Carrier Types | Legacy PBX

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


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.


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.


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.


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 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.


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.


System Cabinets

There are several types of PBX system cabinets, categorized by carrier size and function. A system cabinet may be a single-carrier or multicarrier cabinet. Most very small PBX models are based on a single-carrier cabinet common equipment design. The wall-mountable single-carrier cabinets are sometimes referred to as modules. Single-carrier cabinet designs are typically targeted at customers with port size requirements of fewer than 100 stations. The Avaya Definity One model is representative of this design type. Customers with port size requirements between 100 and 400 stations are usually supported by stackable carri- er designs including a control carrier and a few expansion port carriers, like the Nortel Networks Meridian 1 Option 11C model. Some PBX manufacturers also use a stackable cabinet design for intermediate and large system configurations, like the NEC NEAX2400 IPX that can support port capacity requirements approaching 20,000 stations. Multicarrier cabinet designs, which are not cost effective for small and intermediate line size customers, are fairly prevalent in the large and very large line size market. For example, the small/intermediate line size Hicom 300H Model 30 from Siemens is based on a stackable carrier design, but the larger Hicom 300H Model 80 is based on a six-carrier shelf cabinet design (Figure 1).

Figure 1: Multicabinet Hicom 300H Model 80.

Major cost benefits of the new client/server IP-PBX system designs are due to minimal telephony system cabinet requirements. A 100 percent IP desktop configuration may consist of a single server cabinet with integrated gateways for PSTN trunk access because port cabinets and port circuit interface cards are not required. The switching functions, however, are dependent on Ethernet switch carrier shelves or stackable carrier cabinets, but in almost all situations these common equipment components have been installed to support data communications applications. The migration from a traditional circuit-switched PBX design to the emerging IP-PBX designs will slowly reduce common equipment requirements dedicated solely to telephony applications.

The first generation of digital PBX systems was based on multicarrier cabinets. Multicarrier cabinet designs are still used for many large and very large PBX system models. A multicarrier cabinet typically has four to six carrier shelves. The primary cabinet, often referred to as the control cabinet, houses the common control complex, the center stage switch complex (if incorporated into the switch network design), and may also house port circuit cards. Expansion cabinets, often referred to as port cabinets, house the bulk of the port circuit interface cards. The typical number of printed circuit board slots is between 16 and 20 per carrier shelf. Although the number of carrier shelves and card slots has remained relatively constant during the past 15 years, port cabinet density has increased significantly because port circuit interface card density has increased. Several currently available expansion port cabinets can house between 60 and 100 port circuit interface cards, with each card typically supporting 24 port interfaces. Simple mathematics indicates that a single port cabinet can support between 1,000 and 2,000 stations with associated trunking. Port cabinets during the early 1980s were usually restricted to a few hundred ports.

The first stackable PBX system cabinet design was introduced by NEC in 1983. The original NEC NEAX2400 was an evolutionary design because it offered an cabinet design alternative to traditional multicarrier cabinets. A stackable cabinet design offers customers several benefits:

  • Optimizes common equipment requirements—Before the stackable cabinet design, a customer might have installed a full-size multicarrier cabinet but use a fraction of the available carriers and card slots. Unless a customer had near-term growth requirements, too much hardware was installed to satisfy port requirements at system installation.

  • Reduces common equipment costs—Stackable cabinets more cost effectively satisfy customer port size requirements.

  • Simplifies system upgrade and enhancements—Customer port growth may be accommodated easily by adding another port carrier cabinet. Manufacturer design changes can focus on individual carriers, so that customers can perform system upgrades by changing out one or two carriers, instead of replacing multicarrier cabinets.

The single-carrier cabinet design is most popular for PBX systems targeted at customers with very small to intermediate line size requirements, although a few manufacturers use the stackable cabinet design for their large and very large system models. NEC, the innovator of the stackable carrier cabinet design, uses stackable single carrier cabinets for its small/intermediate NEAX2000 IVS2 system and its intermediate to very large NEAX2400 IPX models.


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.


PBX Call Processing Power: BHC Rating

PBX system call processing capability is rated with the BHC benchmark. BHC is simply defined as the maximum of number of calls processed in 1 hour by the PBX system. There are two types of BHC measurements, Busy Hour Call Attempts (BHCAs) and Busy Hour Call Completions (BHCCs). BHCAs indicate the total number of placed calls that can be processed by the PBX system. BHCA calls include successful and unsuccessful calls. Unsuccessful call attempts include the following call types: no answer, busy, misdial, and abandoned. BHCCs indicate the only total number of successful placed calls. Measured call attempts and completions include station-originated calls and incoming trunk calls.

