An access point (AP) can create two or more networks on the same Wi-Fi radio. This capability, known as Virtual AP (VAP), makes a single physical AP appear as if it were multiple APs. It allows APs to create separate wireless networks to accommodate different user and security requirements. 

A typical example is a “guest” network that runs on the same AP as the home or business network. In this case, the AP advertises two SSIDs over the same channel, one for the guest network and one for the home or business network. Creating multiple networks on a single AP is very common, but comes with a price tag: beacon overhead.

Beacon overhead

An AP announces a wireless network by transmitting a beacon frame (about) ten times per second (using the default beacon interval). The AP will transmit multiple beacon frames for multiple networks, which means that as the number of networks (SSIDs) increases, the airtime consumed by beacons also increases (Who hasn’t seen Andrew von Nagy’s SSID Overhead Calculator? Check out the iOS app, too.)

The beacon size and the data rates used to send management frames also impact the beacon overhead. As networks become more and more complex, beacon frames grow in size. Also, by design, management frames are usually transmitted at lower data rates, which means beacon frames will consume more airtime than other frames of similar size sent at higher data rates.

Let’s look at the following example using WiFi Explorer Pro 3’s Utilization Inspector. The Utilization Inspector displays the beacon overhead for each Wi-Fi channel. In this case, we have a packet capture containing the beacons transmitted by a single AP on channel 1. The AP has four SSIDs, and it’s configured with a minimum basic rate of 1 Mbps. The estimated beacon overhead is 11%. In other words, the AP uses 11% of the available airtime to transmit beacons.

The beacon overhead of an AP transmitting four separate beacons, ten times per second, is 11%.

Improving airtime efficiency

Ratified on February 9, 2021, the IEEE 802.11ax-2021 amendment, also known as Wi-Fi 6, is a collection of enhancements for high-efficiency wireless networks. One of those enhancements is using the Multiple BSSID element to save the AP from transmitting a beacon frame per SSID, reducing beacon overhead when using virtual APs. The Mulitple BSSID element allows an AP to collapse information from multiple networks into a single beacon or probe response.

Revisiting our last example, if the same AP included a Multiple BSSID element to announce the four SSIDs in a single beacon, it would only incur a 4% beacon overhead. That’s a difference of 7% when compared to sending four separate beacon frames, one for each SSID.

The beacon overhead of an AP sending a single beacon, ten times per second, is 4%.

This capability is not new to 802.11ax, though. It was defined in 802.11v-2011. However, the 802.11ax amendment has made it mandatory for APs operating in the 6 GHz band as vendors can implement support for parsing the Multiple BSSID element in all the new 6 GHz clients. A Multiple BSSID element may be used in the 2.4 or 5 GHz bands; nevertheless, legacy clients will not recognize the Multiple BSSID element unless their drivers are updated. Therefore, we expect APs to continue sending separate beacons for each BSSID when operating in these bands.

The Multiple BSSID element

The Multiple BSSID element (ID 71) allows an AP to collapse information for collocated networks running on the same Wi-Fi channel into a single beacon or probe response frame. It avoids sending the same information elements (e.g., Supported Rates, HE Capabilities, HE Operation, etc.) in separate beacons or probe responses, unnecessarily consuming more airtime. 

The BSSID that carries the Multiple BSSID element is called the transmitted BSSID (also known as the reference BSSID), and the BSSIDs carried by the Multiple BSSID element are known as the nontransmitted BSSIDs.

The Multiple BSSID element contains a field called Max BSSID Indicator, which specifies the maximum number of BSSIDs in the multiple BSSID set (do not confuse with the number of active BSSIDs, which the AP does not indicate). It also contains a list of optional subelements. There are two types of subelements: Nontransmitted BSSID Profile and Vendor Specific. 

The Multiple BSSID element (ID 71).

The Nontransmitted BSSID Profile subelement contains a list of information elements that, together with the elements sent in the beacon or probe response, define the set of elements for the nontransmitted BSSID.

The Nontransmitted BSSID Profile subelement.

The Multiple BSSID element includes a Nontransmitted BSSID Profile subelement for each nontransmitted BSSID. However, the AP may choose to advertise only a subset of BSSIDs on a Multiple BSSID element.

The Nontransmitted BSSID Profile always includes two mandatory information elements: the SSID and Multiple BSSID-Index elements. Two additional elements are optional: the Nontransmitted BSSID Capability element and the Non-Inheritance element extension. The remaining elements contain information that differs between the transmitted and nontransmitted BSSIDs, such as security configuration. The elements found in the profile replace the same elements present in the beacon carrying the Multiple BSSID element for the nontransmitted BSSID.

