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.