Digital Switching/Transmission

PBXs based on digital switching and transmission technology debuted in the mid-1970s. Between 1974 and 1976 several communications system manufacturers claimed to be the first to announce a digital PBX system, including Northern Telecom (currently known as Nortel Networks), Rolm (acquired by IBM and then sold to Siemens), and Digital Telephone Systems (later acquired by Harris Corporation and known as Harris Digital until withdrawing from the market in 2000). The stated driving factor for developing a digital PBX system was to support desktop data communications without a modem, although data communications options would not be widely available until the early 1980s. Other benefits of digital switching/transmission included improved system quality and reliability levels and lower potential manufacturing costs.

There were no established standards for designing a digital PBX system in the 1970s, and the resulting systems reflected each manufacturer’s individual design biases. The preferred method of digital transmission used by almost all PBX designers was TDM. TDM is simply described as the sharing of a common transmission bus by many peripheral endpoints. Transmission of digital signals by each endpoint is based on assigned time slots by the PBX common control system. Although TDM was used for transmission of digital signals across the internal PBX switching network, it was possible to use different encoding schemes to convert the original analog signals into a digital format. Although most of the early digital PBXs used an 8-bit word PCM formatting scheme, including Northern Telecom’s SL-1 PBX, the first-generation Rolm CBX used a 16-bit word. The typical sampling rate used to convert analog signals to digital format was 8 KHz (a sampling rate double the maximum frequency of a human voice communications signal), but the Rolm CBX used a 12-KHz sampling scheme.

Encoding schemes other than PCM could also be used. In the early 1980s the first-generation Lexar LBX system used a Delta Modulation (DM) sampling/encoding scheme. Some manufacturers evaluated using Adaptive Differential Pulse Code Modulation (ADPCM), based on a 4-bit word encoding format, but no product was ever announced. Although no written industry standard existed, by the early 1980s it became obvious that the 8 KHz sampling using 8-bit word encoding was the preferred digital PBX switching platform. It took Rolm 8 years after its original CBX system made its debute to change its digital switching platform to conform to the 8-KHz, 8-bit word format; Lexar also converted to 8-bit PCM in the late 1980s. By 1990 100 percent of all new PBXs sold in North America were based on digital switching platforms using 8-KHz, 8-bit word TDM/PCM.

The first digital PBX systems digitized the analog voice signal at the port circuit card. Analog voice transmission signals were digitized for transmission across the internal switching network, mostly through the use of a TDM transmission scheme. After being transmitted across the internal switch network, the digitized transmission signal was reconverted back to analog format at the destination port circuit card. Analog station port cards were used to transmit communications to desktop devices, such as telephones or modems, and analog trunk circuit port cards were used to connect to telephone company trunk carrier circuits.

When Intecom introduced the first digital telephone in 1980 for its IBX communications system, the digitization process was performed with a codec in the telephone. Voice signals were digitized and transmitted over the local loop wiring from the telephone to the port circuit card. The first digital telephones used a multiple-channel communications link between the codec and the port circuit card. One channel was used for digitized voice signals and another channel was used for control and signaling functions. A third channel was also available for data communications devices attached to the digital telephone via a data module. Stand-alone data modules for data-only desktops were also available (Figure 1).


Figure 1: Digital PBX data communications.
Desktop-to-desktop digital communications was a major breakthrough for PBX systems. In addition to using the telephony communications network for voice communications, customers could use the PBX system as a local area data communications network. Very expensive modems would no longer be required to convert digital data communications to analog format, and transmission rates up to 64 Kbps could be achieved. Accessing a centralized computer mainframe system would be simplified—no more modems or coaxial cable cluster controllers. LAN technology in 1980 was in its infancy and very expensive. The early Ethernet Network Interface Cards (NICs) were more than double the cost of a digital PBX datastation. A PBX system could support an entire network of data workstations across the entire enterprise when an Ethernet LAN was limited to 50 workstations with major distance limitations. Great things were predicted for the integrated voice/data PBX system because transmission and switching could be all digital. We now know that LAN technology improved; NIC prices rapidly declined; bridges, hubs, Fast Ethernet, and routers were developed; and PBXs as data networking systems never caught on. The irony of the situation is that the digital PBX transmission and switching infrastructure is evolving toward an Ethernet LAN/IP WAN design.