Before the 1970s, companies had two main options for installing phones in an office environment: Connect each phone to its own private phone line or route all calls through a switchboard. Neither was completely desirable. Private phone lines made it easier for someone on the outside to get to your desk phone day or night without an operator, but calls to other offices had to go outside the office to the CO (central office) and come back in. Often, people with private lines also had a second phone on their desks for internal calls. Phones connected to the switchboard could only be used as long as an operator was on duty to place outgoing calls or connect incoming calls.

PBXes were created to reduce the number of phone lines needed in an office environment and the amount of human intervention required to place and receive calls. They began to appear within offices during the early 1970s, once digital transmission lines became dominant in the PSTN.

Early PBXes were rudimentary in operation and no different featurewise from a home telephone. Using PCM (pulse-code modulation), calls were digitized and transmitted over a 64-Kbps channel. These early devices were also completely proprietary, down to the phone that connected to them. Even if a business upgraded its PBX with new equipment from the same vendor, the company often had to replace everything from the PBX to the phones. Therefore, upgrading a corporate PBX was an expensive proposition--one that most businesses could not afford to do often. This brought about the frequent need to integrate old equipment serving a portion of a company with new equipment to handle corporate growth.

PBXes are no more than smaller versions of the same phone switches located within a CO. Instead of needing one line for every phone in the office connected to the PSTN, organizations can route calls from the CO to the PBX over trunk lines, in effect extending the edge to the office instead of to the CO. Each phone in the office is then connected via its own "line" to the PBX. Alcatel, Ericsson, Lucent Technologies and Nortel Networks have been the PBX vendor leaders over the years by providing businesses large and small with feature-rich switches.

Although the early PBXes could route calls to office extensions, they didn't have any extras, such as voicemail or conference call capabilities. An operator was required to handle all the calls that came in to the main office number. It took another 10 years before the PBX became an office extension rather than just a piece of hardware.

Explosive Features

The 1980s saw an explosion of features for the office PBX. Voicemail offered employees their own answering machines--contained within one box, without the need for messy tapes. Call forwarding let users reroute calls within an office just by punching a few buttons. This eliminated the necessity of intercom systems to inform employees of a call holding for them.

The 1980s also saw the emergence of auto attendants that could direct most calls without human intervention. Digital attendants reduced staffing needs while engendering a hatred of those wonderful "numbered options" for locating the correct person within a company. PBXes also became more open as vendors began manufacturing devices that were extendable or even upgradable. While these still were mostly single-vendor solutions, they did allow some third-party phones to be used with them.

PBXes that connect to analog phones can accept phones from just about any manufacturer, but if digital handsets are your thing, the choices may be more limited, depending on the PBX manufacturer. If your company insists on keeping its analog handsets for use with the PBX, be prepared for some complexity. To access PBX features, users must employ inconvenient, cryptic "star" commands. Although most digital handsets usually mu stbe matched to the PBX, they eliminate the need for the cryptic commands, placing most functions at users' fingertips through programmable keys.

PBXes have gained features over the years and have become more than an extension of the CO switch, but they have not become able to support an additional number of user phones. The first PBXes supported up to 5,000 phones--that's still the number supported by most large PBXes today. While microprocessors have become smaller and more powerful over the years, this technology has been applied to the features, not to the density, of the PBX. Port density is still at the 1970s level, mostly because of the physical space requirements of all those twisted-pair connections.

With their continued development in the 1990s, PBXes began to play a more active role in the rest of the network. Applications that used PBXes to form directory connections or integrated voicemail systems appeared in corporate networks. Integration was--and continues to be--the name of the game. PBXes are no longer destined to provide just voice services.

Today that integration means the PBX becomes a part of the traditional data network. The VoIP (voice over IP) revolution has given the traditional PBX vendor a new arena in which to play. While PBX vendors have been adding IP capabilities to their products, other vendors have been adding PBX features to their VoIP gateways. VoIP gateways are designed to translate between VoIP and the TDM (time division multiplexing) signals of the PSTN. Companies such as Praxon and Vertical Networks are bringing about enterprise-class PBX systems that can be integrated into the IP network.

Praxon and Vertical both offer systems that combine traditional PBX features with those of an IP router. Praxon's PDX (phone data exchange) offers analog connections for up to 64 phones. For slightly larger installations, Vertical's InstantOffice 5000 supports up to 84 telephone connections. You can send faxes from a desktop computer, through the IP network and directly out to the PSTN without ever leaving your desk. Data connections to the Internet and the corporate network are available via frame relay and DSL connections. Tie lines can be connected to other IP PBXes, traditional PBXes and/or the PSTN.

On the IP side, the units combine POP and SMTP e-mail services for users along with DNS, DHCP/ NAT and routing abilities. If your enterprise is stretched for 10/100 ports, you can add Ethernet interface cards to the units. If that's not enough, many IP PBXes are building in features that integrate voice- and e-mail. Users can call in to retrieve voicemail and also access e-mail messages using built-in text-to-speech software. This can be a great feature unless you receive several spam e-mails a day. Luckily, you can review e-mail by subject line.

No Longer Just a PBX

One of the most recent innovations with PBXes is the IP PBX. The IP PBX offers call logs and accounting details along with graphical configurations. Even the switchboard is being replaced by PC software. The operator can answer, forward and even tell who in the office is on the line or forwarding calls to voicemail all from the screen of his desktop computer. Even the phone interface can be transferred to a graphical interface. Calls can be dialed from the computer, with the PBX automatically placing the phone in speakerphone mode.

The larger PBX vendors, however, aren't taking the entrance of IP PBXes into their territory lightly. Alcatel is developing systems that integrate phone books. Instead of trying to remember all your contacts' phone numbers, you need only remember the name of the person you're attempting to reach. Type the name on the integrated keypad, and the phone will dial the number. The LDAP-accessible database can store up to 60,000 names. Names and numbers can either be entered and stored manually or saved after the person calls you.

Similar functions have been found on digital cellular phones for years but without the integration of a true alphanumeric keypad. Alcatel and others prefer to use the name PCX (private communications exchange) for these next-generation devices.

The PBX's next evolution is soft switches. The real difference between a soft switch and an IP PBX is location. These devices are built around software instead of hardware switching. A soft switch is placed within a service provider or carrier's network to bring large-scale PBX features to dozens of customers over an IP network. This type of deployment also can be applied to customers of large enterprises who want to locate PBX functions for branch and remote offices in one central location. Vendors such as VocalData provide Sun Solaris-based soft switches capable of handling up to 2,000 lines, along with digital IP handsets.

Traditional PBXes can also be extended to remote and branch offices. MCK Communications offers a line of EXTender products that take PBX connections and route them over WAN networks to remote desktop telephones. The EXTenders connects home or remote-office users to the same PBX as their internal counterparts. Employees no longer have to worry about dialing extra digits for workers at either end of an EXTender. Instead, one is accessible via the same internal-dialing plan and lets remote workers have voicemail and other PBX features from their off-site locations.

In the near future, PBXes have more of an open architecture that integrates multivendor components to form a single solution. The next generation of PBXes will allow telephones to become a PDAs (personal digital assistant) with integrated databases, day planners and calendars. You'll have everything in one package, with cellular access to voice and data networks. What could be better?