The Cisco PBX Balloon

Happy February! If you enjoy these posts please read Post One for my motivation. I have gotten nowhere on my CCVP studies so no Gatekeeper notes yet. So close yet so far! Anyway I have been contemplating how Cisco took the traditional centralized PBX architecture and modified it for today’s distributed environment.

The traditional Nortel PBX was and I am guessing in most cases still is divided into several basic functional areas. These are Common Equipment, Network Equipment and Peripheral Equipment Looking at a PBX this way made it much easier to understand, operate and troubleshoot.

Common equipment or CE provides processor control, software storage and execution as well as memory functions and I/0 access

Network Equipment or NE provides interfaces under the control of the Common Equipment that performs the digital switching functions of the system. Time Division Multiplexed (TDM) timeslots are connected between various station and trunking Peripheral endpoints.

Peripheral Equipment or PE provides the interface between the NE and the telephones and trunk endpoints. These cards extend audio and signaling to these devices.

The CE, NE and PE components are housed in individual modules which are cabled together to provide a highly reliable, redundant, and feature rich system. The only problem with this design is that it was generally restricted to one site, used proprietary hardware/software and was expensive to purchase and maintain.

Then Cisco (or Selsious, I am not sure which company should be given credit) came along and blew this architecture up like a balloon! Common Equipment functionality is provided by Call Manager servers, uses off the shelf hardware components and a locked down version of Windows. Network components are moved out to...the network! Routers are used as voice gateways and provide analog and digital connectivity to PSTN or PBX systems. Switch ports replace the traditional Peripheral equipment ports. Interconnecting all of this is IP, which uses ports and flows to replace TDM and timeslots.

This takes the traditional PBX model and allows it to be distributed as wide as the network itself. Using a distributed design components can be located pretty much anywhere as long as delay parameters are followed and QoS is applied. Needless to say this complicates troubleshooting so documentation and a full understanding of this environment is critical to maintaining system reliability. We (the voice guys) used stand around and roll our eyes as sites went down due to network issues I find that the division between voice and data networks is getting very blurry and I suggest to all voice people to not only learn VoIP but also begin to gain a full understanding of the supporting infrastructure.