QSIG (Q-Signalling) is a set of signalling protocols designed to enable communication between corporate PBX (Private Branch Exchange) systems in private digital ISDN networks. QSIG allows PBXs at multiple offices to exchange signalling information and provide telephony features as if users were part of a single corporate telephone network.
The technology was developed as an open standard to enable interoperability between telephone systems from different manufacturers. This was particularly important for large organizations with multiple offices where PBX systems from different vendors could be used within the same network.
QSIG was widely used in enterprise telephony before the large-scale transition to IP telephony and SIP.
How Does QSIG Work?
QSIG transmits signalling information between nodes in a corporate telephone network. Voice or other user data is carried over the appropriate communication channels, while signalling is responsible for establishing and terminating connections and managing supplementary telephony features. The QSIG architecture is based on ISDN standards and uses several protocol layers. In particular:
- Q.921 is used at the data link layer and is based on LAPD (Link Access Procedure for the D-channel)
- Q.931 provides the basis for signalling used to establish, manage, and terminate connections
- additional QSIG protocols carry information required for advanced enterprise telephony services
QSIG typically operates across the Q reference point, which connects private telephone network nodes within a corporate network.
For example, when an employee in one office calls an employee in another, QSIG allows the PBX systems to exchange the information required to establish the call and provide the associated telephony features.
What Features Does QSIG Support?
One of the advantages of QSIG is its ability to exchange not only basic call information between PBXs but also signalling data required for supplementary services. Depending on the equipment and implementation, QSIG can support:
- calling and called party identification
- call forwarding and call diversion
- call transfer between users
- call completion and callback features
- calling party name information
- additional enterprise call-handling features
Support for specific features depends on how QSIG is implemented by the equipment manufacturer. Compliance with the standard therefore does not necessarily guarantee full interoperability of all supplementary features between PBXs from different vendors.
Where Was QSIG Used?
QSIG was primarily used in Private Integrated Services Networks (PISNs). It enabled organizations to connect telephone systems located at different branches and sites into a single communications infrastructure.
For example, a company with offices in several cities could connect its local PBXs over dedicated lines. With QSIG, internal calls between offices could be handled as part of the corporate telephone system, while users could access supplementary features supported by the interconnected PBXs.
QSIG was particularly common in large enterprise networks, government institutions, university campuses, hotel complexes, and other organizations with distributed telephone infrastructure.
QSIG and ISDN
QSIG is closely related to ISDN (Integrated Services Digital Network), but the two terms are not synonymous. ISDN is a broader set of digital communications standards used, among other purposes, to connect telephone equipment to telecommunications networks.
QSIG was developed primarily for signalling between private telephone systems. It uses ISDN principles and protocols while adapting them to corporate networks and providing additional capabilities for communication between PBXs.
In other words, ISDN defines the broader technological environment for digital telephony, while QSIG addresses the specific task of inter-node signalling within a private network.
Is QSIG Still Used Today?
Today, QSIG is primarily considered a legacy technology. Modern enterprise communications systems mainly rely on IP telephony, Unified Communications, and SIP (Session Initiation Protocol).
However, QSIG can still be found in organizations that continue to operate traditional digital PBX systems. During gradual migrations to VoIP, gateways may also be used to integrate existing QSIG infrastructure with IP-based telephony.
Knowledge of QSIG therefore remains relevant for maintaining legacy enterprise telephone networks and for migration projects involving the transition from traditional telephony to modern IP-based communications platforms.