Cabinet Backplane | Legacy PBX

The carrier shelves in a multicarrier cabinet connect to a common backplane at the rear of the cabinet. At the rear of each printed circuit board card slot is a connector that attaches to the backplane. The backplane wiring, which uses wire-wrapped connections or printed circuit board technology, contains the system bus structure for intra- and intercarrier processor control and communications signaling transmission. The processor control signals are multiplexed onto the backplane bus structure via time slot assignments. The time slot assignments to the port circuit card slots are usually arranged to allow maximum flexibility in the placement of different device boards in the carrier, although some systems are designed with a fixed number of time slot access connections per port card slot. Physical backplane connectors, such as 25-pair connectors, are used to interface the port circuit card packs. There are also connectors for intercabinet or intercarrier connections. The shelf backplane of a control carrier may have multiple RS-232C connectors to support management and maintenance terminals or call detail recording equipment. Another connector is used for connecting a tip-and-ring pair and maintenance leads to a crossconnect field.

PBX cabinets and/or distributed carriers are usually connected with optical fiber cabling links. Optical fiber links provide very high transmission bandwidth in support of switch network traffic requirements and call processor signaling. Optical fiber distance limitations between cabinets vary from system to system, but the typical loop length is about 1,000 feet without repeaters or special interface cards.

Cabinet Power System | Legacy PBX

Alternating (AC) or direct (DC) current power sources may power PBX systems. Of the two, the current popular power option is AC. AC-powered systems plug directly into commercial AC power outlets. DC-powered systems require external rectifiers to convert the incoming AC current provided by the electrical utility carriers to DC voltage. The external current flows into a centralized power distribution unit that distributes power to the common equipment carriers/cabinets. If reserve power is required, customers can install an Uninterruptible Power Supply (UPS), with its associated batteries, in series with the utility AC power feed. Figure 1 shows a Meridian 1 DC-powered cabinet.

Figure 1: Meridian 1 DC-powered cabinet.

AC-powered systems always require internal rectifiers to convert commercial AC power into the distributed DC power used to support internal common equipment hardware via carrier shelf power units. Power units convert AC input to DC output power for distribution to carrier backplanes to support the housed printed circuit boards. AC-powered system power units typically convert AC commercial power into –48 V DC for internal system distribution. External AC/DC rectifiers distribute –48 V DC voltage directly to the centralized power distribution unit, thus eliminating the need for internal rectifier equipment. Carriers and cabinets are equipped with power filter units to control power distribution. Circuit breakers or electronic fuses are located in the centralized unit to prevent short circuits in one carrier from affect- ing power distribution to other carriers. Figure 2 shows a Meridian 1 AC-powered cabinet.

Figure 2: Meridian 1 AC-powered cabinet with reserve power.

At the carrier shelf level, the power unit converters produce the DC operating voltages required by the various circuit boards. Most printed circuit boards use +5 V DC power. If AC commercial power fails, batteries in the power units provide the –48 V DC necessary to support system operation. The batteries are connected in parallel to provide required voltage. When relay circuits detect a commercial power failure, the battery backup system can continue to power the entire the system for a very limited period, typically 5 to 15 seconds, to avoid interruptions in service. A standard battery backup system powers the control complex for a longer period, from 5 to 30 minutes for most systems, to avoid loss of stored database memory. A UPS option can be used to support full system operation when commercial AC power is lost for extended periods. Relay circuits and circuit breakers are used to detect commercial power loss and distribute UPS power.

Carrier Types | Legacy PBX

A PBX cabinet or a single carrier may support one or more of the following functions.

