacon frames for a period. When changing its operating bandwidth, an AP indicates the new operating channel width in the operation elements included in the Beacon and Probe Response frames. The OMN feature adds the flexibility to dynamically update the operating mode parameters for both the AP and the non-AP STA, and also applies to HE STAs and EHT STAs. However, this mechanism has a notable limitation: The scope of operating mode changes is limited to receive mode only.
The 802.11ax/HE added another, faster mechanism for indicating operating mode updates called the operating mode indication (OMI). OMI supports updates to both receive and transmit operating mode parameters. This mechanism enables an OMI initiator (either an AP or a non-AP STA) to signal a change in its receive operating mode and/or its transmit operating mode, in-band within a data or management frame. Specifically, an OM Control field is included in the A-Control field in the HT Control field in the MAC header of that frame. The OM Control field indicates the channel width and maximum number of NSS that the OMI initiator supports for receiving and transmitting PPDUs. The changes to operating mode parameters for Rx and/or Tx apply only after the TXOP in which the acknowledgment from the OMI responder is received for the frame carrying the OM Control subfield. A non-AP STA can also use the OM Control field to dynamically update its transmit operation to single-user (SU) versus multiuser (MU) UL OFDMA operation. A STA can set the UL MU Disable field and/or UL MU Data Disable field in the OM Control to signal that it has suspended trigger-based UL MU transmissions and will not respond to a trigger frame, and can set these fields to 0 to indicate that UL MU transmissions are enabled by the STA.
In EHT, the OMI mechanism is extended to add support for signaling the 320 MHz channel width as well as support for dynamic updates for the operating mode. EHT adds a new EHT OM Control field that works in conjunction with the HE OM Control field to indicate an EHT STA’s channel width plus its receive and transmit NSS.
Preamble Puncturing
As you learned in Chapter 3, “Building on the Wi-Fi 6 Revolution,” and Chapter 5, “EHT Physical Layer Enhancements,” the preamble puncturing feature added in 802.11ax enables more efficient utilization of available channel bandwidth when an AP is operating on a wider bandwidth and interference (e.g., incumbents) is present in one or more 20 MHz channel blocks within the AP’s operating bandwidth. This scenario is particularly applicable to the 6 GHz spectrum, where there is good chance that incumbents will be present. Such incumbents may include providers of primary services being offered on that channel and typically have narrowband signals (e.g., 10 MHz wide). As described in Chapter 3, the automated frequency control (AFC) system identifies channel widths where incumbent systems are in use. The AP can use this information to determine which channels should be punctured in a wider channel bandwidth. Other sources of constant interference could include unmanaged Wi-Fi networks or unmanaged 5G (NR-U) systems, and their presence could feed into the determination of which channels will be punctured from a BSS with a wide operating channel bandwidth.
EHT STAs puncture channels by skipping preamble transmission on those channels in DL and UL transmissions (hence the name preamble puncturing). Moreover, no data RUs are transmitted on the punctured channels. The 802.11ax amendment and Wi-Fi 6 enabled preamble puncturing only for OFDMA transmissions. In EHT, preamble puncturing is supported for both OFDMA and non-OFDMA transmissions, and support for puncturing is mandatory for non-OFDMA SU transmissions. This approach enables improved channel efficiency for both SU and MU transmissions when wider channel widths are deployed in an EHT BSS. In addition, puncturing is supported for the EHT sounding procedure.
In EHT, puncturing is supported for 80 MHz and higher-bandwidth PPDUs (20 and 40 MHz PPDUs cannot be punctured). For an 80 MHz PPDU, only 20 MHz is allowed to be punctured. For higher bandwidths PPDUs, larger channel bandwidths can be punctured (40, 80, 40 + 80 MHz). Table 7-1 summarizes the allowed puncturing bandwidths for different operating bandwidths of an EHT BSS. Figure 7-4 provides examples of preamble puncturing for each of the bandwidths (80, 160, and 320 MHz). Refer to Chapter 5 for details on all possible preamble puncturing patterns and large MRU allocations for different patterns.
TABLE 7-1 Allowed Preamble Puncturing Bandwidths
BSS Bandwidth |
Puncturing Bandwidth Allowed |
|---|---|
20 and 40 MHz |
Puncturing not allowed |
80 MHz |
20 MHz |
160 MHz |
20 or 40 MHz |
320 MHz |
40, 80, or 40 + 80 MHz |
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FIGURE 7-4 Preamble Puncturing Examples for Different BSS Bandwidths
An EHT AP signals the punctured channels of the BSS in the EHT Operation element. The AP indicates the set of 20 MHz subchannels that are punctured in the Disabled Subchannel Bitmap field in the EHT Operation element, which the AP transmits in Beacon and Probe Responses. The EHT STAs in the BSS do not use the set of punctured 20 MHz channels advertised by the AP for any PPDU transmissions within the BSS. In some scenarios, an EHT STA may also puncture other subchannels, besides the subchannels indicated in the Disabled Subchannel Bitmap, in an EHT MU PPDU or a non-HT duplicate PPDU as per the rules defined in the 802.11be amendment.4 An EHT AP updates the advertised puncturing pattern in the EHT Operation element if the BSS’s set of punctured subchannels changes.
