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Centrex

As a concept, Centrex (central exchange), is something of a step back in time, providing PBX-like features through special software loaded into the Central Office Exchange. Generally speaking, each Centrex station is generally connected to the central exchange via an individual twisted pair local loop (see Figure 4.7). While multichannel local loop connections are available, they require an expensive remote CO line shelf to be installed at the customer premise; therefore, multichannel access is cost-effective only in applications where there exist large numbers of users at a single site. All switching of calls is accomplished in the CO, which also serves all features to the users population.


Figure 4.7  Centrex CO with individual local loops to Centrex stations and with remote CO line shelf.

Centrex first was made available in the United States and Canada in the early 1960s, through SxS offices. In the United States, it was de-emphasized by AT&T and GTE in the 1970s, in favor of PBXs. This was largely to impending deregulation and divestiture, which took full effect on January 1, 1984. Modern Centrex became available in 1984 through the first generation of digital COs [4-13]. AT&T immediately reversed its position, developing sophisticated digital Centrex software for marketing to its former subsidiaries, the Bell Operating Companies (BOCs), as well as to the independent telephone companies. In addition to Lucent Technologies, Nortel and Siemens are notable manufacturers of Centrex COs [4-3].

Although most commonly provided by Local Exchange Carriers (LECs), Centrex service recently has been offered by IntereXchange Carriers (IXCs) and Competitive Access Providers (CAPs) as well. Although traditionally a U.S. domestic phenomenon, Centrex more recently has found market acceptance in the U.K., Europe, Japan and much of Asia. Centrex is attractive to the carriers, as it effectively competes against PBX technology provided by third parties. The great majority of CO Centrex systems installed in the United States are manufactured by Lucent Technologies (5ESS) and Nortel (DMS-100) [4-14].

Centrex Features

Centrex is capable of offering the same feature content as PBX technology. Indeed, the platform inherently is much more substantial in terms of traffic capacity, memory and virtually every respect. Ultimately, however, the carrier must consider the general market for such features, given an appropriate pricing strategy. Additionally, the impact of features on the switch must be considered in terms of memory, traffic, processor load, etc. Finally, the regulators all too often get involved in determining pricing, availability, impact on the rate base and other factors. Therefore, not all technically feasible Centrex features are available; certainly not on a consistent basis. In any event, Centrex features can include Direct Inward Dialing (DID), Automatic Route Selection (ARS)/Least Cost Routing (LCR), call pick-up groups, call forward, conference calling, Automatic Call Distribution (ACD), and Call Detail Recording (CDR). Centrex also generally is capable of data switching, which may effectively deliver CO-based LAN functionality. Additionally, the Centrex CO generally houses a voice processing system or provides remote access to one, thereby delivering highly effective voice mail and other related functionality [4-15] and [4-13].

The Centrex CO also may offer access to a large number of CLASS (Customer Local Access Signaling Services), which best can be described as advanced custom calling features. Such services might include Name Delivery, Calling Number Delivery, Calling Number Blocking, Continuous Redial/Repeat Dial/Automatic Recall, Distinctive Ringing, Call Return/Automatic Callback, Call Trace, Selective Call Screening, Selective Call Forwarding, Selective Call Rejection, and Anonymous Call Rejection [4-15].

Finally, Centrex offerings increasingly include customer rearrangement capability. This feature allows the user to manage its Centrex service as it might manage its own PBX. Through a computer terminal located on the customer premise, access is afforded to the switch database on a partitioned basis. Thereby, the system administrator can accomplish Moves, Adds and Changes (MACs); disconnect Centrex stations; change Class of Service (COS); file trouble reports and work orders and monitor their status; and perhaps even perform various system and network diagnostic tests. Leading providers are Bellcore with the CCRS system and CommTech Corporation, previously AT&T, with the Macstar system [4-16].


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