分类： LINUX
2012-11-03 16:38:27
iwconfig ath0
ath0 IEEE 802.11ng ESSID:"Atheros_AP51"
.......
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
.........
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
最关键的就是rssi和noise这两个东西是从哪里来的，先看看作者的注释
/*
* 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,
* , or . These differences are at most 2db and
* can be documented.
*
* NB: various calculations are based on the orinoco/wavelan
* drivers for compatibility
*/
看代码：
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;
这个函数把avgrssi圆整一下。
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来自哪里呢？
来自ath_rx_poll
ATH_RSSI_LPF(an->an_avgrssi, rxstat->rs_rssi);
其中rxstat->rs_rssi就是descriptor中提取出来的，
ds->ds_rxstat->rs_rssi = MS(ads->ds_rxstatus0, AR_RcvSigStrength);
MS宏的作用是把rssi从descriptor中取出来，根据AR5416的datasheet，11n 2T3R配置有3个rssi，8bit 范围就是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)
好复杂的变换，不知道美国人搞那么复杂干什么，他们不是连加法都要用计算器的吗？
举个例子吧，假设驱动开始两次rx_poll Descriptor中的rssi是0x60 和 0x60
开始an->an_avgrssi=0x127不合法的值。
第一次计算：
ATH_RSSI_LPF(an->an_avgrssi, 0x60)
设x= an->an_avgrssi;
x = ATH_LPF_RSSI((x), ATH_RSSI_IN((0x60)), 10);
an->an_avgrssi =0x60*(1<<7)=0x3000;
第二次计算：
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
总结一下：
1. Rx Descriptor中的rssi(0 ~ 127)
2. ATH_RSSI_LPF 将rssi 转化为avgrssi
3. ath_node_getrssi将avgrssi圆整后报告给上层。
圆整后的avgrssi的意义：SNR即信噪比 噪声功率通常假设为-95dbm 那么信号功率就是：-95dbm + avgrssi
那么信号强度和传输速率的关系是什么呢，香侬定理可以回答这个问题。54Mbps需要的SNR是7.4db 这只是个理论值，在实际的环境中 估计还要乘个不小的系数呢。