Definitions. There are three power levels in common use, dBr, dBm and dBmO

There are three power levels in common use, dBr, dBm and dBmO.

The OdBr point is the level at a reference point within the system that all the transmission levels refer. This used to be the 2 wire audio point or the 2 wire point of origin, as it was sometimes called.

This physical entity within a network has all but disappeared and a point of reference is now normally taken as the virtual outgoing switch point and is set at -4dBr. From the transmissions point of view the audio output from the channel translating equipment is adjusted to suit the required stated dBr level at that point and this becomes the level to which all the transmission levels within the system refer. Transmission levels are referred to in dBr.

The power at a various points in the transmission are referred to in dBm. If a test signal is injected into a OdBr point at OdBm then the test signal level throughout the transmission can be referred to in dBm.

It is sometimes the case that for noise calculations where the transmission level is required in dBm rather than dBr it is assumed that the power at the OdBr point is OdBm.

This is the power of a signal in dBm referred to a point where the transmission level is OdBm.

For instance if a signal power is -80dBm at a -30dBm trans­mission level point, then the signal is defined as -50dBmO.