Control

The control carrier contains printed circuit board slots that house specific control circuit cards or modules supporting processing and memory functions. A single circuit pack may include processor and memory chips. Additional circuit cards may include tone clocks, maintenance and diagnostic circuit packs, auxiliary equipment circuit packs, and a variety of expansion interface boards for system designs with multiple cabinets and/or stackable carriers. An expansion interface board may be used to link to a duplicated control carrier shelf in a fully redundant common control complex design or to other cabinet carriers, such as switching network, port, and application carriers. Tone clock and maintenance and diagnostic functions may be embedded in the processor/memory circuit pack in very small system designs. The control carrier shelf is equipped with one or two local power supply modules, if a centralized power supply carrier shelf is not included in the design.

Switching

The switching carrier contains printed circuit board slots that house switch network circuit cards or modules. The switch network circuit cards or modules may support center stage switching or local TDM bus functions. Switch network function may be embedded on the processor/memory circuit pack in small system designs. For a dedicated switching carrier shelf expansion, interface boards are also required to provide control and signaling links to the control carrier and TDM bus transmission links to expansion port carriers/cabinets. A dedicated switching carrier shelf is equipped with one or two local power supply modules, if a centralized power supply carrier is not included in the design. The Definity G3r ECS Switch Node Interface Carrier is an example of a dedicated switch carrier (Figure 1).

Figure 1: Definity G3r ECS switch node interface carrier.

Port

The port carrier contains printed circuit board slots that house port interface circuit cards. There are a variety of port interface cards supporting the many types of station and trunk peripherals. Some port carrier shelves may also support application circuit packs/modules. A maintenance/diagnostics circuit pack may also be supported on the carrier shelf. The carrier shelf is also equipped with one or two local power supply modules, if a centralized power supply carrier is not included in the design. Port carriers typically support between 16 and 20 port circuit card slots, although some PBX system model port carriers may support fewer or more slots. The number of port card slots has remained relatively constant during the past 15 years, although port circuit card density (port terminations per card) has increased significantly. The Definity G3si/r ECS Port Carrier is an example of a dedicated port carrier (Figure 2).

Figure 2: Definity G3si/r ECS port carrier.

Auxiliary

A dedicated auxiliary carrier/cabinet is available as a common equipment option and typically available only with large PBX system designs. Auxiliary equipment housed in the carrier/cabinet may include call accounting systems, recorders, announcers, paging systems (loudspeaker, code calling), and music-on-hold. Small and intermediate PBX system designs may support some of these functions with the use of circuit packs housed in the control carrier shelf. Local power supply modules power the carrier/cabinet.

Application

Application carriers contain printed circuit board slots that house circuit card packs/modules dedicated to specific system applications, such as system administration, messaging (voice, integrated, unified), or call center (processing, reporting). The carrier shelf or cabinet is also equipped with one or two local power supply modules, if a centralized power supply carrier is not included in the design. The industry design trend has been to use an adjunct server cabinet, instead of an integrated application carrier/cabinet, to support advanced optional application requirements. The early adjunct server options were linked to the PBX common equipment via a proprietary cable, but the more common current method is a TCP/IP-based Ethernet LAN interface. The first adjunct server cabinet options were usually limited to a single applica- tion. Current adjunct servers now support multiple application software programs. For example, the Siemens HiPath AllServe 150 applications server supports system administration, ACD, messaging, and call accounting.

Power

The power carrier distributes power from the main power distribution to the PBX common equipment components. Dedicated power carriers are usually available with large systems only. Based on the port size configuration and number of port cabinets, more than one power carrier may be required.

A very small or small PBX system may have a cabinet design based on a single multifunction carrier that supports the functions mentioned above. In this type of PBX cabinet design, all the basic control, switching, and port interface boards are housed in a single carrier sharing a common backplane, including power converters distributing current to the provisioned circuit cards. Intermediate line size systems are more typically based on stackable carrier or multicarrier cabinet designs that include a control/switching carrier and one or more expansion carriers. Each carrier shelf in an intermediate/large system design would include power supply modules. The control/switching carrier may also support a limited number of port interface card slots for customers with limited station/trunk requirements at system installation to minimize upfront common equipment requirements. Large/very large PBX system models are commonly designed with dedicated carriers for control and/or switching functions. These dedicated control/switch carriers do not support port interface cards, which are housed in dedicated port carriers/cabinets.

