why the obtained rxPower is always equal to zero everywhere??!!

[Kurdman]

void Ieee80211AgentSTA::dumpAPList(Ieee80211Prim_ScanConfirm *resp)
{
EV << "Received AP list:\n";
for (int i = 0; i < (int)resp->getBssListArraySize(); i++) {
    Ieee80211Prim_BSSDescription& bssDesc = resp->getBssList(i);
    EV << "    " << i << ". "
       << " address=" << bssDesc.getBSSID()
       << " channel=" << bssDesc.getChannelNumber()
       << " SSID=" << bssDesc.getSSID()
       << " beaconIntvl=" << bssDesc.getBeaconInterval()
       << " rxPower=" << bssDesc.getRxPower()
       << endl;
    // later: supportedRates
}
}

my question is about above code , I executed the existing movileIPv6 example, noticed that everywhere the returned value of rxpower is equal to zero, I changed the MN's position near and far from the access point but nothing change about the rxpower!!! an other thing I want to know is: is the rxpowr is equal to RSSI (recived signal strength indicatore)? or can I get RSSI from rePower? the attached image is a snap-shoot of a simulation while MN is associated with an AP