Determining 802.11 Signal Strength for Pocket PC

A very common request from Pocket PC developers is the ability to query the signal strength of an 802.11 wireless network signal being received by a WiFi enabled Pocket PC device. Here is some code that accomplishes this task for you.

This code basically queries the RSSI (Received Signal Strength Indicator) via the NDIS protocol. In your application, you call the GetFirstRFEthernetSignalStrength function to kick this all off as follows:

int nStrength = 0;
int nQuality = 0;
GetFirstRFEthernetSignalStrength(&nStrength, &nQuality);

The nStrength valule returned can then be used by your application to update a signal strength indcator such as a track bar or other UI element.

#include <ntddndis.h>
#include <nuiouser.h>
#include <winioctl.h>
#include <iphlpapi.h>

HRESULT GetSignalStrength(
 TCHAR *ptcDeviceName,
 int* piSignalStrength,
 int* piSignalQuality
 )
{
 PNDISUIO_QUERY_OID queryOID;
 DWORD  dwBytesReturned = 0;
 UCHAR  QueryBuffer[sizeof(NDISUIO_QUERY_OID)
     +sizeof(DWORD)];
 HANDLE  ndisAccess = INVALID_HANDLE_VALUE;
 BOOL  retval;
 HRESULT  hr;

 // Attach to NDISUIO.
 ndisAccess = CreateFile( NDISUIO_DEVICE_NAME,
    0, 0, NULL, OPEN_EXISTING,
  FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED,
    INVALID_HANDLE_VALUE );
 if ( ndisAccess == INVALID_HANDLE_VALUE )
  return E_FAIL; // ????

 // Get Signal strength
 queryOID = (PNDISUIO_QUERY_OID)&QueryBuffer[0];
 queryOID->ptcDeviceName = ptcDeviceName;
 queryOID->Oid = OID_802_11_RSSI;

 retval = DeviceIoControl( ndisAccess,
  IOCTL_NDISUIO_QUERY_OID_VALUE,
  (LPVOID) queryOID,
  sizeof(NDISUIO_QUERY_OID) + sizeof(DWORD),
  (LPVOID) queryOID,
  sizeof(NDISUIO_QUERY_OID) + sizeof(DWORD),
  &dwBytesReturned,
  NULL);
 if( retval && piSignalStrength && piSignalQuality )
 {
  hr = S_OK;
  *piSignalStrength = *(DWORD *)&queryOID->Data;

  //Here's Microsoft's interpretation
  //of the return value
  if(*piSignalStrength < -90)
   *piSignalQuality = 0; // No signal
  else if(*piSignalStrength < -81)
   *piSignalQuality  = 1; // Very low
  else if(*piSignalStrength < -71)
   *piSignalQuality  = 2; // Low
  else if(*piSignalStrength < -67)
   *piSignalQuality  = 3; // Good
  else if(*piSignalStrength < -57)
   *piSignalQuality  = 4; // Very good
  else
   *piSignalQuality  = 5; // Excellent
 }
 else
 {
  hr = E_FAIL;
 }

 CloseHandle( ndisAccess );

 return hr;

}

BOOL GetFirstRFEthernetSignalStrength(
 int* pSignalStrength,
 int* pSignalQuality
 )
{
 BOOL retval = FALSE;

 // Get the size of the adapters list.
 DWORD size = 0;

 GetAdaptersInfo( NULL, &size );

 // Allocate space for the list.
 IP_ADAPTER_INFO *info =
    (IP_ADAPTER_INFO*)new char[ size ];

 DWORD err;
 if ((err = GetAdaptersInfo(info, &size ))
    == NO_ERROR )
 {
  IP_ADAPTER_INFO *cinfo = info;
  while ( cinfo )
  {
   // Check the interface media type for
   // Ethernet, which both
   // standard Ethernet and RF Ethernet will return.
   if ( cinfo->Type == MIB_IF_TYPE_ETHERNET )
   {
    TCHAR wName[ MAX_ADAPTER_NAME_LENGTH + 4 ];
    MultiByteToWideChar( CP_ACP, 0,
    cinfo->AdapterName, -1,
     wName,
     sizeof( wName ) / sizeof( wName[ 0 ] ) );

    // Try to get the signal strength.
    //If it works, we
    // return the information.
    //If not, we move on.
    if ( GetSignalStrength( wName,
     pSignalStrength, pSignalQuality ) == S_OK )
    {
     retval = TRUE;
     break;
    }
   }

   // Go to the next adapter in the list.
   cinfo = cinfo->Next;
  }
 }

 // Don't forget to delete the allocation.
 delete [] info;

 return retval;
}