PBX networking has evolved dramatically during the past 25 years. The earliest PBX networking arrangements consisted of two switch nodes linked by a dedicated, private line facility (E&M tie trunk) to save on long distance toll charges. The primary benefit was cost savings. When customers began to use multiple long distance carriers in the late 1970s it became necessary for PBX systems to analyze each placed long distance call to determine which carrier service should handle the call.
The preferred carrier service was usually the lowest priced for the call. A new PBX feature was developed and known by several names, including least cost routing (LCR), ARS, and most economical route selection (MERS). Implementing the feature required the system administrator to enter the call destination route and routing pattern data into a database that would price each call based on tariff pricing data. The tariff data was obtained through a service bureau and needed to be updated regularly.
The benefit to the customer was to reduce long distance toll expenses. Expensive calling routes were restricted to callers only with permitted network classes of service levels; callers with the lowest service level rating could place calls only when the lowest cost route was available.
Also in the late 1970s, AT&T announced its Electronic Tandem Network (ETN) offering, and PBX networks acquired a greater degree of complexity and functionality. ETN was a private tandem network consisting of a meshed network of private line facilities linking tandem switch PBX nodes, main PBX nodes, and satellite systems. In-band signaling techniques supported a network dialing plan and automatic alternate routing between nodes within the network. In addition to cost-savings benefits using fixed tariff private line carrier facilities, customers enjoyed greater control over network operation and use. All of this was initially done with the use of narrowband analog trunking facilities.
The next step up the evolutionary PBX networking ladder was establishing an intelligent network signaling to support transparent feature/function operations between discrete locations served by independent PBX systems. AT&T’s Distributed Communications System (DCS) offering was introduced in 1982 for its Dimension PBX. The first intelligent signaling link required an expensive private data circuit; analog private lines were used to carry voice traffic. The DCS intelligent networking solution allowed customers to use simplified dialing plans (i.e., four- or five-digits) for calls across PBX systems; supported transport of caller name/number display information between telephone desktops working behind different switching nodes; and provided a basic level of transparency within the network for many of the most commonly used station features, such as call transfer, call forwarding, and multiparty conferencing. Shared applications were also supported across a network of PBX systems, such as centralized voice messaging.
The arrival of digital T1-carrier trunk services in the mid-1980s changed the rules for PBX networking because in-band signaling was replaced by out-of-band signaling, and new networking solutions became possible. The same digital trunk circuit used for voice traffic could also be used to support the intelligent signaling link between PBXs. Digital voice carrier services using T1-carrier circuits made out-of-band signaling a more economic and feasible solution for implementing an intelligent network of PBXs. Use of an out-of-band signaling channel allowed PBX systems to communicate with one another at a much higher level than before. The resulting intelligent network configuration could offer customers traditional network transmission costs savings and provide significant productivity gains and additional cost savings through the use of shared application features/functions.
Each PBX manufacturer’s intelligent networking solution was proprietary and caused problems for customers with a mix of PBX systems in their networks. An initiative was begun in the late 1980s in Europe by the leading PBX suppliers to create a standardized network signaling protocol to intelligently link dissimilar PBXs. The signaling standard is commonly known as Qsig, and was originally developed under the auspices of the ISDN Private Network Systems (IPNS) Forum. PBX systems that support Qsig can interwork intelligently with each other; support basic call set-up and tear-down across the network between dissimilar PBX system platforms; conform to a common dialing plan for limited digit dialing across PBXs; transmit and accept telephone display information, such as calling name and number, and call redirection data between desktops; and sup- port feature-transparent operations for a defined set of features, such as call forwarding, call transfer, conferencing, and network attendant service. Although most leading PBX suppliers support Qsig as part of their networking solutions, the degree of transparency between systems remains limited. Manufacturers must do continual testing of their systems to correct Qsig message and signaling problems.
PBX networking advancements in the late 1980s included support of ISDN PRI services. ISDN PRI service circuits became the preferred trunking solution for implementing an intelligent feature-transparent network because the D channel was a natural communications channel for handling signaling and control data across distributed PBXs. New PBX networking features based on ISDN PRI services included support of incoming ANI, and call-by-call service selection (CBCSS). CBCSS allows a PBX system administrator to define the communications service supported by individual ISDN PRI bearer communications channels. A single T1-carrier trunk circuit, supported by ISDN PRI service, could be used for a variety of services between the PBX system and the network exchange carrier’s central office switching system. For example, several bearer channels could be designated incoming DID trunks, others could be designated two-way CO trunks, and others could be designated clear channel data circuits. Using programming tools, the administrator could reconfigure the mix of trunk services on demand or by a schedule, or could even program the channels to reconfigure themselves based on real-time traffic conditions.
Network carrier services in the 1990s designed to support data communications were also supported by PBX systems. Nortel Networks redesigned its Magellan Passport Asynchronous Transfer Mode (ATM) switching system as a Meridian 1 gateway module to support a mix of voice, data, and video communications over broadband trunk carrier circuits. Lucent Technologies, NEC, and Siemens also introduced ATM network interface options for their respective PBX systems. One of the most important PBX networking advancements in the late 1990s was the introduction of external IP telephony gateways, closely followed by integrated IP trunk gateway port circuit cards. Lucent Technologies was the first to market an integrated IP trunk option and was closely followed by other market leaders, including Nortel Networks and Alcatel.
