Date: 29 Jul 2002 08:43:06 -0400 From: Greg Troxel <gdt@ir.bbn.com> To: Dan Pelleg <daniel+bsd@pelleg.org> Cc: freebsd-mobile@freebsd.org, freebsd-questions@freebsd.org Subject: Re: Thinkpad overheating when talking to a D-Link wireless AP Message-ID: <rmiwure7lid.fsf@fnord.ir.bbn.com> In-Reply-To: Dan Pelleg's message of "Mon, 15 Jul 2002 09:20:34 -0400" References: <15666.52258.20053.339128@gs166.sp.cs.cmu.edu>
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The 802.11 specification (now freely available, but a bit hard to read
- so I may be slightly off in terminology here since I don't have it
handy) describes several ways an access point can operate. All of
these are part of "BSS" mode and I believe are typically handled in
firmware without the knowledge of the driver.
The most basic is the Distributed Coordination Function (DCF), in
which a station (STA) transmits frames to the AP after performing
Clear Channel Assessment (basically listen for below-threshold energy
in the channel). There are randomized backoffs for retransmission.
This is basically a contention mechanism.
Then, there is the Point Coordination Function (PCF), where the AP
controls who gets to transmit. This is either associatied with or
implemented by the "Contention-Free Period", where the AP controls who
transmits. Basically the AP sends a packet to a station and lets the
station send a reply packet with the ack. The AP can also poll
stations to see if they have traffic. So an AP could run in a mode
where most of the time is PCF, which can get much higher channel
usage.
This all interacts with power saving, which can work with both modes.
(getting fuzzier) There is an announcement period where stations
indicate that they have traffic (or the AP says what STAs it has
traffic for), followed by time to exchange traffic. This enables a
STA to shut down its receiver if the announcement traffic (from the
AP) told it that there was no traffic for it. APs never shut down. I
am pretty sure this works in both DCF and PCF. This is the basis of
power saving mode - a STA informs the AP on association that it wants
to be able to go into doze mode, and wakes up to hear the beacons from
the AP. If there is no traffic for that STA, it goes back to sleep
until the next beacon. I think the interval is on the order of
multiple tens of milliseconds, and that this is an AP configurable
value (in theory; not sure about particular APs).
So, I would wonder if that AP either does not implement power-saving
mode (I believe this is not optional, but I don't remember), or if it
is configured off. Is your card in power-save mode, and does it get
hot with the Lucent aps if you turn power-save mode off?
If you really want to understand this, I recommend that you read the
802.11 spec (802.11b is just a 'Higher-rate PHY in the 2.4 GHz band' -
the stuff above is in the base 802.11 document), but be warned that it
is not easy reading.
Then, try the dachd0den labs prism2dump, or get tcpdump.org tcpdump to
decode 802.11 management frames, so you can see what's going on.
Greg Troxel <gdt@ir.bbn.com>
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