5.6.2. Application Field of PBXXX 
The PBXXX gateway module is used to link the OpenScape 4000 system
to private systems and networks via DPNSS1 or public exchanges via DASS2.
With PBXXX it is possible to link the OpenScape 4000 system with:
- iSLX, iSDX, EMS 601 or other DPNSS based PBX systems via DPNSS1 protocol or
- public exchanges via DASS2 protocol
- other PABX systems via DPNSS1 protocol if certificated.
DPNSS1 (Digital Private Network Signaling System No. 1) is a British Telecom
(BT) standard private network protocol used on many PBX systems, mostly
in the UK, although it is also supported in many other countries. The
DPNSS1 specification is a published standard in the United Kingdom.
IMPORTANT:
Most DPNSS1 implementations of the various PBX
vendors are a subset of the global DPNSS specification (BTNR188). Cooperation
with other vendor PBXs must be tested first.
DASS2 (Digital Access Signaling System No 2) describes the standard
interfaces of the British ISDN access.