Differences between TDM PBX-Hybrid PBX-IP PBX

In the context of OCS 2007 voice deployments it is essential to understand the fundamentals of existing corporate telephone systems. This article describes on a high-level how a TDM (Time Division Multiplexing) PBX, a Hybrid PBX and an IP PBX look like.

TDM PBX architecture

A TDM PBX is still the most commonly used corporate telephone system in the world. The initial design of TDM PBX systems was done in a time where there were no server-like systems that we know today with e.g. PCI bus-like standardized architectures available. Therefore PBX manufactures put lots of money and engineering work in creating proprietary system architectures with self-made cabinets, bus systems to allow flexibility and extensibility, matching proprietary endpoint hardware and firmware as well as protocols for the communication between the different components. The results of excellent hardware and software engineering as well as manufacturing were proprietary systems, designed to provide enterprise-grade telephony in a reliable high quality fashion.

The General architecture of TDM PBX systems were cabinets that provided power supply, operating system, occasionally hard drives, RAM, CPUs … for all the different boards that could be bought for this particular PBX and plugged into a proprietary bus system within a cabinet. The following picture shows a Nortel Meridian system with its cabinet and a couple of mostly extension boards plugged into the cabinet:

Over the years, more and more boards became available and each board served a specific purpose. There were for example boards providing:

- 16 analog extensions

- 8 digital extensions ISDN-BRI

- 8 proprietary digital extensions

- PRI (Primary Rate Interface) cards with 1 or 2 E1/T1 interfaces

- Management boards

- Boards to connect attendant consoles

- Interface boards to provide relay functionality for e.g. door opener

- Intercom functionality boards

- TAPI boards

- Music on hold boards

- Special extension boards to connect e.g. trading systems, hospital systems…

- …

Downsides of this architecture from today’s perspective were of course that customers had to buy boards from vendor A if you had a PBX from vendor A. Basically no chance to buy boards from vendor B for a vendor A PBX. Also extensibility was only possible in stages, as customers had to buy a 16 analog extension board if they had no more analog extension available and needed one more analog extension. This became very expensive when they needed another cabinet just to plug in the new extension board because there was no space anymore in the existing cabinet(s). Also not just hardware needed to be bought but software licenses as well, depending on the type of card. Vendors tried to mitigate this pain by offering per Port pricing models. The following Figure shows a simplified view of a TDM PBX:

Hybrid PBX architecture

Now what happened to the TDM PBX systems once Voice-over-IP technology could not be ignored anymore by the PBX vendors? PBX vendors designed a new board. Not just one but depending on the manufacturer a couple of boards that provided H.323 registrar or SIP proxy functionalities and had an Ethernet plug in order to allow IP phones to connect to the PBX (well now it’s a “hybrid” PBX as it provides IP and TDM extensions). But not just extensions became connected via IP to the cabinets but also entire cabinets could be placed in a distributed fashion, connecting with the main system via IP. Proprietary protocols tunneled in H.323 or SIP packets (in order to claim to be standard compliant) provided the same feature set to users as they were used with their previous phone. The good thing was that savings for the company by just using one common data/voice cabling system could be realized, no user trainings needed to be performed as the user experience basically was that they have a new looking phone now plugged into a different wall socket. I don’t want to sound sarcastic here: There was definitely the advantage for administrators as well as users to become familiar with VoIP technology and to figure out the right migration path for companies to move to VoIP. At the same time VoIP technology in General had the chance to mature. For hybrid PBX systems as an example, the following boards for the existing cabinets were offered:

- H.323 gatekeeper or SIP registrar boards

- Call server boards

- Signaling server boards

- …

The following Figure shows a simplified view of a hybrid PBX:

IP PBX architecture

Initial idea behind the design of IP PBX systems was to build corporate telephone system using VoIP technology, standard server hardware and existing corporate IP network infrastructure. Gateways were built to establish connections between the TDM world and the IP world. Similar to the board idea in the TDM world, Gateways became available that allowed connections to most endpoints were a direct IP connection wasn’t immediately possible (standalone FAX machines, modems, analog phones, door opener solutions, PSTN connections, PRI trunk connections to existing TDM PBX systems…). The heart of the system was usually one or multiple servers that were running on state-of-the-art server hardware. New functionality was added by creating new application servers. Depending on the manufacturer either proprietary endpoint hardware was needed to buy in order to gain most functionality out of the system (sadly, most of the time proprietary endpoints were designed to match a certain SW server release so that occasionally you had to buy new endpoint HW in order to be able to use new server functionalities) or standard endpoint HW could be connected with limited functionalities. Sometimes even dedicated logical IP networks for voice became necessary to eliminate conflicts with other business-critical IP applications. The following Figure shows a simplified view of an IP PBX:

Summary

It is essential to understand roughly the fundamentals of TDM PBX, IP PBX and PBX system in order to figure out the best migration path for an OCS 2007 voice deployment as no deployment should be designed following a “rip-and-replace” approach.

Comments

• Anonymous
  January 01, 2003
  Good Informative article. This was quite helpful!. No doubt that PBX systems are necessary for managing internal and external telephone calls within an office.http://www.smesauda.com/office-equipments/epabxintercom-system/36.html

• Anonymous
  February 16, 2011
  The Article is pretty much useful. Thank you very much.

• Anonymous
  March 11, 2013
  Great article!  I'm trying to explain to a TDM customer how they can interface with an IP system.  This made things a lot clearer to me.  Thank you very much!

• Anonymous
  December 03, 2013
  Very useful information written in an understandable way. Wish you would have provided definitions for the abbreviations, though.

• Anonymous
  May 08, 2014
  Easy to understand for layman as well -Thanks

• Anonymous
  July 06, 2014
  good information