How does Wi-Fi ad-hoc mode react when 802.11n and legacy peers are present?

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Wi-Fi ad-hoc mode involves direct peer-to-peer communication between 802.11 stations. (In a previous Ask the Expert answer, you can read my response on how Wi-Fi ad-hoc mode works.) The 802.11n standard includes several new rules pertaining specifically to this mode of operation, technically referred to as an independent basic service set (IBSS).

In an ordinary infrastructure mode WLAN, an access point (AP) is responsible for sending beacons that define the capabilities of the WLAN, including supported data rates and required security ciphers. As a central point of control over the BSS, the AP can also make unilateral decisions, like changing the channel (to avoid interference) or turning on protection (when legacy stations are detected).

802.11n wireless LAN deployment resources Learn more about deploying 802.11n WLANs in Lisa Phifer's SearchNetworking.com tips on 802.11n AP deployment best practices and 802.11n wireless LAN upgrade planning.

However, an IBSS lacks this centralized control; decision-making is distributed across all stations participating in the IBSS. For example, in a secure infrastructure mode WLAN, each station completes a four-way authentication handshake with the AP, deriving encryption keys to be used with AES or TKIP. The final 802.11i standard defines a modified version of this four-way handshake that must be exchanged between every pair of communicating stations participating in an IBSS. In effect, every pair of ad-hoc stations makes their own agreement about how to secure the frames they exchange, instead of having one AP establish a uniform security policy used by everyone.

So how does 802.11n impact IBSS operation? Areas given special attention in the 802.11n standard include power management (controlling how long stations doze to conserve battery life) and dynamic frequency selection (controlling when stations should start using a different channel). For example, if IBSS stations made independent decisions about switching channels in the presence of radar, several stations might detect interference simultaneously and announce a switch to different channels. To avoid this, 802.11n specifies rules whereby an IBSS dynamic frequency selection owner station (IDO STA) uses beacons, probe responses, or an extended channel switch announcement to inform other 802.11n stations about an impending channel change. Furthermore, a 20/40 MHz IBSS cannot be changed to a 20 MHz IBSS and a 20 MHz IBSS cannot be changed to a 20/40 MHz IBSS.

In addition, certain elements of high-throughput (HT) operation cannot be changed by beaconing different capabilities, including elements that control the use of reduced interframe spacing, legacy protection, Greenfield mode, and dual CTS protection. For example, 802.11n stations that are members of an IBSS are required to protect HT transmissions, HT Greenfield format frames, and reduced interface spacing sequences as though HT Protection were set to non-HT mixed mode. This requirement ensures compatibility with legacy stations, whether any legacy stations are present or not when 802.11n stations start to form IBSS associations.