The BHCA rating of a PBX system will always be greater than its BHCC rating because unsuccessful calls included in the BHCA measurement have a lesser call processing burden than successful calls. The larger the number of unsuccessful call attempts, the greater the BHCA rating. BHCCs are successful calls that require a switched call connection, which is continually supervised and monitored by the call processing system, and require a teardown process when the call is terminated. PBX manufacturers may provide data on either BHC measurement but do not describe the benchmark tests used to determine the rating.

A manufacturer can test its PBX system to determine its BHCA or BHCC rating by using the system test parameter of provisioning a dial tone within a target period. Station users expect to receive dial tone immediately upon picking up their handsets, but some manufacturers perform BHC rating tests with acceptable dial tone delays of 1.5 seconds. In addition to dial tone delay parameters, BHC ratings are heavily dependent on the following three parameters:

  1. System design configuration

  2. Call connection type

  3. Feature/function activity

The system design configuration defines the number and type of station terminals used for call testing purposes, the types of trunk circuits used for incoming or outgoing calls, and adjunct equipment that may be used to answer or support calls, such as VMSs and recorded announcers. Terminal type and complexity may have a major effect on the call processing rating. For example, a digital multiline telephone model with a softkey display field and add-on module options typically will require more processing power to place and receive calls than a basic analog telephone with no display or options. There are different call processing load factors associated with two-way analog trunks as compared with digital trunk circuits used for ISDN PRI services. An intercom call between two analog telephones is more likely to use less processing resources than an incoming ISDN PRI trunk circuit call, with ANI received by a digital displayphone.

The type of call connections determining the BHC rating will also affect the results. For example, trunk calls are usually more processing intensive than intercom calls. Calls between stations connected to the same local TDM bus may require less processing than calls between stations connected to different local TDM buses that require a center stage switched connection. Availability and physical location of incoming registers, outgoing registers, and sender tone receivers also will affect the BHC rating.

The call processing burden imposed by some PBX features and functions may be the most significant factor affecting BHC ratings. ACD and networking features require significantly greater processing resources than simpler features such as hold or transfer. ACD operations typically will require call screening, routing, queuing, and treatment operations before a connection is completed to an agent position. Intensive implementation of complex ACD features can reduce BHC ratings by factors greater than 50 percent. Figure 1 shows feature/function load factor effects on BHC rating.

Figure 1: Busy hour call processing

Networking operations typically require routing table look-up and analysis before trunk circuits can be selected and calls are routed. If a manufacturer’s benchmark testing procedure does not include some type of feature/function activation and implementation during a reasonable percentage of placed calls, the resulting BHC rating will not mirror the reality of a customer’s actual system installation.

The published PBX system BHC ratings, BHCA or BHCC, based on testing procedures may not adequately reflect the call processing capability of a customer-installed system configuration. The actual rating of an installed PBX system likely will be less than the published number. Fortunately for customers today, even if the true installed system rating is half of the published BHC ratings, the call processing capacity will be far greater than required by the customer. For example, small PBX system models from the leading manufacturers typically have call processing ratings in the range of 10,000 to 50,000 BHCC. Assuming a system configuration of 100 stations with associated trunking, the call processing rating of the system will far exceed the realistic call handling requirements. It is highly unlikely that, during the BHC period, all 100 stations will place 100 or 500 calls per hour. It is more likely that the total number of BHCs for a typical PBX system with 100 stations will be a few hundred.

Many of today’s intermediate/large PBX systems have call processing ratings greater 100,000 BHCC, and some are greater than 500,000 BHCC. Only very large systems approaching maximum port capacity will come close to approaching the maximum call processing capacity of the system. If the PBX is used for ACD applications, customers may need to install a larger system model than necessary for its greater BHCC rating, but it is highly unlikely that call processing limits will be reached, except in extreme circumstances.


Call Processing Redundancy Issues

One of the most important call processing design attributes for a large number of customers is redundancy. The term redundancy can be an ambiguous term if not properly defined. For example, a PBX with dispersed local controllers can be characterized as a redundant call processing design, simply because failure of one local controller usually has no effect on the other local controllers. A localized failure that does not affect systemwide operations can be considered a form of redundant design because the loss of 100 ports due to local controller failure is not as catastrophic as 100 percent system loss should the Main System Processor fail. Loosely defined, all dispersed control call processing designs are redundant call processing designs. This definition, however, may not satisfy the reliability and survivability requirements of many PBX customers.

If call processing redundancy is more clearly defined by a customer as having a readily available back-up processor element should an active processor element fail, then redundancy requires a duplicate local controller for each local processor element in the dispersed control design example. Duplication of processing elements is a high degree of redundancy.

