Multiple System Private Networking

PBX private 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, such as an E&M tie trunk, to save on long distance toll charges. The primary benefit was cost savings. During the late 1970s more complex private tandem network configurations were made available, consisting of a meshed network of private line facilities linking tandem switch PBX nodes, main PBX nodes, and satellite systems. 

AT&T’s first modern private PBX networking option was called Enhanced Private Switched Communications Service (EPSCS). First tariffed in the mid-1970s, it was quickly replaced by the better known and higher-performance ETN offering in the late 1970s. AT&T’s innovative PBX private networking option was initially proprietary, but other leading PBX manufacturers at the time, including Northern Telecom (ESN), NEC (EPN), and Rolm (RolmNet), soon offered similar PBX options. After a few years, the competitive PBX offerings became compatible with ETN. By the mid-1980s, a customer could configure a network with a mix of PBX tandem switch nodes from multiple manufacturers.

The original ETN options were based on in-band signaling techniques supporting 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 done initially with narrowband analog trunking facilities. The availability of digital T1-carrier trunk services in the mid-1980s would soon change the rules for PBX networking because in-band signaling would be replaced by out-of-band signaling.

The next step up the evolutionary PBX networking ladder was intelligent network signaling to support transparent feature/function operation between discrete locations served by independent PBX systems. AT&T’s DCS was the first intelligent, feature-transparent PBX networking option. The first implementation of DCS required an expensive private data circuit (a 9.6-Kbps DDS line) for the intelligent signaling link between PBX network nodes, but digital voice carrier services using T1-carrier circuits made out-of-band signaling a more economic and feasible solution for implementing intelligent PBX networks. Out-of-band signaling channel PBX systems could communicate with one another at a much higher level than before. The resulting intelligent network configuration would continue to offer customers traditional network transmission cost savings but also provide significant productivity gains and additional cost savings through the use of shared application features/ functions. Today’s intelligent private networking options may be configured with a variety of digital trunk services based on a variety of transmission and signaling protocols, including ATM and IP.

Multiple PBX system private networks are designed and implemented for a variety of reasons:

1. Reduction of communications expenses (transmission services, maintenance and administrative support)
2. Improved management and control of communications operations
3. Enhanced communications capabilities among station users
4. Shared applications resources

Reduced costs are always a motivating factor for any business venture, and a private network configuration can have a significant effect on monthly telecommunications service bills and personnel outlays. With private lines or a virtual network service offering, PBX networking features can cost-effectively route calls between endpoints over least-cost routes, minimize trunk circuit requirements, monitor and diagnose transmission carrier facilities for efficient operation and problem areas, and minimize support personnel through centralized management centers. PBX traffic carried over private line facilities incurs a fixed monthly cost regardless of the amount of traffic. Private tandem networks are based on concentrated trunk carrier interfaces at select switching node locations; large trunk groups translate into greater traffic handling capacity per individual trunk circuit. If a virtual networking service is used, interexchange carrier tariffs reward customers by decreasing billing costs as traffic volume increases, making it an attractive alternative for customers who cannot financially justify dedicated private line facilities.

When the customer communications system infrastructure expands in size and geographic scope, it is vitally important that there is a sense of uniformity across the network. Network heterogeneity requires fewer management tools and reduces personnel and training expenses. Network heterogeneity allows for the dispersion of system administrators across the enterprise, with the capability to provide individual administrators with global responsibilities regardless of their physical locations. Dispersing administrators means fewer administrators because there is no need to have local administrators dedicated to a single system. Network administrators can be centralized in a single location for an entire network of communications systems or distributed among several strategic locations, according to the organizational policies of the corporate enterprise.

The fundamental characteristic of a centralized network management center is the support of multiple communications systems and associated administrative/maintenance functions. Integrated network management service tools include fault management, system administration, traffic analysis, directory services, call accounting, support inventory management, and database import/export. Centralization of resources, combined with remote management services, can reduce personnel requirements, the largest single cost item in operating a communications system network.