Inside the sewers delivering terabit broadband


Inside the sewers delivering terabit broadband

Even several metres underground, with thigh-high rubber boots on and breathing shallowly to avoid choking on the foul-smelling air, it’s impossible not to admire the ingenuity of running broadband down a sewer.

To fully understand the depths to which people in this country will go to boost connection speeds, you need to visit business broadband supplier Geo Networks. Not its offices, which are surely clean and comfortable, but its infrastructure: its fibre cables run through London’s sewers. As far as press tours go, it’s not exactly Vegas, but not an invite one can turn down either.

One of the biggest issues holding back fibre broadband is where to put it and how to quickly and cheaply roll it out. Digging is annoying to drivers and expensive and difficult for companies to do, and although BT has been forced by Ofcom to grant access to its ducts and poles, there are other solutions, such as making use of the holes in the ground we’ve already got.

And there are other benefits to sewer broadband, Geo Networks points out, aside from leaving roads untouched.  There’s loads of space, thanks to the foresight of Victorian engineers, with many tunnels wide enough to walk through; there’s solid security, as Thames Water keeps a close eye and people don’t tend to wander down holes filled with raw sewage; and they have a wide reach — by running cables through sewage tunnels, Geo can cover all of the “metro” area of London with its 120km of cabling, and already has 3,000km of cable laid across the UK, with some into rural areas.

Each of the four mud-covered cables in the tunnel we visited holds 432 fibres, and engineer Matt Adams told me a pair of dedicated fibres, which he said most businesses take, effectively gives unlimited bandwidth. Geo claims its recent tests showed each individual fibre offered 2.5TB/sec. Of the four cables running through our sewer, two were full of fibre and ready to go, while the other two were about half full, Adams said — meaning Geo can easily blow fibre through to boost capacity when needed.


Rob Smith, the catchment engineer with Thames Water who took us on our underground tour, proudly pointed out that the sewers were built 150 years ago —  as they’re still in use, that shows remarkable engineering foresight. He pointed out that’s what Geo is trying to do with its broadband: roll out infrastructure that will offer enough capacity for decades to come. Whether the four fibre-filled cables in this sewer offer bandwidth enough for 2062 remains to be seen, but it’s clear that chucking cables through a tunnel wide enough to walk a group of journalists through is much simpler than digging up pavement.

There are some challenges to rolling out sewer broadband — aside from the smell of the stuff you’re sloshing through — but once it’s in place there’s very little that can interrupt it. The cables are rodent-proof and water-proof, so the usual sewer inhabitants shouldn’t be able to cause much trouble, nor should a particularly, ahem, full tunnel.


Problems can be caused by builders drilling new tunnels to the main sewers, such as to connect a new block of flats; if they don’t know where cables are, they can potentially slice through them. Another potential trouble is “rags”. At first, I thought Smith said “rats” — a word guaranteed to induce instant terror in someone standing in knee-high sewer water, I can exclusively report, alongside a sudden desire to know whether rodents can swim and how quickly.

Thankfully, Smith said “rags” ; it’s the sewer worker’s euphemism for bits of material — he suggested “wet wipes” were a major culprit, but I didn’t ask for further examples — that are flushed down the toilet, but rather than being washed away, catch in corners of the pipes and build up into huge piles. Huge, incredibly rank smelling piles (there’s no photo as I was busy gagging). If those rags catch on a cable, they could in theory pull it down and take out connectivity — so far it hasn’t happened, but it’s something Thames Water’s engineers watch out for. In other words, be careful what you flush, or it could be your broadband that goes down the drain.

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