TBR21 is a European telecommunications standard published by

ETSI The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization operating in the field of Information and communications technology, information and communications. ETSI supports the de ...

, and is the standard to which all telephone equipment must adhere to, to be allowed connection to Europe's

public switched telephone network The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists o ...

. It is somewhat equivalent to the US part 68 standard. TBR-21 has been replaced by the new ETSI standard ES 203-021 The "advisory notes" (ATAAB notes) are no longer being maintained.

References

ETSI Telecommunications standards {{Measurement-stub