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Centrex is provided by the carrier and resides on a very robust computer platform which is an integral part of the network. As a result, Centrex offers a number of unique advantages, including reduction of investment and operating cost, and avoidance of responsibility for system acquisition and management. Additionally, system capacity generally is not an issue, and the systems often can be networked to great advantage.
As the capital investment in the switching platform and associated application processors is the responsibility of the carrier, the user avoids the financial pain and associated risks of acquiring what can be a very substantial capital asset. Generally speaking, however, the user remains responsible for the acquisition and of deployment associated station equipment and inside wire and cable systems. Additionally, the user organization avoids a host of operating responsibilities and related costs, as those burdens are shifted to the carrier. Such responsibilities can include system maintenance, system administration (MAC activity), system upgrade, insurance, physical space, physical and network security, power and redundancy.
As the risk of ownership is shifted to the carrier, issues of obsolescence and replacement are obviated. Further, COs are relatively unlimited in terms of port, memory and processor capacity; therefore, it is unlikely that a typical user organization might outgrow the system. This characteristic of Centrex can be extremely important to businesses that tend to expand and contract, perhaps on a seasonal basis. Rather than investing in a PBX switch which is sized to accommodate the maximum size of the organization, additional lines can be added as required and disconnected as that requirement relaxes, with the associated capacity and telephone numbers perhaps being reserved until such time as the requirement resurfaces.
Finally, Centrex allows the user organization to network a number of locations through the serving CO, without the requirement for leasing circuits to connect multiple PBX or Key systems. In fact, virtual Centrex can be provided by networking multiple Centrex COs of the same type; this approach offers the advantage of allowing multiple, distant offices to communicate as though they were collocated and served by a single switch.
While Centrex offers a number of unique and very significant advantages over PBXs. there also exist clear disadvantages, including certain cost issues and considerations of control. In terms of the cost of the service, the recurring charges for the circuits and features continue forever, although special tariffs for very large customers can serve to mitigate this factor. Further, Subscriber Line Charges (SLC), also known as Customer Access Line Charges (CALC), apply to each local loop circuit or defined group of loops; as Centrex generally involves a separate local loop for each Centrex station, the typical Centrex user pays a higher price for local access than would a PBX user organization. Finally, rate stability has always been a Centrex issue, as all local rates may be subject to review and adjustment every few years. In many jurisdictions, rate stability plans have served to mitigate this concern, at least in the short term. PBX users, on the other hand, tend to be impacted to a lesser degree, as relatively few high-capacity circuits are shared on a pooled basis.
As we have noted, standard Centrex feature content is determined by the service provider based on market demand in consideration of the resources available in the serving CO switches and with the oversight of the state regulatory agencies. As the offered features are geared toward the average user, the overall feature set tends to be much less complete than is the case in a PBX environment. Therefore, user organizations with requirements for more exotic features may find Centrex to be unacceptable. Additionally, feature content and feature access may vary by carrier, regulatory jurisdiction, exchange CO, and switch origin (manufacturer and generic software load). Features such as ACD are unusual, while others, such as message waiting, are not well presented. Still other features, such as voice-over paging are not available at all. As a result, some users still place small PBXs, Hybrid KTSs, and other CPE (Customer Premise Equipment) systems on premise to interface the station equipment to the Centrex lines to extend more exotic features to select work groups.
Proprietary voice terminals (P-phones) generally are required, although the manufacturers finally have yielded to user pressure in the last few years and provided interfaces to nonproprietary sets. As P-phones are switch-specific, a multilocation company can face additional inventory costs associated with the requirement to stock multiple set types. User confusion also results, because users must deal with multiple set interfaces.
Perhaps the most significant disadvantage of Centrex revolve around issues of control, as the carriers and regulators determine the nature of the offerings. Special features require lengthy tariff approval processes, assuming that the user is successful in negotiating the support of the carrier and assuming that the regulators are sympathetic. Certainly, the user is dependent on the carrier for switch administration, maintenance, and enhancement. Considering that many LECs have a less-than-stunning reputation for responsiveness, this absolute level of dependency is of great concern to many companies.
The inherent cost and flexibility limitations of Centrex have given rise to a new breed of Centrex CPE which significantly enhances Centrex capabilities. Although the equipment must be matched carefully to the Centrex switch, such CPE includes Centrex-compatible electronic voice terminals (telephones), ACDs, voice processors, paging systems, attendant consoles, and message lamp controllers. Recently, set handlers, or mediation devices, have been developed which act as protocol converters to effect a proper interface between desired electronic station sets and the Centrex system.
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