Voice Terminal Categories

PBX voice terminals can be classified into several basic telephone categories:

• 2500-Type analog telephone

• Digital telephone

• Mobile

• PC client softphone

Within the latter three categories are several subcategories. The distinguishing technical difference between an analog telephone and a digital telephone is that the latter terminal has an integrated codec that digitizes voice signals for transmission to the PBX system over the inside wiring system. The digital transmission format can provide a higher degree of feature and function performance at the desktop level, instead of at the PBX system common equipment. Before the design, development, and widespread availability of digital telephones, the first generation of stored program control PBX systems supported electronic telephones, sometimes referred to as hybrid telephones. Voice transmission between the desktop and the PBX system was analog, but the telephone design included integrated circuitry, sometimes a microprocessor chip, that could support multiple line appearances. Programmable feature/function buttons and limited function display fields were supported, but the performance capabilities were limited due to in-band signaling techniques between the port circuit card and the desktop. Digital telephones, supported by an out-of-band signaling channel, were capable of far greater performance potential.

Mobile telephones is a term used to categorize cordless, wireless, and cellular telephone handsets. Cordless telephones working behind a PBX system are likely to use digital radio transmission between the handset and the base station, but analog voice transmission is supported between the base station and the PBX port circuit card. The terms wireless and cellular usually describe telephones that do not require any local loop wiring, although the base station transceiver is hardwired back to a central switching system, such as a PBX system.

PC client softphones are based on a CTI platform from which the PBX common control complex functions as a server to the desktop terminal. With a client softphone option, communications control and signaling between the desktop and the PBX system is handled over a CTI link (desktop or client/server configuration) behind a traditional common control complex or LAN-based call server. The former type of softphone application requires an associated telephone instrument for voice communications transmission at the desktop. The latter is an example of a softphone based on an IP telephony platform, without a traditional desktop telephone. An IP-based softphone requires the PC client be equipped with a sound board with a combination microphone/speaker or a computer handset.

Analog Telephones

DTMF analog telephones that conform to the old Bell System 2500-type standard specifications are nonproprietary and can be supported by all PBX systems. However, the analog port circuit cards for each PBX system are proprietary. Transmission between the telephone and the PBX is analog based, with in-band signaling for all dialing and feature activation operations. Analog-based voice communications and signaling, transmitted over a 4-KHz transmission line, is carried over two-wire (single pair) UTP telephone wiring between the wall jack and the PBX system. The embedded signaling bandwidth limits support of integrated desktop features and functions, particularly display-based information. Analog telephones typically have a single line appearance, although a second virtual line may be supported for answering incoming calls after the original connection is placed on hold. Two line appearance analog telephones require multipair wiring and multiple port circuit card terminations. Analog telephone in-band signaling over a single wiring pair does not support multiple line appearances behind a PBX system.

Analog telephones may be equipped with a limited number of fixed feature buttons, such as hold, and an array of programmable speed dial buttons. The instrument may be equipped with a message indicator and limited function display field. Displays are usually limited to dialing and call duration information, time clock, and caller line ID (CLID) incoming call directory numbers. More detailed information such as name display, call diversion information, or feature/function menus commonly available in more sophisticated, and more costly, digital telephones are not available.

Some analog telephones have an integrated hands-free answer intercom speaker or a two-way simplex speakerphone. Almost all currently available desktop audioconferencing products are based on standard analog transmission standards between the desktop and the PBX system and supported with the same analog station port circuit used for 2500-type analog telephones. Audiconferencing products are typically equipped with a DTMF keypad and several fixed feature buttons, e.g., mute, and support full duplex speakerphone operation. The Polycom Soundstation is an example of an analog-based desktop audioconferencing product.

Digital Telephones

The second category of voice terminals is digital telephones. Digital telephones have a codec that digitizes analog voice signals at the desktop, using PCM as the encoding scheme. Excluding IP telephones, digital telephones are supported through an out-of-band signaling and control channel between the desktop and PBX port circuit card. There are several subcategories of digital telephones: PBX systems, it is used to support advanced features unique to each system, particularly display-based system capabilities and functions. Figure 1 illustrates a typical multiple line digital telephone instrument from Avaya.

Figure 1: Typical multiple line appearance digital telephone.

• Proprietary

• Universal Serial Bus (USB)

• ISDN BRI

• IP

Proprietary. Most digital telephones currently working behind a PBX system are proprietary, and work exclusively with the manufacturer’s PBX system(s). Some manufacturers, such as Siemens and NEC, have designed their digital telephones to work across different product families of communications system (KTS/Hybrid and PBX systems). All current proprietary digital telephones are supported by a 2B+D communications/signaling transmission format between the desktop and port circuit card. Although the 2B+D transmission format is most closely associated with ISDN BRI services, the dual communication channel/dedicated signaling channel format was first implemented on a proprietary digital PBX telephone in 1980, before there were ISDN standards. The first digital PBX telephone was the Intecom ITE model, capable of supporting digital communications from desktop to desktop across the PBX cabling infrastructure and switching network, with an optional data module for supporting modemless data communications from the desktop. Each B channel can support 64-Kbps communications transmission for voice, data, or video signals. What makes a digital telephone proprietary is the D-channel signaling protocol. The protocol is proprietary and unique for each manufacturer’s PBX system. Although the proprietary nature of the digital telephone’s D-channel format restricts customer flexibility in selecting telephones for use behind their PBX systems, it is used to support advanced features unique to each system, particularly display-based system capabilities and functions. Figure 2 illustrates a typical multiple line digital telephone instrument from Avaya.

