Note that there are two protocols which have been called 'adhoc mode'
over the years.  The names have changed to match what is normal
practice.

A brief list of modes: 

* Infrastructure mode as a client
This is the normal mode to talk to an AP.  Port type is 1, I think on
all cards (I'm not sure port type means anything to ath(4) hardware,
but this notion from wi(4) hardware is now pervasive).

* Ad-hoc demo mode
This was a feature of the early Lucent wavelan cards, back when they
were calling then "WaveLAN IEEE" in perhaps 1998.  There are no ACKS,
and packets are just sent, regardless of whether they are broadcast or
unicast.  There is no concept of SSID, and all 0s is used for the
BSSID.  So basically this ignores all the upper MAC part of 802.11,
just using the PHY and lower MAC.  This is port type 3.
In current/2.0, wi(4) explains that 'adhoc,flag0' selects this mode.

* IBSS mode
This is specified by 802.11.  There are still acks, and power saving
is even supposed to work in theory (not sure about practice).
This is mode 1 still on lucent cards, and mode 0 on prism2 cards.

* hostap mode
[description omitted - not relevant to the above]

IMHO, the word 'adhoc mode' should never be used because it is
confusing.  I'd like to see 'ibss' (a term that appears in the
standard - I believe adhoc mode does not) be the word used in
mediopts, and perhaps adhoc-demo for the old mode.

I'm not sure you can talk to wi(4)'s adhoc-demo mode with ath at all,
but try it.  You can put the 1.6.2 machine in IBSS mode by doing

ifconfig wi0 MYSSID
# set to channel 1
/usr/sbin/wiconfig wi0 -f 1
# create ibss
/usr/sbin/wiconfig wi0 -c 1

IBSS mode does not necessarily work with old lucent firmware.  I am
using

wi0: using Lucent Technologies, WaveLAN/IEEE
wi0: Lucent Firmware: Station (6.16.1)

under 1.6.2 (on an ibm thinkpad 600).

With IBSS mode, there is a BSSID formed from the mac address of one
station.  When two such nets come into mutual range, an "IBSS merge"
is supposed to happen.  This didn't work right on ath0 with
februaryish current, and I'm unsure of the current state.


-- 
        Greg Troxel <gdt@ir.bbn.com>