Analogue VoIP gateways: FXS and FXO to SIP

Analogue is where an all-IP migration usually gets stuck, and not because of telephones. It gets stuck at the fax machine in the accounts department, the alarm dialler with a maintenance contract, the door intercom that was wired into the building, and the lift emergency telephone that is required by law to work. None of those will ever speak SIP, and none of them can simply be switched off.

An FXS gateway gives them a port that behaves exactly as before while the line behind it is a SIP trunk. What makes that work for the awkward cases is transparent hardware bridging: fax and modem traffic crosses without a codec in the path, so the tones arrive as they were sent. Where a compressing codec would have destroyed the transmission, a clear channel per RFC 4040 carries it through.

FXO is the other direction. Instead of terminating equipment, the gateway terminates a line — an analogue provider line, or an analogue extension port on a PBX that has to stay in service. That is the usual bridge during a migration, and the usual failover path afterwards.

Four models are the modular eSBC platform with an analogue module fitted, so the interface can change later without changing the device. The BNSBC-12FXS is the exception: twelve fixed ports, built for hotels, student residences, hospitals and fax service providers, where the port count is the point. All of them are managed through beroCloud like the rest of the range.

Licensing, once more clearly: calls between the analogue ports and SIP need no session licence. Only SIP-to-SIP traffic consumes transparent or transcoded sessions.

For BRI and PRI on the same platform see ISDN gateways. The platform, its session licences and the full module range are on the Enterprise SBC and add-on modules pages.