Digital Desktop

In the late 1970s and early 1980s most PBX manufacturers developed an electronic telephone for use behind their systems. The electronic telephone’s primary benefit was support of multiple-line appearances. Instead of using KTS equipment behind a PBX to support station user requirements for multiple line appearances, an alternative option was available. The majority of station users at the time used single-line appearance analog telephones, with no feature/function buttons, no speakerphones, and no displays. Only a few lucky station users qualified for multiple-line appearance telephone instruments. Today, of course, the typical PBX telephone instrument looks slightly less complicated than the cockpit of a Boeing 777, with more buttons, bells, and whistles than one knows what to do with.

Like the basic 2500-type analog telephone, voice transmission from the electronic telephone to the port circuit card over the inside wiring was analog, but the built-in intelligence of the telephone instrument provided an array of programmable feature/line buttons and a limited function display. A signaling link between the telephone and the PBX provided the intelligence to identify which line appearance button was being used to place the call or which feature button was depressed for activation. Control signaling between the electronic telephone and the port circuit card was embedded within the instrument’s 4-KHz voice transmission channel. The low-frequency signaling stream constrained feature/function development, but was a first step in the evolution of intelligent digital desktops behind the PBX.

The evolutionary step made by electronic telephones was a break from the traditional DTMF signaling techniques for communicating with the PBX common control equipment, as was done with traditional analog telephones. Each PBX manufacturer used a proprietary signaling scheme and dedicated station line circuit cards to support electronic telephones. An industry standard for electronic telephones was not developed for a variety of reasons, although it may have led to more sophisticated desktop terminals. Maintaining a proprietary signaling link meant that electronic telephones could be sold at a high price, with a significant profit margin, if customers required multiple-line appearances. Third-party telephones could not be manufactured unless the signaling scheme specifications were published (which they weren’t).

When Intecom introduced the first digital PBX telephone, the product marketing materials emphasized its potential for integrated voice/data communications with an optional data module. Two communications channels were available to the desktop station user, one for voice and the other for data. Little mention was made of the dedicated signaling channel used to link the telephone with the PBX common control equipment. The digital signaling channel was the major breakthrough that would be the distinguishing factor between analog transmission telephones (industry standard, 2500-type, electronic) and digital transmission telephones. The out-of-band signaling channel, operating at transmission rates between 16 and 64 Kbps (based on the individual manufacturer’s design specifications), could be used for a variety of new, advanced desktop capabilities.

The primary function of the signaling channel was to alert the PBX common control equipment when the telephone handset was taken off the hook to prepare a voice call. The signaling channel was designed to transmit keypad dialing signals and feature/function activation and implementation signals. Display information, such as calling name and number, was carried over the signaling channel, including call redirection information for forwarded calls. Station users could self-program their telephone instruments with software programs residing in the main PBX control complex but accessible via the signaling channel.

The second communications channel originally developed for desktop data communications applications was rarely used because LANs became the dominant enterprise data communications network. Eventually telephone designers were able to program the PBX to support a second voice channel to the individual station user desktop in support of an adjunct voice terminal. The intelligent signaling channel can distinguish between voice calls placed to different directory numbers and support simultaneous calls to and from discrete desktop devices. Using a special analog line adapter module, a digital telephone can be used to support an adjunct analog telephone, modem, facsimile terminal, or audioconferencing station. The adapter module converts signals from the adjunct analog communications device signals into the proprietary PBX digital format. Other uses of the second communications channel include support of a second digital telephone (using a digital line adapter module) off a single PBX communications port interface and bonding of the two channels for high-speed transmission in support of data or video applications using an ISDN Basic Rate Interface (BRI) type of adapter module.

The most impressive use of the signaling channel is the support of sophisticated display information fields and associated context-sensitive softkey feature/function access. The current generation of digital telephones have large multiple-line display fields that are used to view directories and call logs, access on-line help programs, read text messages, and perform station and/or system management operations.

One of the criticisms of traditional PBXs has been the use of a proprietary control signaling to support digital desktop equipment. The recent development of LAN telephony systems using IP signaling standards someday will eliminate the proprietary signaling link between the call processing system and the telephone, but standards are still in development. Cisco’s IP telephone currently does not work on a 3Com IP telephony system, and Avaya’s and Nortel’s IP telephones interwork do not work on each manufacturer’s respective IP-PBX offering. When a high-performance industry standard IP telephone is available to work behind a multitude of IP-PBX systems, it will be possible only because of a standardized signaling link between the call processing server and the desktop, a design specification first developed 20 years ago in the first-generation digital PBXs.