What is “PSTN Replacement”?
PSTN is Public Switched Telephone Network – an analogue technology that most of us will be familiar with, as it is what’s used in old-fashioned telephone landlines. The technology works by making a physical circuit between the two communicating end points, via an exchange.
In the UK, this service has been operated by British Telecom (BT) for decades. Much of the infrastructure is very old and is unsupportable. This, combined with the move to digital, IP-based technologies, has led BT to declare the cessation of the PSTN service in December 2025.
For those of us who are domestic landline customers, the changeover is relatively simple. A new handset (often issued by your provider) and an extra cable converter or two can be readily installed in minutes and will enable a digital handset. This will provide the same functionality as the analogue one but using digital, IP-based technologies.
Why is it a big deal in the Operational Technology (OT) world?
Many companies running large OT estates depend on PSTN comms to communicate with their remote operational sites. These sites are running autonomously and in the majority of cases will be unmanned. They often number in their thousands for a large utility and provide critical data from the plant and instrumentation at those sites (such as alarms). If communications to those sites is unavailable, the best that the utility could do is to send a person in a van to check on the health of the site – but utilities don’t have that kind of manpower and this would not be sustainable. In most cases, the company would be blind to what’s happening on site for long periods.
Upgrading the comms infrastructure to move away from PSTN has a number of challenges and many of these are likely to be common across many businesses. This article explores a few of those challenges, based on my experience of working in a PSTN replacement programme for a major UK utility. The challenges are listed below.
Criticality
I mentioned this earlier, but this is likely to be a critical project. The consequences of failing, or not completing fast enough, would be very serious for an OT-dependent business. They would likely be blind across most of their OT estate which means they wouldn’t receive alarms and other data necessary for the safe and optimised running of their business. Years ago, large sites would be manned either during the working week or 24/7 for critical sites. However, these days OT is used to monitor the sites remotely and the business won’t have the manpower to have people on sites, even temporarily. For these reasons PSTN replacement is nearly always a “must do” project.
Scale
Many businesses with large asset bases will likely have thousands of OT sites. Some sites will be large facilities and might be manned. Some are likely to be very small, perhaps only monitoring a single physical process. There will be sites which are in between these sizes. A site visit will be required for each one. Equipment will need to be purchased. A programme to deliver this work will be required. The budgets for all of these things will be substantial and will need to be planned and approved well in advance.
Working to a fixed deadline
This challenge is fairly self-explanatory. BT have a sunset date for PSTN of December 2025. This shapes the project and means that the sooner the project is started, the less pressure it will be under. A project typically has two stages:
- Designing and proving one or more solutions
- Rolling out the solution to every operational site
The second of those phases, the “rollout” will need to operate at a run-rate which, on average, is the total number of sites divided by the time available before Dec 2025. A sensible unit of measurement is “sites per week” – the number of sites, on average, that need to be converted from PSTN to another solution, every week until the deadline.
The run- rate that can be achieved will depend on the resources available, but the later the start, the higher the run-rate, the greater the pressure to achieve it and the less wiggle room there is if things go awry.
Time on site
Starting earlier, and getting to the rollout phase earlier, obviously helps. However, designing a solution which means that the time required on site is minimised will also help the run-rate – a field team will be able to convert more sites in a week if the time on site is short.
The age and variety of field equipment
OT equipment isn’t like IT equipment. It’s not normally refreshed every 5 or so years; it’s usually expected to last much longer (and does!). A lot of OT equipment working over PSTN won’t be ready, out-of-the-box, to use IP comms. There may not be the time or the budget to change all of this legacy OT equipment, so creative retrofit solutions may be needed. Alternatively, if time / budgets allow, there is an opportunity to change other legacy equipment on the site whilst the site is being worked on and service outages are already planned.
Supply chain
There are some standard challenges for a project this size. Matching rates of supply of equipment to the rate of installation is one. However, in the post-covid world, lead times for electronic products have become very long. Combine this with other large companies also doing similar large projects at the same time, there is considerable pressure on supply chains and competition for resources. Similarly, given the same end-date that all UK companies will need to work to, there could be a competition for staff availability to perform all of the site visits required.
Skills mix
A programme would need to be developed and budgeted. It would have to develop a robust technical solution, to manage the supply chain, and to visit all of the sites. All to an immutable deadline. This would need strong and committed programme governance and wide range of skills and experience.
Security considerations
The PSTN network being replaced had a certain amount of security and resilience built-in. As this network is replaced by a digital, IP-based network, it must be secured like any other IT network. Depending on the complexity of the replacement network’s architecture, this might be far from a simple task and would require specialist network security knowledge.
Conclusion
A PSTN replacement programme has a number of significant areas of challenge. These all need to be met as part of a “must do” project against a fixed deadline whilst there is competition for resources. The sooner the project is underway, the greater the chance of success!