Dotted around London, there are a number of private hotels that aren’t the sort of hotel you might expect – they’re needed to make mobile phones work on the London Underground.

They’re not for humans, but for huge racks of computer and electronics equipment – and each of the UK’s mobile networks is renting space inside them, as this was one of the innovations that allowed phones to work underground in the first place.

While many newer underground railways have had mobile coverage in their tunnels for years, fitting it into the London Underground kept running into the same problem – it costs a lot of money and needs a lot more space in the stations than is available.

Classically, each of the mobile networks would install its own kit in each station and manage it, but there simply wasn’t enough space for that in London’s old tube stations, many of which were built before the wireless telegraph was even invented.

However, as radio equipment has become smaller and cleverer, it’s now possible for several networks to share the same equipment, and in 2021 Boldyn Networks (then BAI Communications) signed a deal to build a “neutral network” that can be leased to mobile networks.

Bodyn has a 20-year concession with Transport for London (TfL) to build and operate the network, and, aside from internal staff time spent managing the project, it’s being delivered at no cost to TfL.

The new network doesn’t just provide phone coverage for customers; it’s also part of the government’s Emergency Services Network and might, in the future, replace staff radios when they’re being upgraded.

In fact, the customer-facing part of the network, while not insubstantial, is the smallest part of the whole project. Given the hundreds of millions it would cost to upgrade the emergency service network and staff back-of-house radio networks, it makes sense to provide a public service as well.

So, for the past four years, hundreds of miles of cabling have been rolled out (literally) into the tunnels and stations, while rooms have been filled with computer racks, air conditioners and a host of related equipment.

Installing all this in old tube stations, in an industrial environment that’s quite hostile to clean equipment and notably lacking in nice straight walls, has not been easy.

A few deadlines were missed because it took longer to find and clear sites for the equipment and get sign-offs on approvals, and there was a very steep learning curve in working in cramped curved spaces, often with just a few hours each night to get in and back out again.

Although it’s taken four years to get about half of the London Underground covered with a phone signal, they now expect the rest to take just a year to complete. That’s in part thanks to experience speeding things up, and also because a lot of the equipment has already gone in over the past four years. All they need are the final bits to complete the job and switch it on.

So, most of it should be live by the end of this year, with an expectation that the installers will be packing up their bags for the last time around the middle of next year. When they pack up, they will leave behind 800km of cabling inside the tunnels, around 1km of cables and several server racks inside each station – oh, and some hotels.

To explain.

The term “hotel” is industry jargon for large data centres with space rented to mobile networks, which install their equipment there. From there, fibre-optic cables snake through London to the tube stations. A master cabinet, usually near the entrance if possible, which then feeds the signal to cabinets dotted around the station. These, in turn, are connected to the small radio antennas that your phone is looking for.

The larger the station, the more cabinets are needed, because of there’s a limit on the length of the coax cable the antennas can use.

Generally, they max out at around 90 metres – but your average tube station platform length ranges from 90 to 150 metres. So they almost always need to have at least two cabinets per station to ensure the signal reaches where the public expects it.

The more complex stations need more – Tottenham Court Road has 18 cabinets, just for the Northern and Central lines and ticket halls.

There’s another distance limit at work here, and that is the speed of light. It takes milliseconds for the signal in your phone to reach the hotel above ground and be handed over to the mobile network. But if it takes too long to get from phone to hotel, then your phone call s..a.rt..s..t o. br..e..ak up.

As it happens, that distance is about 12km, so Boldyn needs nine hotels around London to cover the whole of the Underground.

Back inside the stations, they’ve had to spend several years planning where to squeeze the cabinets in, often removing old, redundant equipment to make space. Sometimes dealing with water ingress, figuring out how to remove heat from the computers, and occasionally rats.

Some of the more modern stations come with much more back-of-house space than their predecessors, but if they wanted to build a room inside a vacant space, it wasn’t just some plasterboard and studwork to build the walls. These have to be fully fire safe, so the rooms could easily cost over £50,000 each.

That’s in part for very sensible safety reasons, but also because these systems are part of the Emergency Services Network (ESN), so needs a lot of resilience built into it.

For example, if one of the “hotels” were taken out of service for some reason, while consumer phones would stop working in the tunnels, the other hotels could pick up the slack for the emergency network. In part, that’s down to the ESN not requiring as intensive a data service as your smartphone. However, some of it is because the ESN radio network runs at a lower radio frequency.

In general, the lower the radio frequency, the farther the signal can travel and penetrate walls, but the less data it can carry. So the ESN in the tunnels runs at 400 MHz, far lower than the 700 to 3,600 MHz range usually used by smartphones.

