Back to my "favorite" Android Issue 82. It seems Android engineers prefer Wi-Fi Direct over ad-hoc as illustrated by this thread.

Ad-hoc has slipped in priority in favor of other superior solutions keeping power constraints and security in mind. Soft Ap support exists and wi-fi direct support will come in the future.

...even though ad-hoc is an age old technology supported by many many devices, and direct wifi is not available yet. But there's hope:

Wifi direct is a layer that auto configures one of the devices as a Soft Ap. The benefits you have with Soft Ap are the same that you will get with wifi direct (wpa2 and power management). Thats not to say there is no good reason to add ad-hoc, there is - compatibility with devices that only support ad-hoc. (...) Expect to see both direct and ad-hoc support in the future.

I wonder though if Wi-Fi Direct will support group communication, or will one device (the soft AP) need to do all the heavy lifting. In an ad-hoc network, no node acts as a relay: all traffic is direct, broadcasts are direct. In an infrastructure (or soft AP) network, the AP relays all traffic: even broadcast packets will be sent to the AP which then broadcasts them to all.

Here's a snippet from Wi-Fi Alliance's FAQ on Wi-Fi Direct:

Is this the same as Ad Hoc mode?

No. Ad Hoc, or IBSS, mode is a legacy protocol for Wi-Fi devices, and Wi-Fi Direct is a new innovation. Wi-Fi Direct brings important security features, ease of setup, and higher performance that is not currently available in Ad Hoc mode. With Wi-Fi Direct, a device can maintain a simultaneous connection to an infrastructure network – this isn’t possible with Ad Hoc.

Is the specification underlying the Wi-Fi Direct certification program

based on the IEEE 802.11s (Mesh) or 802.11z (Direct Link Setup) standards?

No. The specification underlying the Wi-Fi Direct certification program was developed within the Wi-Fi Alliance by member companies. It operates on 802.11 devices but is not linked to any specific IEEE 802.11 amendment.

This means, Wi-Fi Direct is not an IEEE standard, but a specification from the Wi-Fi Alliance. It's not necessarily a bad thing. The rationale here seems to be that the reason why ad-hoc failed despite Wi-Fi certification was because it wasn't "easy enough for home users", and so Wi-Fi Direct is essentially built on top of Wi-Fi Protected Setup.

I have to say I'm rather disappointed by the arguments that legacy ad-hoc, or IBSS, deserves to be abandoned now because it was insecure or slow. Ad-hoc is basically defined above the MAC, so there's no fundamental reason why ad-hoc would not be able to support any of the data-rates or data encryption algorithms that are supported by the client device. That ad-hoc never evolved beyond 802.11b is to be blamed on the Wi-Fi Alliance for not updating the certification to allow 802.11g data-rates or WPA2 AES-based encryption.

Although I can see how WPS will make the "nightmare of setting up ad-hoc" go away for many users, I don't see any substantial advantages of the AP - STA approach taken in Wi-Fi Direct. It almost seems like this is a "lazy" solution along the lines of: since soft AP is becoming more proliferate (ironically to support the crippled STA devices that cannot do ad-hoc, like PS3, Wii, etc.), why not come up with a protocol that will establish which station should become a soft AP on-demand?