Interoperability is predictable

Way back in 2007, while presenting a seminar in Prague, someone asked me for my prediction on when SS7 would no longer be in use. My answer was suitably vague, but something on the lines of, “at least 10 to 15 years.” Ten years on, I wasn’t wrong. Still, I may not be right. SS7 is showing its age, but it’s not about to draw its pension just yet.


SIP endpoints predominate for new installs, PBX replacement, and next generation public safety answering points. All the way from enterprise VoIP through to WiFi calling and a plethora of smartphone apps, SIP prevails. That trend is inexorable.

However, there exists a significant global rump of PSTN or landline endpoints, and the fact remains that a heap of 3G mobile or cell-phone calls are facilitated by legacy SS7 infrastructure. And, in addition to that, many telco interconnects are still provided via SS7. Couple that with no clear prediction on their demise, and you have to admit SS7 will be handling phone calls for some time to come.

What is clear from that is that there will be an ongoing need for calls between those network types – essentially, SIP and SS7. Interoperability is an important consideration at all levels, from telco interconnects and emergency calls, on down to the smartphone app in your pocket. Notably, interoperability is achieved by gateways.

Despite the stated intentions of the likes of AT&T (2020), BT (2023), and Deutsche Telekom (2018) to withdraw all ISDN services in favour of SIP, you will be forgiven for calling their bluff. Even if those ambitions are realised, in due course and not necessarily on target, you won’t be left high and dry without an option but to follow the SIP route.

Contrary to the significant trend elsewhere, SIP hasn’t achieved dominance in the UK, albeit it’s getting there. That’s because enterprise users and the channel that serves them aren't convinced of the reliability of SIP compared to ISDN. That means if they have an IP-PBX, they use it with an integral ISDN connection – or at least a gateway – or they keep their old PBX and retain its ISDN connection.

There is an inference that when BT switches off the last ISDN circuit, everyone will have to switch to SIP. I guess that’s fair enough, and is unlikely to be an issue for anyone with an IP-PBX. However, what of those SMEs still hanging on to the TDM-based PBX they’ve had for years? Lack of maintenance and support may be seen as a lever to persuade them to swap it out, but if it’s been working reliably, where’s the incentive? And what’s the alternative?

Gateways are the alternative. Whether SS7 or ISDN, interoperability with SIP is what gateways provide. Gateways solve the problem of incompatible networks. That’s for why they were conceived and designed. That’s what they do.

It can be said that when everyone is using SIP, there will be no need for gateways. I foresee that no-one can predict when or whether that eventuality will occur. I might be wrong. I might be right.

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