Centrex

Centrex (from Central Office Exchange) provides the functions of a telephone system for private individuals or companies with the help of elements of a public telecommunications network ( e.g. ISDN ). The terms virtual telephone system, Hosted PBX (Private Branch Exchange) and NetPBX have established themselves as synonymous terms .

The technology was developed in the USA at the end of the 1950s and was available on the market there at the beginning of the 1960s. In Germany, the first offers for this technology were only drawn up with the deregulation of the telephone market at the beginning of the 1990s.

principle

Every telephone is connected directly to the public telephone network of a network operator like a simple private connection. The network operator reserves phone numbers and structure according to user requirements and grants a special tariff for internal calls. In contrast to telephone systems , with Centrex the user ideally does not need to procure and maintain their own systems.

services

All services provided by Centrex are initially based on the conditions of the telephone systems on the market. This includes standard telephony services as well as extended group services as well as value-added and additional services. In addition, with Centrex - since these services are provided in public network nodes anyway - there are also special transport services that are always considered separately in private networks. This statement also contains one of the most important differences between private and managed services. These groups of services can be distinguished in detail:

Standard services provide basic functions for handling telephony and are therefore also available in all public networks. This includes such tasks as establishing and clearing connections and managing phone numbers.
Group services expand the existing standard services with special services that take into account the extensive communication behavior of a private group and are therefore only available to a limited extent in public networks. The majority of these services were also placed on the market specifically in the history of the development of telephone systems. These services then include features such as an internal number plan or call transfers and forwarding.
Value-added services offer a summary of services that are tailored to a specific user group. Call centers and teleworking places in particular play an important role.
Additional services complement the standard and group services in the form of specially hosted systems that, for example, provide centralized unified messaging or voice mailboxes. This means that convenience services in particular can be easily provided.
Administration of the respective services and groups is a basic requirement for setting up Centrex. In contrast to local applications such as telephone systems, a number of special features must be taken into account when using central systems, as different active and passive nodes can be located in a group.

Group services

The general participant functions belong to the standard of modern communication and should not be considered further here. In addition to the services also available in telephone systems, the centrex-specific properties include the following features:

Internal numbering (intercom number) - In addition to a telephone number from the public network, each participant receives an internal telephone number with which he can be reached within the group. This function is of particular interest when using Wide Area Centrex and is presented there in more detail using an example.
Integration of private telephone systems - In addition to individual end devices, it is also possible to integrate telephone systems into Centrex groups. Telephone systems can thus be expanded, virtually networked across several locations or gradually replaced by Centrex connections.
Direct dialing to the public network - While telephone systems can only reach your own local network, Centrex allows you to design a wide range of options for accessing the public network.
Mini-group - Each participant in a so-called mini-group remains a member of the public network with their telephone connection and only enters their Centrex group after entering a combination.
Automatic call distribution ACD - With Centrex, calls can be distributed within the entire group according to an automatic procedure.
Manual call distribution - Within a Centrex group, depending on authorization, all calls can be distributed, taken over or forwarded to any location.

Mediations

Even in the age of speech recognition systems, attendant consoles are still an important means of communication. With Centrex, these switchboards can be distributed over several locations or also serve several private groups - the latter refers to a so-called Centrex complex and is currently only possible to a very limited extent with telephone systems. With access to the resources of the public network, there are no restrictions on the transfer of calls within the network for operator consoles at several locations. It is expected that switchboards will continue to be of great importance within the framework of Centrex and offer new functionalities.

administration

With the administration of his private group, the user is given an important tool with which he can design his own communication requirements. Whereas in the past the administration at Centrex was dependent on special end devices and the possible functions were limited, the transition to new types of administration is being discussed with Centrex. Web-based solutions will play a special role here in the future.

technology

Centrex is based on the components of the public network. These generally include:

Public network nodes as carriers of the control functions,
Networks for the connection between network nodes and users,
Terminal equipment as a direct interface to the user.

The carrier of every Centrex solution is the switching system in which the necessary processes are controlled and administered. For this purpose, a Centrex group is formed that can be identified within the system using a unique Business Group Identification (BGID). This identifier is requested during the placement process; if a connection belongs to a BGID, the system also makes the services and applications of the private group stored there available. This identification is also important when setting up cross-location groups in which several network nodes are involved.

