IP Centrex System : Cisco Systems & Sylantro Systems

Cisco Systems
Cisco Systems' CallManager server had been installed by at least three service providers as of mid-2002, to provide IP-Centrex services. Pingtel, one of these competitive service providers, delivers IP-Centrex from CallManager installations in New York City, Philadelphia, and Washington, DC. This service has proved most successful with small businesses that have from 100 to 500 users at a site.

This is a development that we expect will be followed by other service providers, both large and small, as an alternative to the use of more powerful softswitches from the established Centrex manufacturers.

Sylantro Systems
Sylantro Systems sells an application switch that offers hosted PBX and IP-Centrex features to competitive service providers. Sylantro claimed to have sold systems with more than 10,000 user ports within the first year that its system was available.

Tacqua Systems
Tacqua Systems has created its OCX softswitch with class 5 capabilities, specifically so that CLECs can deliver local telecom services with significantly reduced initial and ongoing costs of ownership. Tacqua delivers a kernel of telephony features, while encouraging other software developers to use its intelligent network (IN), application programming interface (API), and SIP interfaces.


IP-Centrex System : Siemens ICN

Siemens ICN sells its Centrex product under the name GeoCENTREX. GeoCENTREX is based on the Siemens EWSD central office switch. The EWSD is the most widely sold such system in the world, supplying 250 million analog and digital ports. A single EWSD system can process 16 million BHCAs, have up to 240,000 ports, and handle 100,000 Erlangs of traffic. Siemens holds a distant third place in the North American market and is still a minor player in the Centrex field.

Siemens intends to provide an end-to-end IP-Centrex solution through a combination of its own technology and vendor partnerships, as shown in Figure 3.5, which illustrates the use of an IAD to link analog stations or KTS/PBX systems to IP-Centrex.

Figure 3.5: Next generation local switch services.

In mid-2002, Siemens sold its controlling share in Unisphere to Juniper Networks and is no longer directly involved in the data router and switch market. However, since Siemens' IP phones work to open standards, the customer and service provider could use any standards-based router. Siemens has retained the name of the Unisphere SRX for its family of softswitches and obtains its IP-Centrex software from Broadsoft Inc.

The Broadworks package can be deployed in a totally packet-based architecture such as full IP-Centrex or work as an extension of a preexisting class 5 switch. The system can support more than 1 million customers per network and process up to 6 million BHCA, based on a cluster of Sun computers, divided into the three classes of application, media, and network servers.

This software enables service providers to manage a nationwide IP-Centrex system centrally, delivering a uniform service to distributed enterprises and optimizing network management resources.

The Broadworks package provides Web portals for five types of service management:

- Service providers with complete system administration capabilities;

- Resellers that can create and manage their own groups of customers, under contract with the service provider;

- Enterprise administrators with access for configuration and management of multilocation dialing plans, for specific organizations:

- Group administrators, within one enterprise, that can perform modification, addition, and deletion of features and users;

- End users within a calling group that can personalize calling services and features.


IP-Centrex System : Nortel Networks

Nortel Networks is the world's largest vendor of Centrex systems and has been responsible for most of the development of these services outside North America over the past decade. There are nearly 100 million lines on Nortel's DMS switches worldwide and of these well over 10 million are for Centrex. Nortel has announced a family of softswitches under the name of Succession, based on Sun computing platforms. The CS2000 can process 300,000 BHCA with a pair of servers, supporting both asynchronous transfer mode (ATM) and IP, together with H.323 and SIP standards. Nortel's concept of IP-Centrex, delivered either from a legacy DMS switch through gateways and from the CS2000 softswitch, is illustrated in Figure 4.

Figure 4: Centrex IP for extended service reach. (Reproduced with the permission of Nortel Networks. Copyright © 2002 Nortel Networks, All rights reserved.)

Nortel Networks conducted a number of IP-Centrex technology trials in North America and Europe during 2001 and 2002. Then, in late 2001, Telstra announced that it would offer CustomNet IP in the major cities of Australia, using Nortel-made gateways to link IP-based endpoints to the existing CustomNet (i.e., digital Centrex) service. Telstra stated that CustomNet IP would also use data services, such as dial connect, cable, asynchronous digital subscriber line (ADSL), or ATM, to provide access to its DMS-based CustomNet. This is the first product set to be released under Telstra's Next Generation Telephony project, but the actual start-up date for this service was not announced.


