Sensitivity setting problem
Tue Jun 24 02:06:33 PDT 2003
On 24 June 2003 09:31, Denis Vlasenko wrote:
> On 24 June 2003 04:55, Jouni Malinen wrote:
> > On Tue, Jun 24, 2003 at 12:04:21AM +0300, insecure wrote:
> > > I tried iwconfig wlan0 sens 1,2,3 but to no avail.
> > > I presume that foreign signal exceeds threshold at any of these
> > > settings.
> > >
> > > I've read the code and it seems hostap does not accept any other
> > > sens values.
> > Current version is mapping sensitivity configuration into Prism2
> > cnfSystemScale with those three options. However, this does not really
> > control what you would like to set with sensitivity threshold. Some of
> > the BBP control registers might be more helpful, but I have not
> > experimented with that kind of configuration.
> I'd be glad to experiment with. I still fail to find Intersil
> docs on prism, anybody feel free to mail me any and all non-NDA
> pdfs you have, because today I am limited to guessing off the
> hostap source.
"PRISM 2.5 Wireless LAN Integrated Medium Access Controller
with Baseband Processor"
Clear Channel Assessment (CCA) and
Energy Detect (ED) Description
lots of stuff ahead...
The Clear Channel Assessment (CCA) circuit implements the
carrier sense portion of a Carrier Sense Multiple Access
(CSMA) networking scheme. The Clear Channel Assessment
(CCA) monitors the environment to determine when it is clear
to transmit. The CCA circuit in the ISL3873B can be
programmed to be a function of RSSI (energy detected on the
channel), CS1, SQ1, or various combinations. The CCA is
used by the Media Access Controller (MAC) in the ISL3873B.
The MAC decides on transmission based on traffic to send
and the CCA indication. The CCA indication can be ignored,
allowing transmissions independent of any channel
conditions. The CCA in combination with the visibility of the
various internal parameters (i.e., Energy detection
measurement results), can assist the MAC in executing
algorithms that can adapt to the environment. These
algorithms can increase network throughput by minimizing
collisions and reducing transmissions liable to errors.
There are three measures that can be used in the CCA
assessment. The Receive Signal Strength Indication (RSSI)
which indicates the energy at the antenna, CS1 and carrier
sense (SQ1). CS1 becomes active anytime the AGC portion
of the circuit becomes unlocked, which is likely at the onset of
a signal that is strong enough to support 11Mbps, but may not
occur with the onset of a signal that is only strong enough to
support 1 or 2MBps. CS1 stays active until the AGC locks and
a SQ1 assessment is done, if SQ1 is false, then CS1 is
cleared, which deasserts CCA. If SQ1 is true, then tracking is
begun, and CCA continues to show the channel busy. CS1
may occur at any time during acquisition as the AGC state
machine runs asynchronously with respect to slot times.
SQ1 becomes active only when a spread signal with the
proper PN code has been detected, and the peak correlation
amplitude to sidelobe ratio exceeds a set threshold, so it
may not be adequate in itself.
A SQ1 evaluation occurs whenever the AGC has remained
locked for the entire data ingest period. When this happens,
SQ1 is updated between 8 and 9us into the 10us dwell. If
CS1 is not active, two consecutive SQ1?s are required to
advance the part to tracking.
The state of CCA is not guaranteed from the time RX_PE
goes high until the first CCA assessment is made. At the end
of a packet, after RXPE has been deasserted, the state of
CCA is also not guaranteed.
The Receive Signal Strength Indication (RSSI) measurement is
derived from the state of the AGC circuit. ED is the comparison
result of RSSI against a threshold. The threshold may be set to
an absolute power value, or it may be set to be N dB above the
measured noise floor. See CR 35. The ISL3873B measures
and stores the RSSI level when it detects no presence of BPSK
or QPSK signals. The average value of a 256 value buffer is
taken to be the noise floor. Thus, the value of the noise floor will
adapt to the environment. A separate noise floor value is
maintained for each antenna. An initial value of the noise floor
is established within 50s of the chip being active and is refined
as time goes on. Deasserting RX_PE does not corrupt the
learned values. If the absolute power metric is chosen, this
threshold is normally set to between -70 and -80dBm.