Interfaces

With Centrex, by definition, all interfaces must be available that are also possible in conventional private groups. The decisive difference to private systems is the architecture of the public network, which is based on publicly available and defined interfaces and protocols. This means that only the publicly defined performance features are available. If special services are to be offered now, as they are common in private systems, the existing protocols must be expanded accordingly and, viewed in this way, they can no longer be used without restrictions.

Logs

One of the core elements in the introduction of Centrex must be the use of the possible protocols in the network. If there was a clear separation between private and public communication protocols in private networks with the use of private nodes, these two worlds are essentially mixed up at Centrex. The DSS-1 ISDN protocol used in Germany is only prepared to a limited extent for the functions of telephone systems. In the world of telephone systems, manufacturers therefore work with their own proprietary protocols, which, however, are not available in the public network with Centrex. This results in a number of restrictions with Centrex compared to the telephone systems available on the market.

Applications

Due to the special architecture of the public network, Centrex can provide additional services in private groups that are usually not possible with private systems. These applications have so far made up a large part of the attractiveness of Centrex and will also play a central role under IP-Centrex .

Wide Area Centrex (WAC)

In the network, this function, in which various locations are combined into a common private group, is also known as inter-site networking or virtual private network (VPN). With this feature, a distributed group appears as if it were in one location. This means that uniform value-added services can be used equally at all connections in this group.

Extended Wide Area Centrex: Several locations in a Centrex group are connected to one network node. Communication is handled within the node.
Network Wide Area Centrex: Several locations of a Centrex group are connected to different network nodes. Communication between the nodes is handled via the SS7.
Intelligent Wide Area Centrex: Several locations of a Centrex group are connected to different network nodes. Communication between the nodes is handled via the IN.
Convergent Wide Area Centrex: Several locations of a Centrex group are connected to different network nodes in different systems (PSTN or NGN). The communication between the nodes is handled either via the SS7, via the IN, via DSS1 or IP protocols.

Special features of all solutions with Wide Area Centrex are:

uniform representation of the entire network under a common number block,
uniform functions and services throughout the network,
Handling of all connections within the WAC group as in a shared private telephone system,
joint administration of all locations,
no bottlenecks or overcapacities due to access to the carrier's network capacities,
reduced risk of disruption between the locations compared to leased fixed connections due to the denser meshing of public network operators,
Customized tariff variants between the locations.

All functions of the Wide Area Centrex are based on communication processes in the systems of the public network operators. The groups in different network nodes in the Network WAC are recognized by evaluating the BGID from the ISDN user part of the signaling protocol SS7. The messages contained there then resolve corresponding functions within the group. At present, however, neither this evaluation of the BGID nor the other functions are standardized, so that only manufacturer-specific messages are exchanged. This means that currently no Centrex groups can be implemented that extend over network nodes from different manufacturers. This also makes it impossible to set up groups that span different network operators. In Germany, only the EWSD systems from Siemens AG are currently in operation across multiple locations as the basis for Centrex. With the use of the SS7 field ISUP V2 (ISUP +), this system provides a manufacturer-specific and limited number of messages for communication between two network nodes. Alternatively, Intelligent WAC would also be possible - there the messages available in INAP are even less designed for the use of a complex Centrex solution, so that above all the numbering between the locations is organized in this way. This means that, as a limited solution, at least the operation of different systems is possible.

If the interconnection between PSTN and NGN now comes into play, the problem of incompatible message formats arises again. If the providers are currently assuming above all that the potential customers of IP-Centrex ideally start out on the greenfield without existing telecommunications infrastructures, the reality is somewhat different. For many users, IP-Centrex would be an ideal migration path from the PSTN to the NGN - but without communication between the two worlds, this remains a pipe dream. One variant of the solution would be to rely more on the protocols of the IP world, which has an open platform with SIP, for example, which could accommodate additional services. Without a uniform approach by all market participants, this project is currently not feasible.