IP-Centrex Systems : Lucent Technologies

Lucent Technologies is the second-largest supplier of Centrex systems, based on its 5ESS digital central office, which has been installed in a number of countries. Lucent made an early start in 2000 with its iCentrex product, which is based on the iMerge gateway, extending Centrex over IP and wireless networks. The iCentrex solution is actually switch independent and could work with other vendors' central offices. This product allows for the use of a mobile phone with Centrex dialing and other features when the user is in the Centrex service area. Sharing one voice mailbox between wireless and wireline Centrex services is also possible, with more than 1,000 wireless handsets feasible in one building.

Lucent claims that its 5ESS switch has the least downtime of any CO system in the U.S. public networks. The iCentrex development relies on the 7R/E Packet Driver software to deliver full PBX-like parity, including call transfer to voice mail and auto attendant. The wireless Centrex features work with either code-division multiple access (CDMA) or time-division multiple access (TDMA) technologies.

The iMerge Centrex Feature Gateway (CFG) enables users to enjoy worldwide Centrex service, as emphasized in Figure 3.

Figure 3.3: Worldwide IP-Centrex.

Lucent identifies the benefits of worldwide IP-Centrex as including a single dialing plan and uniform telephone system at all locations, including mobile and at-home workers, as well as branch plants or offices. IMerge enables a customer to be freed from service area boundaries with the delivery of existing Centrex and new services across a packet network.

Based on the encouraging results from field trials of Lucent's iMerge product SBC Communications launched commercial IP-Centrex service in September 2002, starting with availability in five major cities across the United States. SBC claimed that it was the first major carrier to bring these new IP-Centrex capabilities to the market.


IP-Centrex Systems : Ericsson

Ericsson became interested in Centrex over 10 years ago, when the service was first introduced to Europe, but has not become a significant player in wireline Centrex. However, Ericsson makes three major contributions to the overall IP-Centrex market, as follows:

- A product known as G-Nexus, or GSM Mobile Centrex, delivers PBX-like functionality over a public mobile phone network, using GSM technology, which is the most widely deployed wireless standard in the world. Ericsson-made equipment supports over one-half of the world's GSM users, and its G-Nexus has IP access facilities, with a managed IP network between the enterprise and the Centrex node at the network operator's site. This mobile or fixed access to G-Nexus uses an H.323 media gateway to link to the AXE-based Centrex node, providing business group communications. The latest version of Ericsson's central office, the AXE Local 7.2, is a UNIX-based system that is really a transition product, partway between a legacy circuit switch and a softswitch. Ericsson's softswitch product, known as the IPT 2.1, has an all-IP architecture, supports H.323 protocol, and is intended as a signaling processor for multimedia service delivery. This is illustrated in Figure 2.

Figure 2: Ericsson's IPT 2.1 domains.

- Ericsson was an early starter in the VoIP (transmission) market during 1997 and is now one of the largest suppliers of VoIP gateways.

- Ericsson has become the most successful supplier of in-building wireless phone sets, which can work with PBXs and Centrex services from other suppliers, such as Siemens and Nortel.


IP-Centrex Systems : Alcatel

Five major telecommunications equipment manufacturers, namely Alcatel, Ericsson, Lucent, Nortel, and Siemens have been able to supply Centrex solutions, based on their respective digital CO systems, for the last decade. Three other significant public exchange manufacturers—Fujitsu, Marconi, and NEC—have not supplied Centrex software in the past and do not appear to be interested in delivering IP-Centrex systems (although NEC was one of the early suppliers of IP-PBXs).

The product descriptions that follow are not intended to be exhaustive but should give a good indication of what systems are available to deliver hybrid and full IP-Centrex from a number of vendors. The hybrid, evolutionary approach extends the life and depreciation time of legacy class 5 switches, but retains proprietary technology in the network and limits the integration of new applications. The softswitch-based, full IP-Centrex, approach can be disruptive for the service provider and its customers. This approach may also be risky if the softswitch comes from a smaller company that has had time only to develop a limited set of the popular Centrex features.

Alcatel includes a Centrex offering on its model 1000 central office, but at the time of this writing had not made any announcement regarding IP-Centrex. Alcatel is not a major vendor of Centrex systems; it has implemented a limited form of Centrex in a few countries and planned for Centrex service with several South American telcos.

