Building a network that can withstand damage, disaster and technical failure is one of the biggest engineering challenges in telecommunications.
It means concentrating on 'resilience' - in other words, designing systems that are hardened against accident and damage - and therefore less likely to fail.
It also means incorporating 'redundancy' - an extra back-up that can fill the gap if anything does go wrong.
The communications network was important in fighting the Cold War, with key sections of infrastructure subtly influenced by the chance that Britain might one day be at war against the Soviet Union.
The Post Office provided communications to the bunkers and nuclear defence command and control shelters across the country, with considerable thought being given to keeping the public networks that they relied on open during a war.
Emergency switchboards were established in the basement of all main telephone exchanges, surrounded by concrete blocks for extra protection against radiation.
Every exchange had an early warning system installed, so that the supervisor could make any necessary preparations before bombs or missiles landed.