Regardless of the call processing design category, a PBX system with a fully duplicated call processing design may include any or all of the following:

  1. Fully duplicated common control complex, including the Main System Processor, the Main System Memory (software program, customer database), and the Mass Storage Device

  2. Fully duplicated local controllers at the port cabinet and/or carrier shelf level

  3. Fully duplicated processor bus, including intercabinet communications links

Although the reliability level of the typical PBX system common control complex is very high, usually 99.999 percent (about 5 minutes average annual downtime), hardware and software failures and problems can occur. If there is a problem with the Main System Processor, then all system operations and all system ports can be affected. For this reason, a duplicated Main System Processor is usually required by customers who wish to avoid even minimal service disruptions. In addition to the Main System Processor, customers may request duplicated Main System Memory elements, especially the generic program. A duplicate Mass Storage Device may also be requested because loss of customer database records will affect call processing operations to the same extant as Main System Processor failure.

In a PBX system with a duplicated common control complex, if the active Main System Processor or Main Memory experiences problems, then the back-up (passive) call processing element should instantaneously take control of system operations without interruption of service; active calls remain connected and all activated features continue operating. This is commonly known as a hot-standby duplicated common control system. The only call processing event that is disrupted when the passive element assumes control is a call in the process of being set-up, before call connection to the called party; otherwise, all system functions and operations continue as if nothing happened. If the passive Main System Processor assumes call processing control but all existing switch connections are lost, then it is said to be a cold-standby duplicated control system. A cold-standby system also may require a few seconds or minutes before it is available to begin new call processing operations.

The passive common control elements in a hot-standby design are said to be shadowing the activities of the active elements. For example, the active and passive Main System Processors monitor port status and switch connections, but only the active Main System Processor issues control commands for call processing operations. The passive Main System Processor is merely an observer. Downloads to the active main customer database are simultaneously downloaded to the passive database. Some duplicated common control system designs support operations between the back-up passive Main System Processor and the active Main System Memory and between the active Main System Processor and the back-up passive Main System Memory. This form of shadowing is known as crossover arbitration. Common control complexes with basic shadowing capability transfer all call processing and system operations to the passive processor and memory elements when any of the active common control elements fails; the more advanced shadowing design allows system operations between active processor or memory elements that do not experience problems and the back-up passive processor or memory element—active processor and passive memory or passive processor and active memory. The crossover operation allows for four modes of full function system operation:

  1. Active elements (only one)

  2. Passive elements (only one)

  3. Mix of active and passive (two)

A duplicate common control complex is usually available only in intermediate/large PBX system models. Almost all intermediate/large PBX system models offer duplicated common control as a standard or optional capability. Small PBX system models have traditionally been designed without a duplicate common control complex, even as an option, because manufacturers originally decided that the added system cost to the customer would result in limited sales potential. Likewise, limited sales would not justify the research and development dollars expended for the design. Small system customers who require a fully duplicated common control complex are forced to buy a larger system model to satisfy that need. These customers pay a price penalty for installing a PBX system model with a greater than needed port capacity, because duplicate common control is not available in small system models better suited (and less costly) for their port capacity requirements. For example, the Avaya, Nortel Networks, Siemens, and NEC small system PBX models targeted primarily at customers with fewer than 200 stations are not available with duplicate common control as a standard or an option. Customers must step up to the larger models, sometimes two models above the entry model, if duplicate common control is a requirement. Avaya, Siemens, and NEC offer duplicate common control only as an option on their intermediate/large system models. The duplicate common control option may add as much as 25 percent to the basic system price for small system customers, in addition to the higher cost for the larger system model. Of the four manufacturers, only Nortel Networks offers duplicate common control standard on its intermediate/large system models (Meridian 1 Options 61C and 81C). Figure 1 shows the core module complex of the large Meridian 1 systems.

Figure 1: Meridian 1 option 61/81C core complex.

Dispersed control designs with duplicated local controllers are available in many intermediate/large system models. Most, but not all, PBX systems using a dispersed control design with a duplicated common control complex capability also offer duplicated local controllers. For example, the Siemens Hicom 300H Model 80 can be equipped with duplicated common control and duplicated local control function as an option, but the Nortel Meridian 1 Options 61C and 81C with duplicated common control modules are not available with a duplicated Controller Card. It is possible to have duplicated common control but nonduplicated local controllers. PBX systems equipped with duplicated local controllers are usually available with duplicated common control.

