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.

Centralized | PBX Switch Network Topologies

A centralized topology is defined simply as a switch network design that requires all calls, regardless of the origination and destination endpoints, to be connected through the same TDM bus or switch matrix or the center stage switch complex.

Figure 1: Centralized switching network topology.

Any PBX system with a switch network system comprised of a single TDM bus or switch matrix is classified into the centralized switch network design category because all calls are handled through the same “centralized” switch network element. Most small PBX systems have a switch network design based on a single TDM bus because the traffic requirements for equipped systems with fewer than 100 ports (stations and trunks) can easily be supported without multiple TDM bus requirements and/or a center stage switch complex. A single TDM bus design may also be used by PBXs with larger port capacity limits, if the TDM bus bandwidth is sufficient to support the port traffic requirements. For example, the Avaya Definity’s switch network design is based on a 32-Mbps TDM bus that can easily support the very small port (20 to 40 stations) traffic requirements of the Definity One model and the larger port (40 to 400 stations) traffic requirements of the Definity ProLogix model.

Many intermediate/large PBX system models have centralized switch network designs because a center stage switch complex handles all call connections regardless of the originating and destination call endpoints. It is easy to see the necessity of using a center stage switch complex to support switch connections between port interface circuit cards housed in different multiple carrier port cabinets because a local TDM bus is not configurable across common cabinets, and the system installation may include many port cabinets. The system switch network architecture is easier to design and program if all calls are connected with a centralized switch network element because the same call processing steps are followed for each and every type of call. It is more difficult to see the necessity of using a center stage switch complex to support switch connections between port interface circuit cards housed on the same port carrier shelf or even between ports on the same port interface circuit card, but a centralized switch network design dictates the same switch network connection protocol (center stage switch complex connection) regardless of originating and destination port interface circuit proximity.

The Nortel Networks Meridian 1 Option 81C, Siemens Hicom 300H, and Fujitsu F9600 XL models are examples of centralized switch network designs. Each of these systems can be installed with multiple port cabinet stacks, with several port carriers per cabinet stack. Each system uses a center stage switch complex to support connections between each port carrier’s local switching network (single or multiple local TDM buses), even if the two connected telephones are supported by the same port circuit interface card and connect to the same local TDM bus that connects to the same the Highway bus that connects to the center stage switch complex. It may appear a waste of switch network resources (talk slots, switch connections) to use the center stage switch complex for a call of this type, but that is the way the system is designed and programmed. Figure 1 illustrates the call communications path for a Meridian 1 Option 81C between port interface circuits in different cabinet stacks and between port circuit interface circuits on the same port circuit interface card. The call connection protocol is similar, if not identical, for the Siemens and Fujitsu systems.

A centralized switch network design offers no customer benefits, but it can be problematic because a large number of potential switch network elements (local TDM buses, Highway buses, switch network interface/buffer, TSI, center stage switch elements) are required to complete any and all calls. This can affect switch network reliability levels because the probability of switch network element failure or error affecting a call connection is increased. For example, center stage switch complex failures or errors affect all system port connections in the PBX system.

A major disadvantage of the centralized switch network design is when a customer needs to install a remote port cabinet option to support multiple location communications with a single PBX system. A remote port cabinet option requires a digital communications path between it and the main PBX system location. Most remote port cabinet installations are supported with digital T1-carrier trunk services. If the PBX switch network topology is centralized, all calls made or received by station users housed in the remote cabinet must be connected through the center stage switch complex at the main location, even if calls (intercom or trunk) are local to the remote port cabinet. A T1-carrier circuit, with a limited number communications channels, must be used for every remote cabinet call to access the center stage switch complex. Most remote PBX cabinet options require two T1-carrier channels per call connection, thus limiting the number of active simultaneous conversations at the remote location. This may force the customer to install additional T-1 carrier circuits to support the port traffic requirements at the remote location, but there are limits on how many T1-carrier interface circuit interfaces can be supported by the remote port cabinet. The limitations of the centralized switch network design may force a customer to install multiple remote port cabinets at the remote location or a standalone PBX system.