Transmission level point

In telecommunication, a transmission level point (TLP) is a physical test point in an electronic circuit, typically a transmission channel, where a test signal may be inserted or measured.[1] Typically, various parameters, such as the power of the signal, noise, or test tones inserted are specified or measured at the TLP.

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 ca. 1000 hertz. The dBm is an absolute level with respect to 1 mW. The TLP is thus characterized by the relation:[1]

TLP = dBm — dBm0

When the nominal signal power is 0 dBm at the TLP, the test point is called a zero transmission level point, or zero-dBm TLP. In general, the term TLP is commonly used as if it were a unit, preceded by the nominal level for the test point. For example, the expression 0 TLP refers to a 0 dBm TLP. At a −16 TLP, the measured level of 0 dBm is +16 dBm0.

The level at a TLP where an end instrument, such as a telephone set, is connected is usually specified as 0 dBm.