USB. A special type of proprietary digital telephone is uses an integrated Universal Serial Bus (USB) interface. USB is an external bus standard capable of very high-speed transmission rates, up to 12 Mbps, and can support a wide variety of communications applications. The original intent of a digital telephone equipped with a USB interface port was for desktop CTI applications. The USB port passes signaling and control messages/commands between the voice terminal and a desktop computer. Each PBX manufacturer originally designed proprietary CTI API port interfaces for implementing desktop PC telephony applications. It was believed that the design, development, and adaptation of USB standards would stimulate the market for desktop CTI installations behind PBX systems, but few manufacturers incorporated the interface port in their telephone instrument designs.

A recent innovation using the USB link supports IP telephony. A digital telephone with a USB port eliminates the need for a RJ-11 jack interface between the phone and the PBX cabling system because communications transmission and signaling can be handled over the LAN infrastructure by using a desktop computer as the intermediary link to the LAN. The failure of CTI as a PBX station option has historically limited demand for USB telephones, but the emergence of ToIP communications may help create future demand for the option. Customers using a USB telephone in a ToIP installation can continue to enjoy the look and feel of a traditional telephone with a traditional keypad and handset for dialing and call answering operations while using the desktop computer to facilitate feature/function access (using CTI client software). Frequent computer processing and software problems, a major reason most station users are reluctant to use a softphone, do not affect most USB telephone functions or operations because the computer serves only as a physical connection to the LAN. Of all the major PBX system manufacturers, Nortel Networks has been the most active in promoting use of its USB telephone model for ToIP applications behind its IP-PBX communications systems. pling before a VoIP audio codec converts the digitized sample to IP format. IP telephones may conform to VoIP protocol and signaling formats standards, but each IP-PBX system uses proprietary signaling bit data in support of unique features and display characteristics. The accompanying photograph of the Cisco 7960 illustrates a current generation IP telephone (Figure 2). Although most IP telephones with Web browser capabilities have similar feature capabilities, the look and feel of each supplier’s telephone models is distinct. The accompanying photographs of three supplier’s high-end IP phone models illustrates this (Figure 3).

Figure 2: IP phone attributes: Cisco 7960.

Figure 3: Contrast in styles: IP telephones with Web browser displays.

ISDN BRI. A third category of digital telephones is ISDN BRI. The ISDN BRI telephone communications link to the desktop is 2B+D, but unlike proprietary sets, its D-channel signaling format conforms to National ISDN (NISDN) specifications. ISDN BRI telephones have limited access to some proprietary PBX features, and the level of display information is also limited compared with proprietary digital telephones. Although ISDN BRI telephones have been available since the early 1990s, support of ISDN BRI telephones offers customers some benefits not available with proprietary digital instruments, such as passive bus operation and bonding of the two communications bearer channels. The former is the ability for a single ISDN BRI 2B+D communications link to support up to eight desktop telephones, each with its own directory number. Although only two telephones can be active simultaneously, customers can save money on port circuit hardware and cabling when these telephones are used in low-traffic environments. The latter application supports high-speed transmission to the desktop, up to 128 Kbps, for data or H.320 video communications applications. pling before a VoIP audio codec converts the digitized sample to IP format. IP telephones may conform to VoIP protocol and signaling formats standards, but each IP-PBX system uses proprietary signaling bit data in support of unique features and display characteristics. The accompanying photograph of the Cisco 7960 illustrates a current generation IP telephone (Figure 2). Although most IP telephones with Web browser capabilities have similar feature capabilities, the look and feel of each supplier’s telephone models is distinct. The accompanying photographs of three supplier’s high-end IP phone models illustrates this (Figure 3).

IP. The fourth digital telephone category, and the most recent, is an IP telephone. IP telephones are included in the digital category because voice signals are digitized with standard 8-bit coding and 8-KHz sam- pling before a VoIP audio codec converts the digitized sample to IP format. IP telephones may conform to VoIP protocol and signaling formats standards, but each IP-PBX system uses proprietary signaling bit data in support of unique features and display characteristics. The accompanying photograph of the Cisco 7960 illustrates a current generation IP telephone (Figure 2). Although most IP telephones with Web browser capabilities have similar feature capabilities, the look and feel of each supplier’s telephone models is distinct. The accompanying photographs of three supplier’s high-end IP phone models illustrates this (Figure 3).