The duplication of the processor bus and intercabinet links is another important redundant call processing system design capability. Processor bus problems can affect call processing operations the same way as Main System Processor or local controller problems. Duplicated processor bus design is inherent to particular PBX system models and is usually available with systems offering a duplicate common control complex. Intercabinet links are less likely to be fully duplicated as a standard design capability, and duplication of the links usually depend on installation of a duplicate common control complex and/or duplicated local controllers. Intercabinet links are part of the system design, whether the call processing design is centralized, dispersed, or distributed, because signaling and communications among the processing elements dispersed among multiple common equipment cabinets may depend on the links for a variety of call processing operations. A single link failure to the Main System Processor may affect hundreds and possibly thousands of system ports housed in the isolated port equipment cabinet. Intercabinet links in a PBX system with a distributed control design may not affect call processing operations in the isolated cabinet, but all intercabinet communications will be affected.


PBX Call Processing Design Topologies

There is no PBX design standard that dictates the topology of the call processing network. Every PBX system has a common control complex that includes a Main System Processor, Main System Memory, and System Control and I/O Interfaces, but that may be the only common design element when comparing any two PBX system models. PBX call processing design topologies can be categorized into three general categories:

  1. Centralized control

  2. Dispersed control

  3. Distributed control

These design topologies are shown in Figure 1.

Figure 1: PBX processing design topologies.

A fourth design topology can be added to this list—adjunct server control—if PBX systems equipped with an adjunct CTI applications server are taken into consideration, but only for support of features and functions beyond fundamental call set-up and connection functions. A CTI application server is totally dependent on the PBX common control complex to execute any and all communications operations. Adjunct server control design will be discussed separately later in this chapter.

Centralized Control

A PBX system with a centralized control call processing design topology includes the following processor elements:

  1. Common control complex, including the Main System Processor

  2. Port circuit card microcontrollers

In a centralized control design, the Main System Processor is responsible for all basic call processing functions and the control and execution of all switch network functions. This design specifically excludes local processors at the port equipment and/or port carrier shelf level. Port circuit card microcontrollers interface directly with the Main System Processor via a Service Control Interface.

A PBX based on a centralized control design may have additional processing elements to perform functions and operations not necessary to execute basic call processing functions, such as call set-up and switch connections, although they are necessary to ongoing call processing activities and system availability and survivability. The additional processing elements are typically used for administration, maintenance, diagnostics, and/or measurement operations. The Main System Processor usually handles some, or all, of these functions in small PBX systems with limited processing elements.

Centralized control designs are used most often by small PBX systems targeted primarily at customers with port requirements fewer than 200 stations, although there are a few notable exceptions. For example, all the Avaya Definity models are based on a centralized control design, including the very large G3r model, which can be equipped with more than 20,000 stations. PBXs based on a centralized control design must be equipped with a Main System Processor capable of handling all call processing and switch network functions, without diminished performance when the system is at maximum port capacity and all ports are active.

A centralized control design offers advantages and disadvantages. The major advantage of a centralized control design is fewer processor elements that can experience problems and disrupt service. Reduced local processor failures can increase overall system reliability and survivability. The major disadvantage is that the Main System Processor has total responsibility for all call processing and switch network functions, with no local processors to offset the processing load. Unless the Main System Processor is powerful enough to handle current and future call processing requirements, including support of new features and applications, there will be limitations on system performance levels. One solution to offload the call processing burden from the Main System Processor in a centralized control design is to install an adjunct CTI applications server in support of advanced feature and function needs that require significant processing power. There is a more detailed discussion of adjunct server control options later in this chapter.

Most PBX systems are configured at less than 50 percent port capacity, and the number of simultaneously active ports is almost always half that of the equipped ports. A Main System Processor should have little difficulty supporting the call processing requirements of 50 or 100 active ports, even if the processing element is a 16-bit or 32-bit microprocessor. Main System Processor problems can occur in large line size systems, with hundreds or thousands of active ports, if the main CPU is not equipped and designed to handle the potential traffic. When the Definity designers chose a centralized control design for the large and very large system models, a major design change from the older System 85, they also spent much time selecting the right processing element for the Main System Processor. The innovative selection of a Reduced Instruction Set Computing (RISC) microprocessor, the MIPS 3000, came at a time when all other PBX systems were based on a Complex Instruction Set Computing (CISC) microprocessor platform, such as the Intel 386 or Motorola 68030. A few years before Intel made its Pentium microprocessor commercially available, the MIPS 3000 was evaluated as the best available microprocessor in a centralized control design because it could handle the potential processing load at maximum equipped port capacities. The centralized control design resulted in call processing limitations for the Definity G3 models when configured for large, complex ACD-based call center installations. Definity is one of the few PBX systems that does not use an adjunct applications server to handle advanced ACD call analysis and routing functions, and the heavy processing load on the Main System Processor limits the system’s optional Expert Agent Selection (EAS) skill assignment programming parame- ters when compared with PBX systems equipped with an adjunct server to offload the processing burden.

