Let’s Get Physical

When you’re playing an online game, it’s easy to forget that there’s a lot of cable, exchanges, and other telecomms hardware between you and the other players. (Hence title: in network parlance, we call the cables ‘the physical layer’ of the OSI model).

We had some connection blips this Wednesday. A lot of players from the UK were suffering constant (and very annoying) disconnects every couple of minutes. On realising that this was a communal and local problem, naturally our first reaction was to blame the local ISP of choice, in this case Virgin.

This is possibly unfair since while their customer support isn’t anything to phone home about (haha) the service has usually been pretty solid.

As it happens, the problem was much simpler and more wide ranging than that. Someone drove a digger through a major cable in London. No major telco (telephone company) relies solely on a single cable, and by the evening they’d rerouted and service from our point of view was as good as usual.

But I just want to spare a thought for the complex system that makes up the internet and our telephone networks, and all the thought and planning that goes into making it so easy to recover from such a major setback. And when I say easy to recover, I mean from the user’s point of view. BT engineers will be working to fix that broken cable and it isn’t by any means a small operation.

I said above that no telco relies on a single cable. That’s not precisely true, and the section of the network from your local exchange to your house (often known as ‘the last mile’) may not have any redundant cables available. So when your net goes down due to technical problems, chances are that it’s a local issue. If it was an issue with the trunk networks, the telco would have just rerouted.

In any case, high traffic routes always have redundant routes and cabling built in. Network planners take the possibility of people digging through cables into account, the redundant cables run through completely different routes. Telcos monitor network traffic constantly from some operation centre and engineers are able to reroute at very short notice. Sometimes traffic can be anticipated — for example, if some popular TV show is having a phone in competition — and lines will be rerouted in anticipation. And yes, the TV companies do tell the telcos when they are doing this.

International routes are even more carefully monitored. These are huge revenue generators for the telcos who operate them (whenever an international call is made, a portion of your phone bill is paid by your local telco to the international one, through a complex system of monitoring and comparing calls in both directions.

Actually, the first¬† telecomms standards organisation was set up back in 1865 for to help international companies figure out how to transfer and bill calls to each other. It’s because of international standards that we can all talk to each other at all via phones or net, but it all began in order to answer the question, “How can we charge for this?” :)

Alas, poor cable

Cables get broken in all sorts of odd and interesting ways. No matter how deep they are buried, how far away from civilisation, and how heavily cladded, someone or something will find a way to get through.

The most infamous of all is poor TAT-8. TAT stands for Trans Atlantic Telephony, and TAT-8 was the eighth cable to be laid across the Atlantic, which was done in 1988 to great excitement.

It was the first transatlantic cable to use optical fibre instead of copper. If you’ve ever seen a section of transatlantic copper cable (unlikely I know, but there is a piece in the Science Museum in London) you’ll be struck¬† by the size of it. The copper coaxial core is as thick as a man’s thigh. It’s covered in layers of plastic insulation to stop the energy from leaking out.

Fibre is one of the great inventions of the 20th century, not only could it carry more information than copper but it was a lot cheaper to make. Copper is expensive, silicon is not. So it’s not surprising that telcos had a mad rush to install fibre as and when they could spring for the expense. TAT-8 was to be a triumph of fibre. In the end, it didn’t quite go according to plan.

Because the cable was made of fibre, it didn’t need to be so heavily lagged. An optical signal can be insulated by just painting the outside of the fibre black. And because the signal was optical, it was thought unnecessary to also shield the power lines that ran along the fibre (an electrical field won’t cause interference with an optical signal). Not only that, but the cable wasn’t buried as is usual, in stead it was laid across the ocean floor.

It was damaged by trawlers. It was attacked by sharks (apparently the electric field from the unshielded power lines drove them into a frenzy … who knew? Link is to a scientific paper on the subject, published in 1989, just one year after the cable was laid). It was unreliable, unsurprisingly.

It was later replaced by TAT-9, an optical cable with better anti-shark shielding, which was duly buried and saw good service.