All-IP migration for a legacy PBX
Published
For years the switch to All-IP was a date in the future. In Germany it is now largely behind us: no new ISDN lines are sold and existing ones have been converted. Other European markets are at different points on the same curve, but the direction is the same everywhere. The question has not gone away, it has moved — from when will this happen to the line is already SIP and the PBX is still in the rack.
Why the PBX can stay
A classic PBX is paid for, it is configured, the staff know it, and replacing it is a project with a budget, training and risk attached. The line behind it is a SIP trunk. A VoIP gateway belongs in between.
The gateway presents an ISDN line to the PBX: the ports run in NT mode, the PBX still sees BRI or PRI and dials the way it always has. Towards the provider the device speaks SIP. Nothing changes for the PBX, nothing changes for the extensions, and the money already spent on the equipment stays where it is.
What the IP side brings with it
- Less effort for maintenance, administration and call charges
- Better interoperability between vendors, because SIP is an open standard
- More collaboration features once the PBX itself is eventually replaced
- New workplaces provisioned faster, without a cable run to the frame
Analogue devices keep working
Fax machines, card terminals, door intercoms, lift emergency phones, alarm diallers: none of them speak SIP and none of them ever will. An analogue gateway with FXS ports puts them behind the SIP line transparently rather than re-encoding them — which is precisely why fax and modem traffic survives the trip.
Common questions
What is All-IP?
A network in which voice, data and video share one transport and telephony is signalled with SIP. The separate ISDN switching path is gone.
What happened to ISDN?
The ISDN line towards the provider disappeared. What did not disappear is ISDN as building wiring: the BRI bus feeding the DECT base stations, the PRI port on the PBX, the ISDN card in the fax server. That side still speaks ISDN — only now it speaks it to a gateway instead of to the exchange.
What does it mean for an older PBX?
Without a gateway an ISDN-based PBX cannot place calls after the conversion, because the line it was built for no longer exists. With one in front of it, the PBX carries on.
Which interface do I need?
That depends on how the PBX is connected: BRI for smaller systems, PRI (E1) for larger ones. The ISDN gateway family covers both, and because every model is the same modular platform, the module can be swapped later when a site is consolidated.
And when the line fails?
Then a second path helps: a VoLTE eSBC that adds a mobile route, or a failover switch that physically moves the line to a backup path when the primary system stops answering.
An overview of the usual starting points is under Solutions; guides and downloads are under Resources.