Client/Server IP-PBX System Design

Overview

The first IP-PBX systems were based on a client/server design. When most telecommunications managers hear the term IP-PBX, they usually envision a client/server design, although the converged system design is gaining in popularity and likely will dominate the market for the next few years. An IP-PBX system based on client/server design fully uses and depends on a LAN/WAN switching network infrastructure for call control and communications signaling. Like converged IP-PBX systems, client/server designs are not standard or uniform across manufacturers, although the competing models share some common design elements.

The term client/server is borrowed from the world of data communications. It describes an IP-PBX system that does not use a traditional PBX common control complex and integrated circuit switched network or traditional common equipment hardware (port cabinets and port interface circuit cards). A client/server data communications design specifies a data processing topology in which a personal computer (client) depends on a centralized computer (server) for applications software and database management functions. For many years the traditional PBX system design was compared with a mainframe computer because all call control and switching functions were centralized and desktop terminals (teleprinters, CRTs) lacked processing functions of their own. As enterprise voice communications systems evolved toward distributed and dispersed modular design topologies, similar to the concurrent evolution of minicomputer and personal computer networks, the term client/server was used more and more to describe the improved PBX system design. The first IP-PBX systems more closely conformed to data communications and processing client/server design topology; hence, the adoption of the term.

The common control complex of a client and server IP-PBX is based on a telephony call processing server that transmits and receives control and status signals to/from LAN-connected peripheral endpoints, known as clients. The telephony server’s primary role is as a gatekeeper to clients for call setup and teardown functions and to manage and control communications bandwidth requirements for each call. In IP-PBX terminology, a telephone terminal is also referred to as a client. The IP telephone client depends on the telephony server for dial tone, call routing, and desktop feature/function implementation, similar to the relation between an analog/digital telephone and a traditional PBX common control complex. The major distinction between a traditional circuit switched PBX system and a client/server IP-PBX is that the LAN/WAN infrastructure, not an internal network of TDM buses, is the primary switching network.

Some current and planned client/server IP-PBX models may be equipped with optional port cabinet/carrier equipment with integrated TDM bus backplanes for circuit switched connections, but only calls between ports connected to the same port cabinet/carrier are circuit switched; all other calls depend on the LAN/WAN infrastructure for transport and switching operations. This type of system also may be classified as a converged IP-PBX, because TDM/PCM circuit and IP packet switching is supported. Arguments can be made for categorizing it in either classification, but if the design primarily depends on IP packet communications, and circuit switching is secondary or optional, the system is best defined as a client/server design. Design differences between individual IP-PBX models, as illustrated by client/server designs with optional circuit switched port carrier equipment, make it difficult to definitely classify any PBX system into one category or another.

There are several hardware layer elements common to all client/server IP-PBX models:

• Call processing layer—Gatekeeper/telephony call server

• Client layer—Voice terminals and other communications devices

• Applications layer—Messaging, contact center

• LAN/WAN infrastructure—Ethernet switches, IP routers, telephony gateways

Differences between client/server IP-PBX models are based on how each design element is configured within each layer and between layers. There are also differences in how some features and functions are provisioned, and whether optional application services are integrated into the overall system design or provided through nonproprietary third-party equipment. Ethernet switches and IP routers are designed to industry standards, and products from different manufacturers are usually interchangeable at the infrastructure layer, except for a few select IP-PBX models that may require proprietary LAN/WAN solutions as part of their overall system design or integrate LAN/WAN interfaces into their hardware design.