If desired, ED may be used in the acquisition process as well
as CCA. ED may be used to mask (squelch) weak signals
and prevent radio reception of signals too weak to support
the high data rates, signals from adjacent cells, networks, or
buildings. See CR 47 (bit 6).
i like this! ;)
The Configuration registers effecting the CCA algorithm
operation are summarized below (more programming details
on these registers can be found under the Control Registers
section of this document).
The CCA output from pin 60 of the device can be defined as
active high or active low through CR 1 (bit 2).
CR9(6:5) allows CCA to be programmed to be a function of ED
only, the logical operation of (CS1 OR SQ1), the logical function
of (ED AND (CS1 OR SQ1)), or (ED OR (CS1 OR SQ1)).
CR9(7) lets the user select from sampled CCA mode, which
means CCA will not glitch, is updated once per symbol and is
valid for reading at 15.8s or 18.7s. In non-sampled mode,
CCA may change at any time, potentially several times per slot,
as ED and CS1 operate asynchronously to slot times.
In a typical system CCA will be monitored to determine when
the channel is clear. Once the channel is detected busy,
CCA should be checked periodically to determine if the
channel becomes clear. Once MD_RDY goes active, CCA
should be ignored for the remainder of the message. Failure
to monitor CCA until MD_RDY goes active (or use of a time-out
circuit) could result in a stalled system as it is possible for
the channel to be busy and then become clear without an
ok, here are the registers:
CONFIGURATION REGISTER 1 ADDRESS (02h) R/W I/O POLARITY
This register is used to define the phase of clocks and other interface signals. 00h is normal setting.
Bit 2 This control bit selects the active level of the Clear Channel Assessment (CCA) output pin.
Logic 1 = CCA Active 1.
Logic 0 = CCA Active 0.
seems like we won't need to touch this
CONFIGURATION REGISTER 9 ADDRESS (12h) R/W TX CONFIGURE
Bit 7 CCA sample mode time.
0 = 18.7us.
1 = 15.8us.
Bits 6:5 CCA mode.
00 - CCA is based only on ED.
01 - CCA is based on (CS1 OR SQ1).
10 - CCA is based on (ED AND (CS1 OR SQ1)).
11 - CCA is based on (ED OR (CS1 OR SQ1)).
well... thats whole lot of options ;)
CONFIGURATION REGISTER ADDRESS 35 (46h) R/W ED THRESHOLD
Bit 7 Energy Detect Threshold control.
0 = threshold is relative to noise floor.
1 = threshold is absolute.
Bits 6:0 ED Threshold. Range 0 - 127dBm. RSSI > threshold triggers ED.
This is exactly what I need! Wow.
CONFIGURATION REGISTER ADDRESS 47 (5Eh) R/W ACQUISITION THRESHOLDS
Bit 7 Disable False alarm Rate Processing.
0 = Enable, SQ1 #1 threshold is adjusted in real time by FAR logic.
1 = Disable, SQ1 #1 threshold is set to value of CR 47 (5:0).
Bit 6 ED and SQ1 control for acquisition.
0 = SQ1.
1 = ED and SQ1.
Bits 5:0 SQ1 threshold #1, range 0 to 7.875. (000.00 - 111.111).
Used for initial detect and initial setting for FAR.
Dunno. Maybe leave alone for now.
Well. There's lots of options, but as a first simple
approximation we can map
iwconfig wlan# sens N (N=0..127)
to CR35.6:0 bits
and make CR9.6:5 settable by prism private ioctls
At least I'd like to be able to set 'em to 00 or 10,
so that any signal below ED is dismissed.
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