In a telecommunications system, a transmission-level point (TLP) is a test point or interface, i.e. a physical point in an electronic circuit where a test signal may be inserted or measured, and for which the nominal power of the test signal is specified.
A zero dBm transmission-level point (or synonym zero-transmission-level point) is a transmission-level point where the nominal test signal power is 0dBm.
In practice, the abbreviation TLP is usually used, and it is modified by the nominal level for the point in question. For example, where the nominal level is 0 dBm, the expression 0 dBm TLP, or simply, 0TLP, is used. Where the nominal level is −16 dBm, the expression −16 dBm TLP, or −16TLP, is used.
The nominal transmission level at a specified TLP is a function of system design and is an expression of the design gain or loss.
Voice-channel transmission levels, i.e. TLPs, are usually specified for a frequency of approximately 1,000 Hz.
The TLP at a point at which an end instrument, e.g. a telephone set, is connected is usually specified as 0 dBm.