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.<ref name=reeve>Whitham D. Reeve (1992) Subscriber Loop Signaling and Transmission Handbook—Analog, IEEE Press, p.229</ref> Various parameters, such as the power of the signal, noise, voltage levels, wave forms, may be measured at the TLP.<ref name="tia">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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 Template:Slink) with respect to 1 mW power. When the nominal signal power is Template:Gaps 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:<ref name="reeve" />
- TLP = dBm — dBm0
The term TLP is commonly used as if it were a unit,<ref name="tia"/> preceded by the nominal level for the test point. For example, the expression Template:Gaps refers to a Template:Gaps. 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The level at a TLP where an end instrument, such as a telephone set, is connected is usually specified as Template:Gaps.