Incidentally, if you read a tabloid headline about the NHS wasting money on old pagers when doctors could use smartphones, it means the person writing it doesn’t understand radio technology. Pagers operate at really low frequencies and work where thick walls and heavy hospital equipment block conventional phone signals. The pager might only be able to say “get to a phone urgently”, but it’ll do it everywhere.

Back to the tube stations, and while they might not need many antennas in public spaces to deliver a reliable signal, the thick walls in the back-of-house areas not seen by the public often require a dedicated antenna in every single room and corridor.

On average, the split is about 60/40 staff/public areas, and every single one of those antennas needs to be wired into place by staff, usually working at night.

Of course, it’s not just inside the stations that phones (and the ESN) work, but inside the tunnels as well.

They don’t really want to install hundreds of antennas inside the tunnels, as that’s not just a maintenance nightmare when one breaks, there’s also a theoretical risk that one could come loose and hit a train.

So they turned to a long-proven idea – the leaky feeder cable.

Quite simply, it’s a very fat cable with holes in the shielding that allow radio waves to leak out (and be picked up inside), acting a bit like a sprinkler hose, spraying radio signals all along the tunnel.

Well, almost all along the tunnel.

Unsurprisingly, the cables have a finite length before they run out of radio signal to broadcast, and some tunnels are longer than they can reach. Sometimes, there’s a place along the tunnel they can use to add another set of kit to run an extra cable or two – such as a ventilation shaft or an old tube station, like Bull & Bush on the Northern line.

If there isn’t something handy to use, then frankly, there might need to be a bit of a gap. However, it’s expected that most people might never even notice, as the trains will be travelling at their fastest in the middle of the tunnel, so any gaps will be short-lived, and most smartphones often buffer their downloads to give the illusion of a constant signal.

Fortunately, the ESN radio reaches where other radios can’t, so the emergency services won’t be cut off.

As it’s classed as critical infrastructure, every tunnel is equipped with two leaky-feeder cables, so if one fails for technical reasons, the second should still keep working until a repair is carried out.

But here’s a question for you.

What happens if a train has to stop in the middle of the tunnel, and you phone for help? Your phone normally sends its location to 999 when you call them, but if they send an ambulance to your location, they could arrive to find you sitting in a tunnel 20 meters beneath them. Which isn’t much help.

So, the system has been deliberately configured to transmit the location of the nearest tube station, where access can be arranged. That’s why sometimes you might check your smartphone map, and it will display the “wrong” location, because that’s the best one for a 999 call to use.

Getting to this point has taken four years.

Most of the tunnels were fully cabled by the end of last year, but as tunnel cables enter the stations, they can’t always be switched on until the station upgrade is also complete.

They’ve just switched on the 62nd station.

However, it can sometimes take a while for the mobile networks to activate their end. It turns out the phone signal inside the station can be better than the one above ground, which might cause issues when your phone swaps between them. That’s why you might lack signal in a station, but the person next to you on a different network has five bars.

Some recent switch-ons include the sub-surface lines between Blackfriars and Canon Street, and between Notting Hill to Bayswater. But as many of the stations are nearly completed, you can expect a cascade of tunnels and stations switching on this year.

The contract with Boldyn covers installations in all underground tunnels and tube stations and was recently extended to include the underground sections of the DLR and London Overground. You might think it’s obvious which stations are underground, but contracts like specific definitions – so in legal terms, the contract covers any station that has to comply with section 12 of the Fire Precautions Act 1971, which governs the rules for underground stations.

If the station is subject to those rules, it will get phone coverage added, otherwise, it’s up to the mobile network operators to deal with any coverage blackspots on the surface platforms.

So while Boldyn isn’t adding coverage to surface stations, they are upgrading coverage around them in some areas. Boldyn and TfL are using TfL’s surface infrastructure to run cables and deploy small cells in areas with high footfall, creating bubbles of higher phone capacity.

Boldyn says that it recently boosted phone coverage around King’s Cross, Waterloo, London Bridge, Old Street, The Shard and Hyde Park Corner. Ultimately, it’s up to the mobile networks to decide if and when they want to use the capacity being created, but it’s there if they want it (or not).

In the meantime, work continues in the tunnels and stations, aiming to complete most of it by the end of this year.

So next time your phone works perfectly 20 metres underground, remember: you’re not roaming at all – you’re just checking in to London’s strangest chain of hotels, where the only guests are signals, servers, and the occasional rat with excellent reception.