Dispersed Control

A PBX system with a dispersed control call processing design topology includes the following processor elements:

  1. Common control complex, including the Main System Processor

  2. Local control processors at the port equipment cabinet and/or shelf level

  3. Port circuit card microcontrollers

In a dispersed control design the Main System Processor is responsible for all basic call processing functions but may not execute all call processing and switch network functions. Local controllers provide the interface link between the Main System Processor and port circuit card microcontrollers and perform some call processing and switch network functions under the supervision and monitoring of the Main System Processor. The local processor elements function as slave controllers under the Main System Processor, which functions as the master controller. Like a PBX system based on a centralized control design, dispersed control designs can include additional processing elements typically used for administration, maintenance, diagnostics, and/or measurement operations.

A dispersed control design is the most common call processing design for intermediate to very large PBX system models. The primary advantage of a dispersed control processing design is that it offloads processing activities from the Main System Processor to increase overall system call processing performance. Call processing capacity is less dependent on the Main System Processor in a dispersed control design than in a centralized control design. The primary disadvantage is that failure or errors at the local processing level can affect service for all ports under its control. Unless the local controller is available in a redundant or duplicated mode, it is a potential major single point of failure for dozens or hundreds of system ports. For example, the Nortel Networks Meridian 1 Option 81C Controller Card can support one or two port carrier shelves, typically equipped with several hundreds of station ports. If the Controller Card, responsible for some call processing and switch network functions, fails, each port will lose service. The Controller Card is not available in a duplicated mode and is a major point of failure in the Meridian 1 Option 81C system design. Competing PBX system models from Siemens (Hicom 300H Model 80), NEC (NEAX2400 IMG), and Fujitsu (F9600 XL), for example, generally or optionally duplicate the local controller card at the port equipment cabinet/carrier shelf level to reduce the probability of service loss.

Distributed Control

A PBX system with a distributed control call processing design topology includes the following processor elements:

  1. Multiple common control complexes, including multiple Main System Processors

  2. Port circuit card microcontrollers

A distributed control design is based on peer-to-peer Main System Processors. It is similar in operation to a centralized control design because each Main System Processor is responsible for all basic call processing functions and the control and execution of all switch network functions. There is a major difference, however, in that each Main System Processor has control over a limited number of system ports. Each Main System Processor has responsibility for one or more port equipment cabinets but not all installed port equipment cabinets in the PBX configuration. This design excludes local processors at the port equipment and/or port carrier shelf level because the Main System Processor functions as a local processor to the one or two port cabinets it controls. In a distributed design port circuit card, microcontrollers interface directly with the Main System Processor via a Service Control Interface, if no local controllers are included in the design.

There are several important distributed control design advantages:

  1. Multiple common control complexes increase system reliability and survivability; each Main System Processor is responsible for a limited number of system ports.

  2. System call processing capacity is a function of the number of installed Main System Processors; adding an additional Main System Processor will increase the total system call processing capacity.

  3. Multiple common control complexes are ideally suited for system configurations with multiple equipment rooms (campus, multilocation) because locations remote from the main equipment room are not dependent on a centralized Main System Processor for call processing operations.

A distributed control design requires synchronization and coordination among the multiple common control complexes for call processing, switching, and administrative functions. A true distributed control design supports transparent features and function operations across the system, with the option of using a single administration and maintenance interface for the entire system. The design is a difficult one to develop and operate successfully. One of the first distributed control designs was attempted by Rolm in the early 1980s, and the technical problems in synchronizing the VLCBX’s multiple Main System Processors and memory databases significantly delayed commercial availability of the product after its announcement. Rolm eventually solved the problems and was successful in marketing and selling its multinode CBX II 9000 system, a successor to the VLCBX, later in the decade.

Another early distributed control design that has been very successful and continues to be marketed and installed today is the Ericsson MD-110. First introduced in the early 1980s, the MD-110 was originally based on the Ericsson AXE 5 central office switching system design. Each LIM port equipment cabinet has a dedicated control complex; LIM cabinets communicate with each other over PCM-based FeatureLinks. In theory, a single MD-110 can be installed with more than 200 LIM cabinets and Main System Processors. LIM cabinets can be centralized or dispersed across multiple customer locations, with each cabinet dependent only on its local Main System Processor for all call processing functions. The MD-110 is currently the only circuit-switched PBX system based on a fully distributed Main System Processor design.


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.


