iwconfig ath0
ath0 IEEE 802.11ng ESSID:"Atheros_AP51"
Mode:Managed Frequency:2.412 GHz Access Point: 00:1F:33:F7:5E:27
Bit Rate:1 Mb/s Tx-Power:17 dBm Sensitivity=0/3
Retry:off RTS thr:off Fragment thr:off
Encryption key:off
Power Management:off
Link Quality=79/94 Signal level=-16 dBm Noise level=-95 dBm
Rx invalid nwid:709 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0
比較關心紅色的結果的由來。
大致流程：
wireless tool iwconfig.c
iw_enum_devices Extract interface name
print_info
get_info
iw_get_stats
iw_get_ext ioctl SIOCGIWSTATS
display_info
iw_print_stats
sprintf(buffer,
"Quality:%d/%d Signal level:%d dBm Noise level:%d dBm%s",
qual->qual, range->max_qual.qual,
qual->level - 0x100, qual->noise - 0x100,
(qual->updated & 0x7) ? " (updated)" : "");
kernel net/core/wireless.c
wireless_process_ioctl
dev_iwstats (case SIOCGIWSTATS)
get_wireless_stats
dev->get_wireless_stats(dev)
madwifi
ieee80211_ioctl_vattach
dev->get_wireless_stats = ieee80211_iw_getstats
set_quality
set_quality(struct iw_quality *iq, u_int rssi, __u8 noise)
{
iq->qual = rssi;
/* NB: max is 94 because noise is hardcoded to 161 */
if (iq->qual > 94)
iq->qual = 94;
iq->noise = noise;
/* This noise is read from the ieee80211com which is updated by the ath layer,
* and the callback function of the ath layer points back to the function
* in the hal layer, in dBm, an thus being casted into __u8.
*/
iq->level = iq->noise + iq->qual;
iq->updated = 7;
}
Link Quality=79/94 Signal level=-16 dBm Noise level=-95 dBm
最關鍵的就是 rssi 和 noise 這兩個東西是從哪裏來的，先看看作者的註釋
/*
* Units are in db above the noise floor. That means the
* rssi values reported in the tx/rx descriptors in the
* driver are the SNR expressed in db.
*
* If you assume that the noise floor is -95, which is an
* excellent assumption 99.5 % of the time, then you can
* derive the absolute signal level (i.e. -95 + rssi).
* There are some other slight factors to take into account
* depending on whether the rssi measurement is from 11b,
* 11g, or 11a. These differences are at most 2db and
* can be documented.
*
* NB: various calculations are based on the orinoco/wavelan
* drivers for compatibility
*/
看代碼：
ieee80211_getrssi
case IEEE80211_M_STA: /* use stats from associated ap */
default:
TAILQ_FOREACH(vap, &ic->ic_vaps, iv_next)
if (vap->iv_bss != NULL) {
rssi_samples++;
rssi_total += ic->ic_node_getrssi(vap->iv_bss);
}
break;
}
ic->ic_node_getrssi = ath_node_getrssi;
這個函數把 avgrssi 圓整一下。
ath_node_getrssi （）
{
#define HAL_EP_RND(x, mul) /
((((x)%(mul)) >= ((mul)/2)) ? ((x) + ((mul) - 1)) / (mul) : (x)/(mul))
int32_t avgrssi = ATH_NODE_CONST(ni)->an_avgrssi;
int32_t rssi;
/*
* When only one frame is received there will be no state in
* avgrssi so fallback on the value recorded by the 802.11 layer.
*/
if (avgrssi != ATH_RSSI_DUMMY_MARKER)
rssi = HAL_EP_RND(avgrssi, HAL_RSSI_EP_MULTIPLIER);
else
rssi = ni->ni_rssi;
/* NB: theoretically we shouldn't need this, but be paranoid */
return rssi;
#undef HAL_EP_RND
}
Avgrssi 來自哪裏呢？
來自 ath_rx_poll
ATH_RSSI_LPF(an->an_avgrssi, rxstat->rs_rssi);
其中 rxstat->rs_rssi 就是 descriptor 中提取出來的，
ds->ds_rxstat->rs_rssi = MS(ads->ds_rxstatus0, AR_RcvSigStrength);
MS 宏的作用是把 rssi 從 descriptor 中取出來，根據 AR5416 的 datasheet ， 11n 2T3R 配置有 3 個 rssi ， 8bit 範圍就是 0~127 0x80 是不合法的值。
ds->ds_rxstat->rs_rssi = MS(ads->ds_rxstatus0, AR_RcvSigStrength);
ATH_RSSI_LPF(an->an_avgrssi, rxstat->rs_rssi);
#define HAL_RSSI_EP_MULTIPLIER (1<<7) /* pow2 to optimize out * and / */
#define ATH_RSSI_LPF_LEN 10
#define ATH_RSSI_DUMMY_MARKER 0x127
#define ATH_EP_MUL(x, mul) ((x) * (mul))
#define ATH_RSSI_IN(x) (ATH_EP_MUL((x), HAL_RSSI_EP_MULTIPLIER))
#define ATH_LPF_RSSI(x, y, len) /
((x != ATH_RSSI_DUMMY_MARKER) ? (((x) * ((len) - 1) + (y)) / (len)) : (y))
#define ATH_RSSI_LPF(x, y) do { /
if ((y) >= -20) /
x = ATH_LPF_RSSI((x), ATH_RSSI_IN((y)), ATH_RSSI_LPF_LEN); /
} while (0)
好複雜的變換，不知道美國人搞那麼複雜幹什麼，他們不是連加法都要用計算器的嗎？
舉個例子吧，假設驅動開始兩次 rx_poll Descriptor 中的 rssi 是 0x60 和 0x60
開始 an->an_avgrssi=0x127 不合法的值。
第一次計算：
ATH_RSSI_LPF(an->an_avgrssi, 0x60)
設 x= an->an_avgrssi;
x = ATH_LPF_RSSI((x), ATH_RSSI_IN((0x60)), 10);
an->an_avgrssi =0x60*(1<<7)=0x3000;
第二次計算：
ATH_RSSI_LPF(an->an_avgrssi, 0x60)
x = ATH_LPF_RSSI((0x3000), ATH_RSSI_IN((60)), 10);
= ATH_LPF_RSSI((0x3000), 0x3000, 10);
= (0x3000*9+0x3000)/0x3000=10
總結一下：
1. Rx Descriptor 中的 rssi(0 ~ 127)
2. ATH_RSSI_LPF 將 rssi 轉化爲 avgrssi
3. ath_node_getrssi 將 avgrssi 圓整後報告給上層。
圓整後的 avgrssi 的意義： SNR 即信噪比 噪聲功率通常假設爲 -95dbm 那麼信號功率就是： -95dbm + avgrssi