Call center

One of the currently insignificant applications is virtual call centers . However, under the conditions of IP-Centrex, this picture should change, since there communication can be better linked with the required data. The focus of the solutions currently in use is the use of a central ACD, which supplies several locations with Wide Area Centrex. A central call management system is provided here, with which the loads on the individual agents can be controlled from one point. The use of CTI is also planned; Due to the still very complicated handling, there have so far been no use cases for it. The following services can be used in conjunction with Centrex and call center solutions:

English local automatic call distribution (L-ACD): This means that the incoming calls are distributed within a location.
English network wide automatic call distribution (N-ACD): This system makes it possible to distribute calls within a WAC group to any number of locations.
English interactive voice response (IVR): The ACD system can also use an additional system to pre-qualify the callers using voice recognition and then assign them to the respective agents. With this solution, users can do without their own speech recognition systems.
English computer telephony integration (CTI): In the past, the interaction of data applications and voice telephony could only be implemented in complex projects, but with IP-Centrex it is possible to link both worlds in a simple way.
Value-added services: In addition to IVR and CTI, other call center services are conceivable as a virtual solution under IP-Centrex.
Call Management (CM): The same applies to querying and influencing the parameters of a call center.

Central operator station

Centralized exchanges, in which active call handling is also possible by simply querying callers as in the call center, are an essential component of many of the Centrex solutions currently in operation. In the Wide Area Centrex it makes sense to process different locations centrally. This not only streamlines communication processes, but also uses personnel more effectively. In connection with network-wide ACD solutions, this system gives users a wide range of uses. Since the ZVPL is based on public networks, the functions differ in some points from those of private systems: In one of the Centrex Group's locations, the central dial-in is defined as a direct connection to the ZVPL. In addition, calls to the ZVPL can also be diverted to the ZVPL at the other workstations using the "Do not disturb" function. If an external subscriber calls one of these connections or wants to send a general request to the central dial-in, this connection is redirected directly to the ZVPL in the network without the involvement of the customer's connection lines. Both the caller's number and the originally dialed destination are displayed there. This means that you can react immediately as if the ZVPL was at the originally selected location. In the next step, the call can be processed at the ZVPL as well as in a call center or passed on within the Centrex group. If necessary, the ZVPL - and only he and only in this case - can break through the "Silence before the phone" function for the originally selected subscriber and transfer the call to this place. With the functions outlined, a number of services are available to the user that cannot be easily produced with a private system. In this context, the function of the Centrex complex also plays an interesting role. This means that several private groups under Centrex can be supplied with one ZVPL from a central service provider - a function that is not possible with a telephone system.

Teleworking

Centrex offers a simple solution in this area that is superior to other systems in many respects. Therefore, the solutions currently available are presented first. The ISDN-based Centrex provides these services:

After the teleworker has registered at his home workplace, all functions of his office workstation are available to him.
Incoming calls at the office workstation are forwarded directly to the teleworker in the network without a line being seized to the company.
As a full member of the private communication group, incoming calls can be forwarded by the teleworker to the company location.
Outgoing calls made by the teleworker are marked with the office number so that the called party cannot identify the origin of the call from a home office.
If the teleworker logs out of the group, his outgoing calls are displayed and billed as private calls again.
In addition, it is possible to integrate data connections into this process and thus set up a VPN in the ISDN. With IP-Centrex this variant is considerably simplified.
There is a signaling for the teleworking station, which indicates the status of the external work station. This can be used to account for the teleworker's presence. With this feature, active teleworking stations can also be integrated into call distribution if required.
It is also possible that the teleworker is only partially involved in the communication processes in his company in a so-called Mini-Centrex solution.

So far, this solution has primarily been used to integrate teleworkers as an "overflow" for virtual call centers . In this way, the operator of a call center can integrate additional workplaces into existing groups with little effort and without having to invest their own money - regardless of their location.