The company has developed the powerful 5000 series of softswitch platforms, of which the 5020 is a signaling server and the 5424 is designed for off-loading switched data traffic from the voice network to the Internet. The model 5000 is a class 5 switch server, with a typical network topology as shown in Figure 1.

Figure 1: Network topology with Alcatel 5000 softswitch.

This 5000 softswitch can process 5 million busy hour call attempts (BHCA) and grow to 1 million virtual ports, delivering voice over IP or ATM and interfaces to the SS7 network. The softswitch in Figure 3.1 is shown with integrated access devices (IAD) on the customers' premises, providing 16 voice channels over one digital subscriber line (DSL).


Deployment Strategies for IP-Centrex

The arrival of credible IP-Centrex services presents different scenarios to corporate decision-makers, depending on the state of their telecom infrastructures and on the alternative offerings in their marketplace. These situations fall into one or more of three classes:

1. Controlled Migration to IP-Centrex. Many organizations will wish to add IP phones to an existing Centrex configuration (i.e., the hybrid approach). Two possible reasons for this might be:

- To implement a contact center as a managed service;

- To extend fully featured, multimedia service to remote or mobile worders.

For these solutions the organization will need to use a wellmanaged IP network, which may be provided by the ILEC, that currently operates the Centrex service, or may be from another service provider (e.g., an international telco with a wider-ranging network, such as AT&T or BT).

The use of two or more IP-WANs presents a further choice to the customer, as the customer may prefer to have access directly to the competitive long-haul network through a switch at the customer's site, rather than via the IP-Centrex service provider's own network access. This alternative is shown in Figure 1.

Figure 1:Controlled migration to IP-Centrex.

2. Greenfield IP-Centrex Implementation. On the reasonable assumption that the average digital PBX needs to be replaced (not just enhanced) after a 10-year life, or that long-term Centrex contracts never last more than a decade, about 10% of all organizations will face a crucial greenfield telephone services decision each year.

3. Move to a Competitive Service Provider. Many organizations obtain their Internet or VPN connections through an ISP and/or rent Web server hosting facilities from a service provider other than the incumbent telco. In many of these cases this service provider is a good candidate to become the customer's communications application service provider (CASP), in preference to the ILEC.

When considering whether or not to entrust its in-house voice and video communications to a CASP, an organization's management must evaluate the following topics:

- The specific IP-Centrex design being deployed by the CASP;

- The match between the feature set available, from the competitive IP-Centrex service, and that needed by the users;

- The local access technology being offered to the WAN and its scalability to accommodate growth (or, occasionally, contraction) of bandwidth needs;

- The perceived financial stability of the service provider, in a time when the finances of some long-established, large telcos also have been seriously questioned.


Network Considerations for IP-Centrex

Requirements for VolP
The IP-based systems and networks that will be used for voice or video transmission over IP must meet several criteria to deliver a signal of the same quality that has been available through circuit-switched, digital systems and with which the users are satisfied.

Because of the real-time, interactive nature of VoIP the average transit time (i.e., latency) between source and destination is most important. As shown in Table 2.1, ideally one-way latency should be less than 100 ms. If the round trip delay on a voice conversation exceeds three-tenths of one second, then it becomes difficult to continue an intelligent dialog, as many have experienced when a telephone call was routed through a geostationary satellite.

Jitter is the variability of packet arrival times at the receiver and is generally caused by large bursts of data interfering with the real-time traffic in the network. Although modest jitter delays have not been considered serious to real-time conversation, yet significant jitter (i.e. delays approaching 60 ms) leads to unintelligible speech patterns and ruins video displays.

Packet loss corresponds to link dropouts, which is a common experience with mobile/cell phones in congested areas and also results in unsatisfactory voice or video communications.

The bandwidth, or bit rate, requirements for VoIP depend on the type of encoding/decoding (i.e., codec) technique that is used in the digital signal processor (DSP) in the end stations. A reasonably good guideline, with current technology, is to allow 20 Kbps in the network or the access link for each simultaneous voice conversation, or 200 Kbps for a small-group video conference session.

LAN Requirements
In a large office complex there may be a correspondingly large number of simultaneous voice calls on the LAN, in the busy hour of the week. This traffic volume depends on the peak telephone utilization level in the organization, which may be 25% in many offices, but rise to nearly 100% of the population in some highly sales-oriented businesses, such as a stock brokerage. Even if there are 1,000 concurrent voice calls on the LAN, this probably represents an added load of only 20 Mbps, which is insignificant on a gigabit backbone.

