BT and a group of PBX vendors developed DPNSS in the 1980s. They designed this open standard to provide digital private networks at a time when ISDN and QSIG standards were still being defined.

DPNSS has rich services and feature sets and provided the basis for much of the work on the QSIG protocols. Also, interoperability between DPNSS and the QSIG signaling system is specified as part of the inter-working of both protocols.

The ISDN and QSIG protocols became more popular since they were developed, and DPNSS is not as widely used in today's private networks. DPNSS specifications are available from BT Plc and are defined in the following four documents:

• BTNR 188-T—DPNSS1 testing schedule

• BTNR 189—Inter-working between DPNSS1 and other signaling systems

• BTNR 189-I—Inter-working between DPNSS1 and ISDN signaling systems

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