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Transmission level point
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{{Short description|Test point for signal monitoring}} 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 (physics)|power]] of the signal, noise, voltage levels, wave forms, may be measured at the TLP.<ref name="tia">{{cite web |url=https://standards.tiaonline.org/market_intelligence_/glossary/index.cfm?term=%27%234SQRB%2BHJDE%24%0A |title=Transmission level point (TLP) |work=Glossary of Telecommunication Terms |publisher=Telecommunication Industry Association (TIA) |access-date=2023-05-19 }}</ref> The nominal transmission level at a TLP is a function of system design and is an expression of the design [[Gain (electronics)|gain]] or [[attenuation]] (loss). [[Voice channel|Voice-channel]] transmission levels at test points are measured in [[dBm|decibel-milliwatts]] (dBm) at a [[frequency]] of ~1000 [[hertz]]. The [[dBm]] is an absolute reference level measurement (see {{Slink|Decibel|Suffixes and reference values}}) with respect to 1 mW power. When the nominal signal power is {{gaps|0|dBm}} 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 ''{{gaps|0|TLP}}'' refers to a ''{{gaps|0|dBm|TLP}}''. 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>{{Cite web |last=Reeve |first=Whitham D. |date=1996 |title=Application Note No. 8 |url=https://reeve.com/Documents/Transmission%20Level%20Point%20No8.pdf |url-status=live |archive-url=https://web.archive.org/web/20230201062929/https://reeve.com/Documents/Transmission%20Level%20Point%20No8.pdf |archive-date=2023-02-01 |access-date=2023-06-26 |website=reeve.com}}</ref> The level at a TLP where an [[end instrument]], such as a [[telephone]] set, is connected is usually specified as {{gaps|0|dBm}}.
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