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In telecommunications, a transmission level point (TLP) is a test point in an electronic circuit that is typically a transmission channel. At the TLP, a test signal may be introduced or measured.[1] Various parameters, such as the power of the signal, noise, voltage levels, wave forms, may be measured at the TLP.[2]
The nominal transmission level at a TLP is a function of system design and is an expression of the design gain or attenuation (loss).
Voice-channel transmission levels at test points are measured in decibel-milliwatts (dBm) at a frequency of ~1000 hertz.
The dBm is an absolute reference level measurement (see Decibel § Suffixes and reference values) with respect to 1 mW power. When the nominal signal power is 0dBm at the TLP, the test point is called a zero transmission level point, or zero-dBm TLP. The abbreviation dBm0 stands for the power in dBm measured at a zero transmission level point. The TLP is thus characterized by the relation:[1]
- TLP = dBm — dBm0
The term TLP is commonly used as if it were a unit,[2] preceded by the nominal level for the test point. For example, the expression 0TLP refers to a 0dBmTLP. If for instance a signal is specified as -13 dBm0 at a particular point and -6 dBm is measured at that point, the TLP is +7 TLP.[3]
The level at a TLP where an end instrument, such as a telephone set, is connected is usually specified as 0dBm.
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