The SSID element

The SSID element (ID 0) contains the identity of the ESS, IBSS, or mesh network. If the network is hidden, the SSID is a zero-length string.

The Multiple BSSID-Index element

The Multiple BSSID-Index element (ID 85) identifies a nontransmitted BSSID within the set of multiple BSSIDs. 

The Multiple BSSID-Index element (ID 85).

The value of the BSSID Index field, the Max BSSID Indicator field of the Multiple BSSID element, and the reference BSSID are used to derive the nontransmitted BSSID. Let’s say A0:A1:A2:A3:A4:A5 is the reference BSSID, then the nontransmitted BSSIDi, where i is the index of the BSSID, and N = 2Max BSSID Indicator, is:

BSSIDi = A0:A1:A2:A3:A4:A5i

where

A5i = A5 - (A5 mod N) + (((A5 mod N) + i) mod N)

For example, if BSSID Index = 2, Max BSSID Indicator = 4 (16 BSSIDs), and reference BSSID = CC:88:C7:41:6D:30, then the nontransmitted BSSID = CC:88:C7:41:6D:32.

The Nontransmitted BSSID Capability element

The Nontransmitted BSSID Capability element (ID 83) includes the contents of the Capability Information field in beacons and probe responses for the BSS. It specifies, among other things, whether the nontransmitted BSSID is part of an ESS, IBSS, or mesh network or whether a short preamble is supported.

The Nontransmitted BSSID Capability element (ID 83).

Non-Inheritance element extension

The Non-Inheritance element (ID 255, ID Extension 56) identifies one or more elements that are not inherited by the nontransmitted BSSID. It specifies two lists of elements: a list of element IDs and a list of element ID extensions. Suppose an ID in either list matches the ID of an information element in the beacon or probe response that carries the Multiple BSSID element. In that case, that information element is not considered an element of the nontransmitted BSSID.

The Non-Inheritance element extension (ID 255, ID Extension 56).

Networks evolve, and so does WiFi Explorer Pro 3

Note that the limitations for legacy 2.4 and 5 GHz clients to recognize and parse the Multiple BSSID element also apply to existing Wi-Fi tools, such as Wi-Fi scanners. Unless Wi-Fi scanners add support for the Multiple BSSID element, they will only see a single network when parsing beacon or probe response frames. In other words, they will not show information about other networks the AP includes in the Multiple BSSID element.

Finding 6 GHz networks in WiFi Explorer and WiFi Explorer Pro 3 is still limited due to the lack of Macs with Wi-Fi 6E hardware. However, we have constantly been updating the apps to fully support 6 GHz and all of the new capabilities, including the Multiple BSSID element. WiFi Explorer Pro 3 can already import packet captures containing 6 GHz networks or do a remote scan in the 2.4, 5, and 6 GHz bands using a compatible Linux computer and an Intel AX210 card. It will also find and display nontransmitted BSSIDs (networks included in the Multiple BSSID element).

For example, the screenshot below shows four networks from a single beacon frame: one transmitted BSSID and three nontransmitted BSSIDs. Note how the beacon airtime for the three nontransmitted BSSID is 0 ms as only the beacon of the transmitted BSSID consumes airtime. Also, the security settings is different for the nontransmitted BSSIDs, which means each nontransmitted BSSID profile contains a RSNE element that specifies a security configuration that’s different to the security configuration of the transmitted BSSID. 

WiFi Explorer Pro 3 showing 4 different networks from a single beacon frame.

Conclusion

In this blog, we’ve described one of the 802.11ax features designed to increase the efficiency of Wi-Fi networks. Beacon frame transmissions may cause a considerable overhead in environments with a large number of SSIDs. By using the Multiple BSSID element, an AP can reduce the beacon overhead by collapsing the information of multiple SSIDs into a single beacon or probe response frame. This capability is mandatory for APs operating in the 6 GHz band, and as we’ve mentioned, it means Wi-Fi tools, such as Wi-Fi scanners, must be updated to find and display all networks advertised by a Wi-Fi 6E AP.

Thanks to Josh Schmelzle for providing the 6 GHz captures used in this blog and the discussions around Wi-Fi 6E.


Sources:

IEEE 802.11-2020 – IEEE Standard for Information Technology–Telecommunications and Information Exchange between Systems – Local and Metropolitan Area Networks–Specific Requirements – Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specificationshttps://standards.ieee.org/standard/802_11-2020.html.

IEEE 802.11ax-2021 – IEEE Standard for Information Technology–Telecommunications and Information Exchange between Systems Local and Metropolitan Area Networks–Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 1: Enhancements for High-Efficiency WLANhttps://standards.ieee.org/standard/802_11ax-2021.html.