Digital Access Signalling System 1
Digital Access Signaling System 1 (DASS1) is a protocol defined by British Telecom for digital links to PSTN based on ISDN.
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DASS2 is a superset of DPNSS which expands its capability to route calls across the PSTN and not just PABX to PABX.
It is defined in BTNR190 however it is very difficult to get a copy of this document.
Much of the data in DASS2 is compatible with DPNSS. For basic calls, only the header information changes.
While developing a generic ISDN interface board, many sources suggested the DASS2 and DPNSS protocols are almost the same as EuroISDN. Extracting data from various ISDN types showed that the similarity ends with layer 2.
Signalling data is in timeslot 16. There are only 8 bits in each timeslot so a complete data message is built up from many frames. Messages are separated by a flag (01111110b).
This is a legacy ISDN standard and all new installations are EuroISDN/ISDN30e. Pete Garner, BSc.
See also
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