Legacy PBX Common Equipment

PBX common equipment can be divided into two main hardware categories: cabinets and printed circuit cards. There are many cabinet and card category types, and each PBX system model is designed and configured with the use of unique and proprietary hardware equipment. Neither cabinets nor cards are interchangeable between different PBX systems from different manufacturers. Proprietary common equipment hardware is currently the major reason the open design platform of new IP-PBX client/server systems is attracting the attention of many customers. Although the industry trend is toward reduced dependence on proprietary common equipment, the very large installed base of traditional PBX systems will guarantee the continued requirement for new equipment cabinets and port circuit interface cards for many years to come.

The PBX starter package is usually called the main system assembly and includes the necessary hardware elements and software for basic system operation. Small port size customers may require only a single cabinet equipped with all of the necessary equipment to support their station and trunk interface needs, but very large port size customers may require a dozen or more colocated or distributed cabinets. For any specific customer port size requirement, common equipment costs are relatively fixed regardless of feature or function requirements because the same core hardware components that support plain old telphone system (POTS) applications are required for advanced call center or networking applications. The advanced application configurations may include adjunct application servers, but the common equipment hardware supporting port station and trunk requirements is usually application independent. For example, desktop telephone instrument prices can differ by hundreds of dollars across a family portfolio, but cabinet and port interface circuit card costs supporting an inexpensive analog telephone are about the same as compared with an expensive ACD-based displayphone model with a headset interface. Figure 1 shows the basic PBX common equipment components and their relation to one another.

Figure 1: Basic PBX architecture.

PBX common equipment installations can range from a wall-mounted small cabinet design supporting fewer than 100 stations to floor-based multiple cabinet designs supporting 20,000 station users, but each configuration provides the same fundamental call processing, switching, port interface, and applications support capabilities. Single-carrier cabinet designs are usually based on a common control and switching complex consisting of a limited number of multifunction printed circuit boards to conserve cabinet real estate and a few port interface card slots. The large system models may have one or more carriers dedicated to common control functions, dedicated switch network carriers, and numerous expansion cabinets primarily supporting port interface circuit cards. The basic PBX operational elements are the same in the small and large models, although the common equipment design is significantly different.

Cabinet/Carrier Shelf Local Controllers | Legacy PBX

Many, but not all, intermediate/large PBX systems have local control cards that support one or a group of port carrier shelves housed in a port equipment cabinet. The primary functions of the local control cards are passing control signals from the Main System Processor to the port circuit cards and providing a signaling link between the peripherals (via the port circuit card microcontrollers) and the main control complex. It effectively analyzes, controls, and supervises the port circuit cards and peripherals at the cabinet or carrier shelf level. Other common functions of the local controller are local TDM bus talk slot assignments and supervising and monitoring local switch network connections and voice signaling transport. Localizing the switch network access function can greatly reduce the call processing load on the Main System Processor. The reduced processing load can be especially significant for PBX systems with a distributed or dispersed switch network design. Local controllers can also perform a variety of diagnostics and circuit test operations such as passing status updates to the main control complex.

Local controllers, like the port circuit card microcontrollers, do not control call processing functions but merely execute operations under the supervision of the Main System Processor. The type of processing element used as a local controller differs from system to system. Based on the localized and limited role it has in the call processing operation, it is not necessary for the processing element to be a current microprocessor platform; many PBX systems have retained the same local control boards for more than 5 years. The local controllers perform their tasks based on firmware programs resident on the printed circuit board.

Some PBX system call processing designs offer fully duplicated local controller function as a standard or optional system attribute. In an intermediate/large line size system, local controller card problems sometimes can affect hundreds of system ports. Processor failure or error will result in loss of service to all ports supported by the local controller, unless a back-up controller is available.