oidal test signal (800 Hz or 1000 Hz) at a point in the channel to the power of that reference signal at the transmission reference point. C.10.2 Definition of Transmission Reference Point In its old transmission plan, the CCITT had defined the zero relative level point as being the two-wire origin of a long-distance (toll) circuit. This is point 0 of Figure C.3a. In the currently recommended transmission plan the relative level is -3.5 dBr at the virtual switching point on the transmitting side of a four-wire international circuit. This is point V in Figure C.3b. The transmission reference point or zero relative level point (point T in Figure C.3b) is a virtual two-wire point which would be connected to V through a hybrid transformer having a loss of 3.5 dB. The conventional load used for computation C.10 RELATIVE POWER LEVEL: dBm0, pWp0, etc. V (-3.5 dBr) Zero relative level point Figure C.3 The zero relative level point. of noise on multichannel carrier systems corresponds to an absolute mean power level of -15 dBm at point T. The 0 TLP (zero test level point) is an important concept. It remains with us even in the age of the all-digital network. The concept seems difficult. It derives from the fact that a telephone network has a loss plan. Thus signal levels will vary at different points in a network, depending on the intervening losses. We quote from an older edition (1st ed.) of Transmission Systems for Telecommunications (Bell Telephone Laboratories, New York, 1959, Vol. I, pp. 2-3): In order to specify the amplitudes of signals or interference, it is convenient to define them at some reference point in the system. The amplitudes at any other physical location can be related to this reference point if we know the loss or gain (in dB) between them. In the local plant, for example, it is customary to make measurements at the jacks of the outgoing trunk test panel, or (if one does not wish to include office effects) at the main frame. For a particular set of measurements, one of these points might be taken as