ISDN VoIP gateways: BRI and PRI to SIP

ISDN was supposed to be gone by now, and in most contracts it is. What is not gone is the equipment behind it: the PBX that still has years of depreciation left, the DECT installation whose base stations hang off an S0 bus, the alarm system that dials out over a B channel. An ISDN gateway is what lets that equipment keep working after the line behind it has become a SIP trunk.

All of these models are the same modular eSBC platform with an ISDN module fitted. That matters for two reasons. The interface can change later without changing the device — a 4 BRI module out, a PRI module in, when the site consolidates. And the SIP side is a full SBC: topology hiding, header rewriting, codec conversion and two network ports, not just a protocol converter.

Every port switches between TE and NT mode in software, pin swap and 100 ohm termination included. That is the difference between a site visit and a browser tab when a site turns out to be wired the other way round than the plan said. The BRI and PRI modules carry a high precision clock, which is what a DECT installation needs before it will stay in sync across base stations.

One licensing point, because it is often assumed the other way round: calls between the ISDN ports and SIP need no session licence. The transparent and transcoded session licences apply to SIP-to-SIP traffic only. A pure ISDN-to-SIP gateway needs none of them.

For the Americas the PRI models can be built with a T1 module instead of E1. Fax servers reach the line through beroCAPI, which keeps the CAPI application on a virtual machine rather than on a server with a card in it.

Analogue interfaces on the same platform are described under analogue gateways. The platform itself, its session licences and the full module range are on the Enterprise SBC and add-on modules pages.