However, if the in-building network is to carry IP-based video traffic, as will increasingly be the case as multimedia applications are implemented, then a major LAN upgrade is much more likely to be needed. If we assume a minimum bit rate of around 200 Kbps for two-way video interaction and up to 2 Mbps for higher definition, full motion, video displays, then a number of simultaneous video sessions will put a heavy load on the LAN. In some early installations for IP-based video conferencing a separate LAN that went all the way to the main edge router for the building was dedicated to the video outlets.

WAN Requirements
An existing metropolitan area network (MAN) or WAN will certainly be more challenged by the added burden of carrying VoIP packets than a LAN. The transit and jitter delays and bit rate requirements will each have to be seriously addressed across all stages of the larger network. Generally a data network that runs on frame relay (FR) technology will not meet the latency criteria that we have specified. There may be some exceptions if the FR virtual circuit involves only two frame switches. The regular Internet is definitely not suitable for corporate VoIP traffic and only a managed network can meet the requirements.

A managed IP network is one where the service provider(s) monitor and control the QoS, outside the customer's own environment, to minimize latency and jitter, while ensuring that sufficient bandwidth is available for busy traffic. In a managed network the VoIP packets pass through the least number of nodes and the QoS is guaranteed, going beyond just "best effort" engineering.

The characteristics of the access links between the LANs and WAN must also be well defined and managed to fit within the overall criteria budget from source to destination.


Softswitch Characteristics

A softswitch is a high-availability computing platform that houses software to control multimedia traffic over an integrated telecom network and mediates the signaling between packet and circuit-switched networks (i.e., between the IP-based and the legacy TDM domains). Softswitches may be considered software-based replacements for class 5 (local exchange) and class 4 (toll exchange) central offices.

These systems promise an excellent opportunity for service providers to deliver new, innovative broadband services, while reducing equipment and personnel costs by up to 50%, compared with conventional, digital CO systems. The legacy switches, made by the eight major telecommunication equipment manufacturers, are highly proprietary in nature.

A softswitch should reside in an extremely reliable (i.e., extensively redundant), industry-grade, rack-mounted server in a secure network data center. Ample power supply and network access backup capabilities are also essential. Softswitches work in close conjunction with modular media gateways and, partly because of the physical separation of control and packet-switching functions, can be scaled in capacity to meet quickly changing traffic patterns more easily than with older circuit switches.

A significant feature of softswitches is the facility for a system administrator to install and manage all the telecom services of the switch through a Web portal. In this users are offered a much better way to control their own network, such as ordering new services, click-to-call on any numbers, modifying call forwarding, and customizing individual telephone sets (whether real or virtual). For these reasons, managing telecom services will become much easier than it has been with on-premise PBXs or off-site Centrex services. With a softswitch it should be just as simple to administer video-conference services as voice calls, which will be a big improvement over legacy class 5 CO capabilities. Some of these softswitch entities are illustrated in Figure 1te.

Figure 1 :Softswitch configuration.

Carrier grade softswitch products have been on the market since late 2001 from established manufacturers, such as Nortel Networks and Siemens, as well as recently started companies including Sonus and Tanqua Systems. Most of the newer companies in this line of business belong to the Softswitch Consortium.

The early applications of softswitches were for off-loading Internet traffic, from more expensive and busy CO systems, primarily with DSL on the line side and IP or ATM on the trunk side. Going forward, the role of softswitches will be much more to replace conventional central office systems and to deliver multimedia IP-Centrex services. Softswitch availability is the key for the development of hosted telephony services by the competitive service providing companies and for the capability of putting all an organization's voice, video, and data traffic onto a single network.


Centrex or PBX : IP Telephony

For several years the term Voice over Internet Protocol (VoIP) has been used to identify the transmission of voice signals over any IP-based network. In the business context, VoIP now refers to the transmission of voice and/or video communications over a managed WAN or LAN.

Some consultants and one or two telecom manufacturers have used the term telephony over Internet protocol (ToIP) to describe the switching of real-time, conversational traffic in systems that are attached to IP data networks.

The words "IP telephony" have emerged as the umbrella term that covers both VoIP and ToIP, which can be delivered either by an IP-PBX (housed on the customer's premises) or by IP-Centrex (for which the call processor is owned and accommodated by the carrier).

