 a reference point, and signal or noise magnitudes at some other point in the plant predicted from a knowledge of the gains or losses involved. In toll telephone practice, it is customary to define the toll transmitting switchboard as the reference point or "zero transmission level" point. To put this in the form of a definition: The transmission level at any point in a transmission system is the ratio of the power of a test signal at that point to the test signal power applied at some point in the system chosen as a reference point. This ratio is expressed in decibels. In toll systems, the transmitting toll switchboard is usually taken as the zero level or reference point. Frequently the specification of transmission level is confused with some absolute mea- sure of power at some point in the system. Let us make this perfectly clear. When we speak of -9-dB transmission level point (often abbreviated "the -9 level"), we simply mean that the signal power at such a point is 9 dB below whatever signal power exists at the zero level point. The transmission level does not specify the absolute power in dBm or in any other such power units. It is relative only. It should also be noted that, although the reference power at the transmitting toll switchboard will be at an audio frequency, the corresponding signal power at any given point in a broadband carrier system may be LEARNING DECIBELS AND THEIR APPLICATIONS at some carrier frequency. We can, nevertheless, measure or compute this signal power and specify its transmission level in accordance with the definition we have quoted. The transmission level at some particular point in a carrier system will often be a function of the carrier frequency associated with a particular channel. Using this concept, the magnitude of a signal, a test tone, or an interference (level) can be specified as having a given power at a designated level point. For example, in the past many long toll systems had 9-dB loss from the transmitting to the receiving switchboard. In other words, the re