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FTZ 1 TR 6 (or 1 TR 6) is the standard for the obsolete German national digital signalling protocol ( D channel protocol) used for the ISDN. It has been superseded by DSS1 but is still in use on some lines and private exchanges.
1 TR 6 did not support multiple subscriber numbers but featured an Endgeräteauswahlziffer (EAZ, German "user device selection digit"). ISDN subscribers would get a sequence of 10 subsequent subscriber numbers, the last digit of which being the EAZ. The EAZ "0" was used for "global calls" to all connected devices.
1 TR 6 also offered semipermanent connections, which were (comparatively cheap) leased lines that could be temporarily disabled to use the B channel for other purposes.
It is said that the "TR" simply stands for "technische Richtlinie" (technical guidelines).