Conditions of use

In contrast to projects with private systems, Centrex requires a different approach. The point of view of the user must always be taken into account, which has grown out of those systems of telephone systems and leased lines. A complete reorientation of the customers ultimately only leads to confusion. Nevertheless, when using Centrex, certain conditions must be taken into account that cannot be found in classic projects.

organization

The focus is on outsourcing, costs and the influence on the processes. The following positions play a role:

no investments by the customer are necessary,
shorter contract periods than with a telephone system are possible,
heterogeneous solutions from the telephone system and Centrex require special support services,
With special function packages, the customer can reduce costs by only paying for what is really needed,
the user is only billed for the use of the network that is actually used,
Special price models can be negotiated which, in contrast to telephone systems, make internal connections chargeable,
The user does not have to provide space for doing without his own technology
internal calls may incur costs and
The classic Centrex is only suitable to a limited extent for companies that are too large.

technology

The use of Centrex offers a number of technical features compared to other systems:

The reliability in public networks and network nodes is significantly higher than would be possible with private systems.
Due to their system structures, public network operators offer greater security than private systems.
They also put a lot of effort into system maintenance, administration and maintenance.
With support on a public network node, even growth without intermediate steps is possible.
As a rule, public networks are significantly less blocked than would ever be possible with private systems.

User equipment

The interaction of Centrex and existing user systems has a number of advantages:

Telephone systems can be expanded with additional connections, regardless of the location of the system.
Virtual networking of existing telephone systems is possible.
Additional application servers in the network expand the functions of existing private systems.

There are also some disadvantages and problems in the interaction of telephone systems and Centrex:

Comfort features are not available across systems as there are no standardized protocols for them.
Call forwarding is not possible in some configurations.
Forwarding of connections or call forwarding between the systems can overload the private routes.
CTI systems are not possible as third-party applications across systems.

distribution

In North America, Centrex has a market share of around 30% of internal lines in companies and administrations. Together with IP Centrex, the “Centrex principle” now dominates around half of the market there. Centrex is also widespread in Scandinavia, Great Britain, New Zealand, Australia and Japan. In Germany, too, the product is offered by various network operators, primarily those that operate regionally. Due to newly developed features and the possible amalgamation with IP technologies (IP-Centrex) and broadband applications, the service is predicted to have a renaissance that was not thought possible until recently.

history

Originally there was only Centrex worldwide, as customers' own telephone systems were not permitted in the early years of telecommunications. Only from 1900 on, telephone systems developed out of technical necessity, which subsequently became the market-determining factor. A look at history is very helpful for understanding current developments. Centrex was originally developed in the USA. At the beginning of the fifties, AT&T found in market research that the service features of private private branch exchanges could only incompletely support customers' business processes, while public switching systems were incomparably more powerful. On the basis of this knowledge, initial investigations began in Bell Labs, as a subsidiary of AT&T, into how the public exchanges can accommodate private group communication. The main aim was to provide high-turnover large companies with more services. The Bell Labs carried out the first tests in Colorado Springs in 1958, which then resulted in regular operations in Chicago a year later. Suddenly there was the completely unexpected situation that instead of the expected target groups among major customers, more and more small and medium-sized companies were interested in Centrex. This demand for inexpensive corporate networks then brought a certain heyday for Centrex in the USA in the following years. In the 1970s this boom broke off with the advent of high-performance private branch exchanges. The market was then stimulated by a turning point in the entire communications market: AT&T was broken up into small business units. These so-called baby bells saw Centrex as an excellent opportunity to access lucrative business customer potential. For some RBOCs, Centrex became a key product for their own survival in the market. Atlantic Bell, for example, reached around half of its business customers with this solution. In the race for market share of public or private solutions, digital systems then brought a new generation of services - again in the USA. Other countries then slowly followed suit in the 1980s and 1990s, but never achieved the density of Centrex connections that can still be found in all of North America today. In Germany, following deregulation in 1998, some regional providers also tried to score points with business customers with Centrex. However, the first promising approaches initially failed due to technical problems and later due to the network operators' lack of understanding of the telephone system market. In the period since 2006, a number of specialized providers have established themselves in a second attempt in the German market. The precondition for their emergence was the meanwhile strongly advanced spread of broadband connections also with small and medium-sized companies. Today's solutions are usually configured by the user himself via the web. The available scope of services corresponds to that of high-quality conventional telephone systems.

literature

Andreas Schulz, Thorsten Thews: Manual Centrex . Hüthig, Heidelberg 2002, ISBN 3-7785-3966-3
CeTeiX - The Centrex Journal . Dresden 2000-2009, ISSN 1617-2930
John B. Abrahams: The manager's guide to Centrex . Artech House, 1988, ISBN 0-89006-330-3

Web links

Development (PDF; 251 kB)