from __future__ import print_function
import struct
from datetime import datetime
from twisted.internet.fdesc import setNonBlocking
[docs]class SpectrumReader(object):
# spectral scan packet format constants
hdrsize = 3
pktsize = 17 + 56
# ieee 802.11 constants
sc_wide = 0.3125 # in MHz
def __init__(self, path):
self.fp = file(path)
if not self.fp:
raise Exception("Cant open file '%s'" % path)
setNonBlocking(self.fp.fileno())
[docs] def read_samples(self):
data = self.fp.read()
if (data):
ts = datetime.now()
return ts, data
else:
return None, None
[docs] def flush(self):
self.fp.read()
[docs] @staticmethod
def decode(data):
"""
For information about the decoding of spectral samples see:
https://wireless.wiki.kernel.org/en/users/drivers/ath9k/spectral_scan
https://github.com/erikarn/ath_radar_stuff/tree/master/lib
and your ath9k implementation in e.g.
/drivers/net/wireless/ath/ath9k/common-spectral.c
"""
pos = 0
while pos < len(data) - SpectrumReader.hdrsize + 1:
(stype, slen) = struct.unpack_from(">BH", data, pos)
if not (stype == 1 and slen == SpectrumReader.pktsize):
print("skip malformed packet")
break # header malformed, discard data. This event is very unlikely (once in ~3h)
# On the other hand, if we buffer the sample in a primitive way, we consume too much cpu
# for only one or two "rescued" samples every 2-3 hours
# We only support 20 MHz
if stype == 1:
if pos >= len(data) - SpectrumReader.hdrsize - SpectrumReader.pktsize + 1:
break
pos += SpectrumReader.hdrsize
packet = data[pos: pos + SpectrumReader.pktsize]
pos += SpectrumReader.pktsize
yield packet
'''
(max_exp, freq, rssi, noise, max_mag, max_index, bitmap_weight, tsf) = \
struct.unpack_from(">BHbbHBBQ", data, pos)
pos += 17
sdata = struct.unpack_from("56B", data, pos)
pos += 56
yield (tsf, max_exp, freq, rssi, noise, max_mag, max_index, bitmap_weight, sdata)
# calculate power in dBm
sumsq_sample = 0
samples = []
for raw_sample in sdata:
if raw_sample == 0:
sample = 1
else:
sample = raw_sample << max_exp
sumsq_sample += sample*sample
samples.append(sample)
if sumsq_sample == 0:
sumsq_sample = 1
sumsq_sample = 10 * math.log10(sumsq_sample)
sc_total = 56 # HT20: 56 OFDM subcarriers
#first_sc = freq - SpectrumReader.sc_wide * (sc_total/2 + 0.5)
pwr = {}
for i, sample in enumerate(samples):
subcarrier_freq = 0
if i < 28:
subcarrier_freq = freq - SpectrumReader.sc_wide * (28 - i)
else:
subcarrier_freq = freq + SpectrumReader.sc_wide * (i - 27)
#subcarrier_freq = first_sc + i*SpectrumReader.sc_wide
sigval = noise + rssi + 20 * math.log10(sample) - sumsq_sample
pwr[subcarrier_freq] = sigval
yield (tsf, freq, noise, rssi, pwr)
'''