EHT Sounding
As described in the “Sounding Procedure” section in Chapter 3, transmit beamforming and DL MU-MIMO operations require the AP to have knowledge of the channel state, so that it can organize its downstream transmissions and optimize the signal for each receiver. To that end, sounding allows a beamformer (typically the AP) to send a sounding PPDU—a null data packet (NDP)—to one or more beamformees (typically a STA or a group of STAs). The receiving STAs respond with compressed steering matrices that indicate the state of the channel for a subset of the tones. (The “Sounding and Beamforming” section in Chapter 5 provides PHY-related details for the sounding exchange and the returned steering matrix.)
The goals of sounding are the same in 802.11be as in 802.11ax, with the addition of two use cases:
Sounding for 320 MHz bandwidths (in 6 GHz).
Sounding for partial bandwidth. This provision is necessary to allow for sounding within a puncturing context, where one or more 20 MHz subchannels have to be bypassed.
Figure 7-5 depicts the format of the EHT sounding PPDU.
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FIGURE 7-5 Format of the EHT Sounding NDP
The general structure of the EHT sounding NDP is similar to that of the previous generation’s (802.11ax) sounding NDP, including the L-STF, L-LTF, L-SIG, and RL-SIG fields; the PE field; and a variable number of protocol-specific LTFs. However, whereas 802.11ax included the HE-SIG-A, HE-STF, and HE-LTFs fields, 802.11be includes the U-SIG, EHT-SIG, EHT-STF, and EHT-LTFs fields:
The U-SIG field indicates that the NDP is EHT (802.11be) and identifies the NDP bandwidth. 802.11ax allowed up to 160 MHz transmissions, and 802.11be adds the 320 MHz option for the 6 GHz band. The U-SIG field also carries a Punctured channel Indication field, which indicates if some of the RUs are punctured. The field includes a code (a number between 0 and 24, depending on the NDP bandwidth5) that represents which 20 MHz, 40 MHz, and/or 80 MHz RU(s) or MRU(s) is/are punctured (when applicable). There is no puncturing for 20 MHz or 40 MHz transmissions. The 80 MHz bandwidth allows for one 20 MHz puncture; a code from 1 to 4 indicates which 20 MHz segment is punctured. The 160 MHz bandwidth allows for 20 MHz or 40 MHz punctures, so in this case the codes range from 1 to 12 (one of 8 possible 20 MHz segments, or one of 4 possible 40 MHz segments). The 320 MHz bandwidth allows for 40 MHz, 80 MHz, or concurrent 80 MHz and 40 MHz puncturing (24 possibilities).6
The EHT-SIG field contains only a Common Field, which indicates if spatial reuse modes are allowed for this transmission, and identifies the number of spatial streams in effect (if spatial reuse is allowed). The Common field also indicates if the NDP is beamformed, and specifies the number of EHT-LTF symbols included in the frame.
The EHT-STF has the same role as the STF in previous generations, albeit this time in an 802.11be transmission context: It is intended to improve the automatic gain control estimation in a MIMO transmission.
Just like 802.11ax, 802.11be allows for trigger-based and non-trigger-based sounding, for a group of STAs or a single STA, respectively. These two modes in 802.11be follow the same choreography and message exchange as in 802.11ax (illustrated in Figure 3-5 in Chapter 3). Just as in 802.11ax, the STAs’ response can be single-user (SU) feedback, multi-user (MU) feedback, or channel quality indication (CQI) feedback.
The 802.11be amendment includes other possibilities for EHT sounding. In particular, EMLSR mode allows a client device to implement a single full-capability radio, and one or more low-capability radios (see the “Enhanced Multi-Link Single Radio” section in Chapter 6, “EHT MAC Enhancements for Multi-Link Operations,” for more details). When a STA receives an initial control frame on one of its links, the STA in EMLSR mode switches its full radio to that link and operates there using all antennas. Therefore, before starting the sounding procedure with STAs operating in EMLSR mode, the AP sends a MU-RTS frame to ensure that the STA is operating in its full capability mode. The AP then sends the EHT NPDA and EHT sounding NDP.
The upper part of Figure 7-6 illustrates this scenario, for non-triggered sounding, where the compressed beamforming/CQI is sent directly after the EHT sounding NDP. Naturally, there is an alternative where multiple non-EMLSR STAs can participate in the conversation. In this case, the MU-RTS is first sent to the STAs in EMLSR mode. Each of the STAs then switches its main radio to that channel and replies with a CTS. Next, the AP sends the subsequent EHT NDPA frame, then the EHT sounding NDP, to the group. At that point, the STAs are triggered and reply with their individual EHT compressed beamforming/CQI information.
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FIGURE 7-6 Examples of Sounding with EMLSR Operation
Many other sounding scenarios are possible with EMLSR operations. The main requirement is the presence of an initial control frame to ensure that the EMLSR STAs are operating in full-capability mode before they receive the EHT NDPA frame. At the bottom of Figure 7-6, you can see an example where n STAs are in the EMLSR mode, and other STAs (n + 1 to x) are not in the EMLSR mode. The initial control frame is a BSRP trigger frame; this frame brings clients 1 to n to operate with full-capability radio on the channel. It is followed by the corresponding PPDUs transmission by those STAs. This transmission is then followed by the EHT NDPA. The AP can also group the STAs (e.g., STAs in EMLSR mode in one group and non-EMLSR STAs in another) when it triggers the group with the BFRP Trigger frame to retrieve the STAs sounding matrices, as shown in Figure 7-6.