Local Processors & Port Circuit Card Microcontrollers | Legacy PBX

Local Processors

The first generation of stored program control PBX systems was based on centralized call processing system designs. Call processing system designs evolved to include a variety of processing elements outside the common control complex but under the control of the Main System Processor. These processing elements are sometimes referred to as slave controllers or local processors. These processing elements may be used for a variety of functions, such as systems administration and maintenance; function-specific applications, such as messaging, or ACD routing, queuing, and reporting; local switch network access; and diagnostics. Small PBX system models usually centralize all call processing, administration, and maintenance functions within the common control complex, but intermediate/large system models may use dedicated processor elements to offload some call processing operations from the Main System Processor or dedicated processors to handle systems administration and maintenance services.

Port Circuit Card Microcontrollers

The most common local processor element is a microprocessor controller resident on a port circuit card. The primary functions of the on-board microcontroller are to pass control signals originating from the Main System Processor to the individual station/trunk circuits and provide a signaling link between the peripherals and the common control complex. The port circuit board microcontrollers function independently of one another and are responsible only for the port circuit terminations on the printed circuit card. The microcontroller has the primary responsibility for monitoring the status of its colocated port circuit terminations and peripherals. It also provides the processing intelligence for the physical link connections to the local TDM bus under the command of the main control complex and/or cabinet/carrier shelf processors. The very localized processing functions performed by the microcontrollers are generally considered mundane and repetitive but are necessary to support the call processing functions of the main control complex. Localizing some processing operations at the level of the port circuit card reduces the processing load of the main system processor and increases the overall system call processing capacity potential.

The AT&T System 75 integrated the first port circuit card microcontroller into a PBX system call processing design in 1984. Today it is a standard port circuit card design element. The current microcontrollers are not based on the latest processor platforms but on older 8-bit, 16-bit, or 32-bit microprocessors. All port terminations on the printed circuit card depend on the local microprocessor, and processor failure or error will result in loss of service. No PBX system design offers a redundant microcontroller design option because service loss is limited to a small percentage of total system ports. For this reason, it is sometimes prudent for a customers to distribute vital peripheral resources across two or more port circuit cards.


Main System Memory | Legacy PBX

The main system memory component of the common control complex consists of several types of memory databases:

  1. Generic program

  2. Operating memory

  3. Customer database

The generic program stores the main call processing program consisting of all operating instructions, provides the main processor element with necessary intelligence to perform the tasks required by the system, and executes continuous diagnostics, system measurements, and fault isolation routines. The generic program also includes all feature and function software codes in support of station- or system-initiated call processing features and functions, including the standard feature set and optional software packages.

The operating memory is also known as the working memory because it stores all data and information related to the real-time operating conditions of the PBX system, including port circuit status, switch network status (time/talk slot availability and usage), and status of activated features and related data.

The customer database memory contains all data and information related to station user profiles, terminal devices, and the system configuration. Customer database information includes customer programmed information such as class of service and restriction assignments; hunt, trunk, and call coverage group assignments; call routes and routing patterns; system dial plan; terminal button assignments; and system access passwords.

There is no standard PBX system memory storage supporting the three basic memory databases. Some PBX systems use a single memory storage element that is partitioned. Some PBX systems dedicate a memory storage element to each memory database or segment the generic program from the operating/customer database memory storage element. PBX systems typically use dynamic random access memory (RAM) for main memory storage. Electronic programmable read only memory (EPROM) might be used by older systems still in operation. Flash ROM is sometimes used in small systems to simplify customer database upgrades and shorten reboot time. Generic programs in small PBX system models typically require at least 24 Mbytes of RAM storage; very large models may require up to 256 Mbytes of RAM. Most PBX systems fall within this memory storage range.

A floppy disk drive unit is typically used to load resident software programs into the mass storage unit. Most current generation PBX systems use hard drives embedded on printed circuit boards; older systems used dedicated hard disk drive units. Other storage options are tape drives, Flash ROM, and magneto-optical drives.


Basic Call Processing Functions

The primary call processing responsibilities of the Main System Processor are provisioning of dial tone, digit reception and analysis, number analysis, TDM bus talk slot assignments and switch connections for intercom and trunk calls, routing analysis, feature provisioning, and call monitoring.

There are several fundamental main processor management functions used to process calls:

  1. Call sequencing control: management of the call sequence logic that takes a call from one state to another

  2. Resource management: management of various system resources, such as DTMF receivers; time/talk slots for call connections; tone generators; and internal software records for call processing (including the system dial plan), messaging, measurements, and call detail records

  3. Terminal handling: management of different desktop terminal models, including support of line appearances, feature buttons, display fields, adapter modules, and other functional components

  4. Routing and termination selection: controls the selection of the terminating endpoint (station, trunk) of the call, including functions such as hunting, bridging, call coverage, and least cost routing.