Some organizations have been using VoIP to reduce the cost of longdistance service, particularly international service, for several years. In countries that have highly competitive interexchange carriers it is generally no longer worthwhile to use VoIP, after allowing for the cost of gateway hardware and software. Several international carriers, such as the T-Systems division of Deutsche Telekom, have specifically promoted the use of IP networks for international traffic.

IP telephony has been validated since the year 2000 by the availability of some IP-PBXs. An IP-PBX system or IP-Centrex service may deliver only VoIP (through appropriate trunk and line interfaces), only ToIP (by retaining interfaces for the existing analog and digital devices), or both in the form of IP telephony. Now that IP has become, by far, the most popular protocol for data transmission and is widely deployed in networks of all sizes, from one room to worldwide, organizations have access to the appropriate transport technology to gain the advantages of convergence. Full convergence between multimedia and data applications requires the availability of both VoIP and ToIP, which are both inherent to the concept of IP-Centrex.

Interface Standards
The key to success with IP-Centrex will be a high level of interoperability between devices and systems or applications. This will be a major move away from the proprietary interfaces that have kept acquisition costs up and made application implementations complex with PBX systems. The acceptance of a limited number of open standards will also facilitate more competition between a broader range of manufacturers and service providers.

The earliest call control standard for mapping users' names or telephone numbers into and IP source or destination addresses was H.323, which was adopted by the International Telecommunications Union (ITU). H.323, was originally intended to define how multimedia communications were to be transmitted over a data network between teleconference units and, as used for VoIP, defines only a restricted feature set, with different enhancements being made by various manufacturers. H.323 does not define transmitted voice quality, is considered to be too processing intensive, and depends on the use of intelligent workstations.

A working group of the IETF created the session initiation protocol (SIP) to lessen call setup times and take better advantage of the Internet infrastructure than H.323. SIP is most likely to become the interface standard of choice between telephone sets and computers with ToIP systems.

A third standard, officially known as H.248 but more generally as MGCP or Megaco, is being jointly developed by the IETF and ITU, with support from some, but not all, major manufacturers. The H.248/ MGCP protocol addresses the needs of multimedia conferencing and is intended for use with media gateway controllers (MGC).

Every communicating device on an Internet protocol-conforming network must have an IP address, so that a desktop with a telephone, a PC, and a softphone (within the PC) or a video terminal needs to be allocated three addresses. There must be a process with the network for mapping telephone numbers to corresponding IP addresses.

Also, any endpoint's address must be known in order to be accessible; in some circumstances, this requirement becomes a security concern. This situation can be problematic in that multiprotocol label switching (MLPS), which is widely used in managed wide area networks (WANs), does not allow for any "spoofing" (i.e., the alteration and retransmission of any part of a signal in order to hide the address contents and therefore discourage hacking).


Blurring the Boundaries

The advent of IP-Centrex will blur the clear distinction between Centrex services and PBX systems in two important ways. With conventional Centrex, call control and switching take place in the CO on telco premises, or in the generally much smaller PBX, both processes occur on the customer's premises.

In a fully IP-Centrex environment, call control remains at the controller in the carrier's CO building, but much of the call switching is done in systems (such as Ethernet switches or routers) on the customer's site. The "payload" packets for an IP telephony call between two phones within the same building will not travel outside the building. In current Centrex systems all voice traffic flows over the local loops from the user's phone to the CO and then either back into the building to another user or off to the PSTN.

The second change, which is already happening, is that service providers (including smaller competitors) can acquire IP-PBXs to provide Centrex-like services, rather than buying a much more expensive CO system. In this way the barriers to entry into the IP-Centrex business are dramatically lower than they were previously.


Development of Internet Protocol Centrex

Ever since Centrex was launched 40 years ago, it seems that technological advances in telecommunications have occurred in premises-based PBX systems before being adopted for Centrex services. This trend is now apparent with Internet protocol-based telephony products, in that IP-PBXs were being sold 2 years before beta testing of Internet Protocol Centrex (IP-Centrex) was really underway. In late 2002 several telcos were moving from pilot trials of IP-Centrex to the early stages of its service delivery.

