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Migrating large TDM PBX systems to VoIP solutions

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This article describes aspects of the migration process from a TDM (Time Division Multiplexing) PBX to a VoIP solution like Office Communications Server.

TDM to VoIP migration using PRI tie lines

A large enterprise TDM PBX consists of one or multiple PBX nodes. All endpoints (mostly PBX phones) on one site are connected to these nodes using two or four wire technology.

Before it is possible to connect a Gateway (GW) to a TDM PBX, a PRI link needs to be provided by the PBX. If no board with a currently unused PRI link is available in the PBX, an investment needs to be taken into this PBX by adding a PRI board. Costs are around 5.000 USD for a board + configuration (also anticipate delivery times for the new board) + sometimes networking licenses that need to be paid to the PBX vendor in order to configure a tie line. Worst case, if there is no space for another PRI board, a new shelf needs to be added or even an entire cabinet to place the PRI board into the PBX.

Now that an unused PRI connection became available in the PBX that can be configured to match a protocol of our GW, the VoIP GW can be connected to the PBX. Since it’s going to be a big VoIP deployment, an 8 PRI GW (=8 x 23 channels (US) or 8 x 30 channels (Europe)) will be purchased right away but in the initial stage only one PRI link will be connected to the GW.

Now it is possible to start moving users over from the TDM PBX to the VoIP solution by rerouting the extension of the TDM PBX user to the tie line where the GW is connected to.

More and more users will be migrated from the TDM PBX to the VoIP solution so that the need for tie line connections (PRI connections) between the TDM PBX and the VoIP solution increases. Why? Because users don’t change their communication behavior, just because they have been migrated. Migrated users will need to communicate with non-migrated users to the same extend as before the migration. Therefore the need for PRI connections (new PRI boards, PBX licenses …) increases while increasing the number of VoIP users.

The need for tie line capacity will increase up to a point where migrated users will start to communicate more and more with other migrated users. These communications will occur on IP without a need for Gateway and tie line capacity. At this point, there is no need for more PRI tie line connections anymore and the more users will be migrated to the VoIP solution less just previously installed PRI boards and sometimes even entire GWs become obsolete. This could be called a “Tie line paradox”.

This means at a given point, it becomes financially attractive to end the “smooth” migration process and do a “rip and replace” migration where the PBX solution will de decommissioned and the VoIP solution will become the only voice solution in the enterprise. At this point, the VoIP solution needs to take care also on the remaining extensions and connections of the legacy TDM PBX. This means the VoIP solution or the GWs need to provide a solution to connect fax machines, alarm systems, elevator phones, modems …. which has to be taken into consideration before pulling the plug on the TDM PBX.

TDM to VoIP migration using IP trunk connections

What happens, if the TDM PBX would be able to provide an IP trunk connection that matches the SIP requirements of the VoIP solution?

Essentially the migration path is the same. PBX needs to be configured for every migrated user and eventually licenses need to be purchased in order to install the IP trunk in the TDM/IP PBX (Hybrid PBX). But a significant difference is that no HW (PRI boards, GWs) needs to be purchased. To be more precise: No HW needs to be purchased that will be thrown away after the migration. The IP trunk connection is scalable.

TDM to VoIP migration using GW fronted PBX approach

Another approach is to place a GW between the PBX and the PSTN. In this approach the PBX needs to be configured for every migrated user to route the call to the PRI connections that were previously directly connected to the PSTN so that a call from a PBX user to a migrated user will pass through the GW. Also it needs to be configured on the GW that calls to migrated users from the PSTN will be routed to the VoIP solution and not to the PBX based on called party number.

Since the extension of users that will be migrated will never be in continuous number ranges, every extension needs to be manually configured in the routing table of the GW (of all GWs if there are multiple).

Please check with the GW vendor on how many routing entries are possible to configure as this number is currently rather limited and would prevent to use this scenario for big migration projects.

Conclusion

A “smooth migration” can be achieved in multiple ways. The PRI tie line approach is only good for the initial rollout. Before significant amounts of users will be migrated from the TDM PBX to a VoIP solution, IP trunk connections could to be used at first. But as soon as the majority of users (maybe >50%) has been migrated to the VoIP solution, the “rip and replace” method is the only financially interesting solution.

Comments

  • Anonymous
    January 01, 2003
    Nice article and a good overview of migration paths! One small comment though, there is HW needed to communicate from VoIP to TDM; DSP ports. Depending on the amount of traditional TDM user on the IP PBX there could be a growth of VoIP traffic, thus using more TDM ports.

  • Anonymous
    January 01, 2003
    Doing this in Europe using E1 instead of T1, the need is approx. 6 E1 GW interfaces instead of 8 GW interfaces. (823=184 channels, 630=180 channels) Since every interface HW (Both PBX and GW) costs about 5000 $ the saving is approx 4 *5000 $ (2 PBX interfaces, 2 GW interfaces). /Torbjorn

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    March 30, 2012
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  • Anonymous
    July 25, 2012
    The comment has been removed

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