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QSIG

QSIG is a Q.931 based signaling protocol usually used for signaling between PBX systems in a private telephony network. QSIG services have rarely been offered by public telephony providers.

Although QSIG is a non-proprietary standard, it has been implemented in various flavors making multi-vendor QSIG interoperability a challenge. In most of the cases, only a very basic set of supplementary services (e.g. name display, call completion on busy sunscriber / call completion on response, call forwarding) will work. However, there is a mechanism available in QSIG to carry proprietary information elements transparently between PBXs. This can be used by VoIP gateways to provide QSIG over VoIP trunking functionality that supports all propiretary features used by PBX system as long as all connected PBXs support the same proprietary QSIG dialect.

Fortunately, legacy PBX vendors have move from their entirely proprietary protocols to QSIG based protocols lately (Siemens Cornet-NQ would be an example).

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QSIG