Qsig

Qsig is an inter-PBX signaling system designed for multiple PBX system platform networks. The proprietary nature of IFTN solutions restricted customer configuration flexibility to a single supplier’s product platform. Qsig in its current form originated during the 1990s as a standardization effort by the IPSN Forum, a group of Western European PBX equipment suppliers, with Siemens and Alcatel at the forefront of the movement. IPSN work efforts were handed off to the ECMA and the International Telecommunications Union (ITU) for the formalization of issuing standards and specifications. Qsig is based on the ITU’s Q.93x series of recommendations for basic services and generic features and Q.95x series for supplementary services.

The major benefits for developing Qsig were outlined in the Qsig handbook originally published by the IPNS Forum.

Vendor independence. The nonproprietary nature of Qsig, based on open international standards and supported by all of the leading global PBX suppliers, allows customers to configure an intelligent communications system network when using equipment from more than one supplier.

Guaranteed interoperability. A memorandum of understanding (MoU) signed by the leading global suppliers signifies commitment to Qsig specifications, facilitates interoperability performance tests, and assures customers that they will be able to operate communications networks with a mix of supplier equipment.

Free-form topology. Qsig does not impose the use of a specific network topology, so it can be implemented with any network configuration: meshed, star, main/satellite, etc. Existing networks can, regardless of their topology, be upgraded to Qsig. Newly designed networks can be installed with the most effective and economical topology.

Unlimited number of nodes. There are no nodal limits for a Qsig network. New nodes can be added as needed.

Flexible numbering plan. Qsig does not impose any number plan restrictions for the network, thereby allowing customers to freely adopt customized numbering plans.

Flexible interconnection. Qsig will work over any type transmission network for linking PBX systems, including two- and four-wire analog tie lines, digital leased lines (including ISDN PRI and BRI), radio and satellite links, and VPN services provided by interchange carriers. Associated transmission delays are managed and controlled according to Qsig specifications.

Public ISDN synergy. There is network service compatibility between public and private ISDN transmission facilities. Applications developed for desktop terminals connected directly to a public ISDN network will also be available to desktop terminals provisioned within the Qsig-based customer private network.

Supplementary services for corporate users. Qsig supports private communications features beyond those defined for public ISDN networks, including caller name ID, call intrusion, do not disturb, path replacement, operator services, mobility services, and call completion on no reply.

Feature transparency. Features and functions supported by any network node can be transparently supported across the network to station users configured behind other network nodes. Qsig is structured and organized to adapt to service levels offered by different PBX systems, and it allows each network node to provide only the required level of service. There is an exchange of high-level services between any two nodes, via transit nodes with lower service levels: transit nodes pass communications and control signals between systems.

Innovation. Qsig does not restrict individual PBX manufacturers from developing customized, unique features. A special mechanism within Qsig, generic functional procedures (QSIG GF), provides a standardized method for transporting nonstandard Qsig features. As defined in Qsig GF, the basic rules related to feature transparency allow end-to-end communication through the network, regardless of network structure. Qsig does not prevent the use of innovative, proprietary system features across the customer network and allows for customized new feature development negotiation between PBX suppliers and customers.

Multiapplication domain. Qsig is not restricted to PBX systems and can support applications requiring other peripheral communications equipment, such as VMS, fax servers, data processing equipment, and multipoint conferencing systems.

Evolution. Qsig has an evolutionary path to support communications features, functions, and applications that are developed in the future.

Qsig Architecture

Qsig standards specify a signaling system at the ITU-T ISDN “Q” reference point, which is intended primarily for use on a common channel, although Qsig can be implemented over any suitable inter-PBX connection platform. The “Q” reference point, the logical signaling point between two PBXs, was defined explicitly for the Qsig. The physical connection point to the PBX system is made at the “C” reference (also a new ISDN reference point). There are three sublayer protocols at Layer 3, including the Qsif GF procedures. Qsig GF protocol provides a standardized mechanism to exchange signaling information for the control of supplementary services and additional network features (ANFs) over customer networks.

Qsig basic call (BC) message sequence is an intermediary transit node linking two endpoint PBX systems. Qsig BC is a symmetrical protocol designed for peer-to-peer networks, and it includes transit node capability.

ECMA also has been working on enhancements to its Qsig specifications to support broadband PBX networks. B-Qsig is an extension of Qsig, using many standards as possible available from the ITU-T and ATM forums.

Qsig Supplementary Services and ANFs

The following is a listing of the Qsig supplementary services and ANFs:

• Advice of charge

• Call completion

• Call forwarding and diversion

• Call interception

• Call intrusion

• Call offer

• Call transfer

• Call waiting

• Direct dialing in

• Do not disturb

• Identification services

• CLID presentation

• Connected line identification presentation

• Calling/connected line identification restriction

• Calling name identification presentation

• Calling/connected name identification restriction

• Mobile

• Multiple subscriber number

• Operator services

• Path replacement

• Recall

• Subaddressing

• User-to-user signaling