In the several years that IP telephony PBX systems have been on the market, they have sold in modest numbers and supported no more than 1% of all PBX-attached stations in the United States by the end of 2002, with a smaller impact in other markets. To some extent, this slow rate of acceptance is certainly due to the overall slackening of investment in telecom equipment since the run-up to the year 2000. However, another serious factor is that the advantages of the IP-PBX over conventional, circuit-switched [or time-division multiplexing (TDM)], PBXs are not obvious to most telecom or IT managers.

This relatively slow acceptance of the IP-PBX is not necessarily a good indicator of the future of IP-Centrex, once that becomes a stable, fullfeatured service that is intelligently and aggressively sold by the service providers, both ILECs and CLECs.

IP-Centrex offers all the advantages that have been well advertised for premise-based IP telephony systems (e.g., ease of administration and ease of access to corporate, LAN-based applications), combined with the present advantages of Centrex (i.e., the complexity of design; implementation and management being outsourced to the telco with the voice switch placed inside the perimeter of the public network).

We are convinced that IP-Centrex will bring a revolution in the delivery of corporate voice communications and that within 10 years as many business-related endpoints will be supported by IP-Centrex services as by premises-based switches.

The package of advantages that can be promised for IP-Centrex is sufficiently attractive and thus in tune with the trend of renting managed services for IT functions (such as Web server hosting), that organizations that retain ownership of their PBX may be in a minority within a decade.

This predicted revolution, produced by the acceptance of IP-Centrex, will have three dimensions:

1. The new IP-Centrex hardware and software will be produced by a wider range of companies than the five large manufacturers of CO systems.

2. Because of the reduced cost and complexity to enter the market, IP-Centrex will be available from more competitive and smaller companies than just the ILECs. Those providers that are now in the managed services business with Web servers and customer relationship management (CRM) software will also be renting integrated voice services.

3. The current best case, business-line market, share ratio of 20% for Centrex to 80% for premises-based switches could be reversed to an 80:20 ratio in favor of IP-Centrex within 10 years. The senior management of the largest Centrex system manufacturer is predicting a 50:50 market share ratio for Centrex versus PBX, within a decade, and we believe that this is a reasonable expectation.


Multiple Centrex Systems

An essential characteristic of Centrex service is its ability to support multiple customers from one CO system, since the software package can be reliably partitioned. After "teething" problems with digital Centrex software were solved in the early 1980s, no case of incorrect connection between different customer services has been reported, which is really a remarkable record. A typical Centrex system based in a large CO can support up to 50,000 phones, so for most purposes the growth potential, for a specific customer, may be considered effectively unlimited.


Integration with On-Premise Switches

Many Centrex lines are rented, instead of regular business lines, to link KTSs to the Public Switched Telephone Network (PSTN). While the features that are available with Centrex may complement the capabilities of the KTS (which are typically used in offices with fewer than 30 lines), the common motivation for this application is the attractive rental rates, because of discounts that are available with Centrex, taking into account the number of lines attached to one CO and the duration of the contract. An organization with more than 100 lines served by one CO and a 3-year contract can obtain an almost 40% discount from the monthly, single-line tariff. These types of contracts are not generally available with conventional, non-Centrex, business lines (which are rented on a month-to-month basis). Centrex lines that are provisioned with the ground-start signaling compatible with PBX systems are also available from some telcos as an alternative to analog trunks.

The attractive margin of up to 50% offered between the short-term, single-line tariffs and those for a 1,000-line and 5-year package have given rise to a significant Centrex reseller business. In this market a competitive local exchange carrier (CLEC) leases a large number of Centrex lines on a wholesale basis and rents these to small businesses and organizations (such as a school board or a bank) with a number of offices in a given area. Two cautionary notes should be sounded, as some Centrex resellers have proven to be financially unstable and the incumbent carrier (ILEC) continues to control the service, rather than the reseller.


Virtual PBX Service

We generally consider Centrex as providing a "virtual PBX" service, with multiple customers and many sites being served by the software that resides in the one CO system, as shown in the upper part of Figure 1.2. A standard characteristic of Centrex is that all its stations have direct inward dialing (DID) access, usually based on seven-digit numbers. Users can employ abbreviated dialing (e.g., four digits) between Centrex-based stations within a local calling area. These features minimize the need for operator consoles within a Centrex system, although some type of operator support, for the occasional caller who needs assistance, is still usually required.

PBX replacement and KTS support.

A business line package with a few popular features, which is available from carriers in some countries, clearly does not qualify as Centrex service when measured against the virtual PBX definition.
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