Call processing is a series of events that result in the completion of a call. Figure 1 illustrates the call dialing and connection process. The process begins when a port (station or trunk) changes from an idle to an active state. Port seizure occurs when a station goes off-hook, a trunk port circuit receives an incoming call signal from the Central Office or network, or an attendant begins dialing. When a station port has been seized, the main processor seizes a register storage record, instructs a tone sender unit to send dial tone to the caller, and instructs the switch network to establish a connection to the port. When a CO or DID trunk port has been seized, the main processor seizes a register storage record, assigns a tone receiver unit register to the port, and instructs the trunk circuit port to signal that the main processor is ready to receive digits.

Figure 1: Call dialing and connection process.

The second step in the call process is digit reception and analysis. Digits are sent by system users to dial another station or activate a specific feature. A call register stores dialed digits or received digits over a trunk circuit. Digits can be received one by one (manual dialing) or in a group if a dialed number has been preprogrammed—speed dial. For station initiated calls, the first digit received by the register suppresses the dial tone. There is typically a time-out period between going off-hook and dialing the first digit or between the first and second dialed digits; if no digits are dialed within the programmed period, an intercept tone is sent to the caller. In addition to seizing a register, a tone sender and receiver are simultaneously seized, a switch connection is made, and the Class of Service of the caller is checked. A no-progress tone is sent to the caller if a tone receiver cannot be seized.

The third step in the call process is number analysis. Number analysis allows the PBX system to identify the number dialed and properly route the call. Internal system number analysis is performed for all calls, both intercom and trunk calls, within the PBX system. Special number analysis processes are performed for DID trunk calls and dialed feature access codes using * and # keys. For incoming DID trunk calls, the main processor analyzes the digits to determine whether the number is an attendant console position. If the number is an attendant console position, the call is processed and routed to the attendant. If the number is not an attendant console position, the internal number analysis program analyzes digits.

The internal number analysis program determines the correct call processing procedure to be implemented. There are many types of dialed or received numbers that can be analyzed for call routing purposes: station number, including hunt group number; individual, group, or emergency attendant number; abbreviated dialing number (speed dial); paging number; automatic route selection access code; Direct Inward System Access (DISA) code; data station number; modem pool group access code; and external destination codes, such as 911. If a number is not defined in the number analysis program, it is treated as a vacant number, and the appropriate intercept treatment is applied.

Another important main processor function is provisioning of call progress tones and indications. Call progress tones and indications are single- and dual-frequency combinations applied in a variety of cadence patterns, such as continuous tone sending, one repeated sequences (tone, pause, tone pause), two repeated sequence (tone 1, pause 1, tone 2, pause 2), or three repeated sequences (tone 1, pause 1, tone 2, pause 2, tone 3, pause 3).

Call progress tone types include:

  • Busy tone

  • Call diversion indication (call forward indication)

  • Call waiting indication

  • Conference tone

  • Confirmation tone

  • Dial tone

  • Expensive route warning tone

  • Intercept tone

  • Intrusion tone

  • Message waiting tone

  • No-progress tone (congestion tone)

  • Off-hook queue tone

  • Recall dial tone

  • Ringback tone

  • Special dial tone (do not disturb tone)

  • Special message waiting tone (message waiting)


Operating System Platform

The first PBX call processing system designs were based on proprietary operating systems before commonly used platforms such as Windows were developed. The operating system required to support a circuit switched PBX system must meet stringent, real-time, multitasking demands. A PBX system may need to support hundreds, maybe thousands, of simulta- neous conversations, with each station user potentially activating a variety of features before or during a call. The real-time nature of PBX-based voice communications requires an operating system designed for its unique operations and features. An operating system such as Windows is not ideally suited for circuit switched, real-time call processing applications. Even today, many years after Windows has become the most popular server operating system for enterprise system applications, most PBX systems use a proprietary operating system. A few small PBX system models designed for customers with fewer than 200 stations are based on a Windows NT or Windows 2000 operating system platform, but there are no announced plans to use a Windows operating system for a large system model based on a circuit switching design.

AT&T was the first PBX manufacturer to use a version of an industry standard operating system, UNIX, when it introduced its System 85 family in 1983. The proprietary operating system developed by AT&T in the early 1980s, known as Oryx/Pecos, is still used by Avaya, the AT&T/Lucent Technologies spin-off, in its current intermediate/large Definity PBX models. In keeping with the company’s tradition of innovative operating system platforms, Avaya’s small system Definity One and IP600 models were the first circuit-switched PBXs to run on a Windows 2000 operating system. The Avaya PBX system platform will be based on a client/server platform using a version of Linux as its operating system and targeted at customers with significant IP telephony requirements.

Alcatel used a version of the UNIX V operating system, known as Chorus, for its 4200/4400 PBX models during the mid-1990s and continues to use it as the foundation for its new OmniPCX 4400. Nortel Networks began using a UNIX derivative, VX Works, for its Meridian 1 system in the early 1990s, and has successfully migrated its software and operating system to its new Succession CSE 1000 client/server IP-PBX system design.

Mitel was the first traditional PBX system manufacturer to use Windows NT server for a circuit switched system design, but recently changed to a VX Works platform for the Ipera 3000, the latest upgrade of its server-based call processing design that supports the traditional circuit switched SX-2000 peripheral equipment cabinets.

The operating system of a traditional PBX system provides services and system resource allocation to the call processing, feature/function, administration, and maintenance software programs. The operating system coordinates all system processing elements and controls CPU bus activities. An operating system program passes signaling and information between high-level programs running on the Main System Processor and lower-level programs running on localized processors (cabinet carrier and/or port circuit level). Other important operating system programs are used to support maintenance and fault processing programs, mass storage (customer and system database) programs, file management system programs, and I/O programs supporting external devices, such as printers, modems, and alarms.


Main System Processor

The Main System Processor is responsible for all call processing activities. It may also be responsible for maintenance and diagnostics activities, although some PBX systems are designed with a dedicated processing element for this function. The Main System Processor executes high-level call processing functions based on computer programs stored in system memory, monitors and controls all port-to-port connections, provides status indications to station users, and initiates the operations necessary to implement system features and functions.

The Main System Processor in current PBX systems is typically a 32-bit microprocessor chip from an outside supplier. Intel and Motorola have been the primary suppliers of microprocessor chips used as Main System Processors during the past decade. Several leading PBX systems are currently using Pentium-level microprocessors, although older microprocessors, such as the Intel 386/486 or Motorola 68030/40 chips, are used in many currently marketed systems. Many installed PBXs still operate on older 8-bit or 16-bit microprocessor technology platforms, proof of the long life cycle viability of traditional PBX system common control design.

The main processing element is an important factor for determining PBX call processing power, but it is not the only factor. Software code, call processing system design, and feature/function implementation play important roles in determining the so-called horse-power of a PBX system, known as Busy Hour Call (BHC) capacity (see below). The feature/function capabilities of a PBX system are also relatively independent of the main processor element because the generic software feature/function program in many current PBX systems is processor independent. It must be emphasized that using the latest generation microprocessor chip does not automatically guarantee a PBX system high call processing capacity or advanced feature/function provisioning.


Legacy PBX Traffic Engineering Analysis

The call processing system of a PBX is responsible for all control functions and operations. It is responsible for monitoring and supervising all system design elements, including the switching network, printed circuit boards, and peripheral equipment, including station terminals and trunk circuits. It is also responsible for basic call set-up and teardown, feature/function provisioning, and systems management and maintenance operations. The PBX system call processing design has evolved from a single processing element responsible for all control functions to many processing elements, each with very specific responsibilities and functions. The PBX call processing design may differ from system to system, but there is a similar set of functional elements and responsibilities that is common to all enterprise voice communications systems, regardless of system size or functional complexity. Figure 1 illustrates the main PBX processing elements in a typical configuration design.

Figure 1: Main PBX processing elements.

Common Control Complex

The PBX common control complex can best be described as the brain of the system. Although other systems and subsystems may be responsible for physical operations, such as switch connections and voice signal transport, the common control complex is the command center responsible for issuing orders and supervising operations. There are several main components in a typical PBX common control complex, including:

  • Main System Processor

  • Main System Memory

  • System Control Interfaces

  • I/O Interfaces

The common control complex can be a single printed circuit board containing all of the listed common control elements, or it can be individual printed circuit boards for processor elements and interfaces and dedicated memory storage elements, such as hard disk or tape drives. The Main System Processor and Main System Memory are the two core elements in the common control. The System Control Interface and I/O Interface provide access to the two main common control elements for other internal system components and external system devices. The System Control Interface provides an intelligent link to the switch network (TDM bus) and processor bus to monitor port circuit activity and pass signals and messages between the main processor and local processors. The System Control Interface also can be used to support external call processing elements or adjunct systems dependent on the PBX common control complex. Examples are a CTI applications server used in call contact centers, and a third-party VMS. The I/O Interface ports typically are used for systems management, maintenance, and reporting functions. Examples are systems management terminals and call accounting systems. Figure 2 illustrates the main design elements of the common control complex.

Figure 2: PBX common controls and interfaces.
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