Tuesday, 3 June 2014

What's special about fibre

In light of my recent blog posts it is about time I was a bit less ranty and a tad more educational, so I decided to try and explain why fibre is important when talking about "fibre optic broadband".

Virgin's explanation of copper being bad and fibre being good talks of your broadband slowing down as it gets further from the exchange (using copper). This is not a bad start to explaining it, but I think I can do a little better.

When we are talking about an Internet connection it means computers passing digital signals over the distance between you and somewhere else in the world (e.g. a web server). Whilst you might see web pages, emails, pictures or movies, it all boils down to ones and zeros carrier over some form of communications link.

There are two main physical communications link that are used (we're not really considering radio / WiFi at this point). The two types of link are copper or fibre.

The picture above shows the two -

The copper is metal, shiny orange-ish metal that conducts electricity. As you know, metal can carry electrical signals. In this picture you see coaxial cable, it means there is a thick copper wire in the middle, an insulation around it, and then a wire mesh around that, which the white plastic around that. This is one common means of carrying electrical signals. The other common means twisted pairs which is two wires, insulated and twisted around each other. These are often with other pairs in a protective outer coating, and usually are colour coded to identify which pair is which.

The fibre looks thin and what you can see in the picture if 4 colours strands. In actual fact that is four glass fibres which themselves consist of an inner and outer layer of glass, and then a protective coating and a colour coding so you can tell which is which of the four, on the left you can see that is all within a further protective coating, and the whole lot goes in plastic tubing. This is one typical way of handling fibre.

Obviously both of these come in various styles. You can get extra thick cooper coax cable for going long distances. You can get different grades of fibre, and they can come in different protective coatings. There are fibres under the sea that need armour coating to avoid damage.

So, it comes down to how these different communications systems carry the digital signals.

When we talk of speed, that si a tad deceptive. The speed of signals is different. The fibre is literally the speed of light (in glass). The copper is a little slower. But that is not actually what concerns you - what matters is the data rate. This is how many bits of digital signal can be carrier per second. So we talk of megabits per second (millions of bits a second) and gigabits per second (billions of bits) and so on. That is what most people think of as "speed" when talking of Internet connections.

Fibre uses light, in fact it uses lasers. The light is not usually visible, and can actually be lots of different frequencies (colours) of light all mixed together in a signal fibre. There are some clever ways to get more signals down a fibre, but even the simplest, which is a single laser that flashes on and off for the ones and zeros, is very reliable and very fast. Whilst the light does get dimmer over distance, the light can go a very long way without any sort of repeater or amplifier and the light can flash very fast allowing very high rates for sending data. It is kind of intuitive that light can go a long way - just look up at the stars and think about it.

Copper, on the other hand, uses electrical signals. These work very well over short distances, and are used for most networking - the cat-5 cables you may see use 4 twisted pairs of copper, and work up to 100m. But making copper carry signals any longer is harder work. There are clever tricks (such as ADSL) for getting signals to go for miles - but the data rates are lower for the longer cables. It is not that the signals slow down as such, but that what you can achieve - how much data per second you can send - is lower on longer lines.

For a copper pair, or coax, going 100m can allow quite respectible data rates. A 100m cat5e cable can easily handle a gigabit per second, and do so very reliably. But going further is hard work, and if you went 10km you would be lucky to get one megabit per second on a copper pair using ADSL.

For a fibre you can easily go 10km, and in fact fibre will quite happily do speeds of gigabits per second at distances of 70km. With more modern systems and multiple lasers a fibre cable can carry terabits per second. So, the distance from an exchange to your home, even if that is 10km, would easily be able to do 10 gigabits with relatively simple equipment these days.

Most of the Internet involves fibres. Any link between any two places that is more than around 100m will almost certainly be fibre (there are point to point microwave links and satellite links in some cases). Often, in equipment racks, the links between equipment, going a few metres, will be copper network patch leads, but even these will sometimes be fibre.

So, when we look at the Internet service you get at home, the hundreds of miles of fibre in the Internet are not the important bit - that just works - that is "fast". The important bit is the last part that gets to your home. In your home you can use fast short network leads or whatever you like, but the last few miles or even the last hundred meters to your home is what matters.

There are tricks done by BT, Virgin, and others. These are to get the fibre that bit closer - some times within a few hundred meters - by having fibre to a street cabinet. That means the last bit is much improved even using copper, but it is still massively different to using fibre.

Fibre to the cabinet and coax or twisted from the cabinet for the last bit can get speeds at a few 100 megabits if you are close. If you truly had a fibre from the exchange all the way to your home then that could ultimately handle terabits (not that any ISP could afford to sell such a service yet).

There is another factor with fibre vs copper. Whilst glass may seam fragile, it gets well protected, but copper suffers from some other problems which don't affect fibre. Copper can pick up radio interference from thin air, be subject to nearby lightening strikes, and get corrosion of joints. Fibre, on the other hand, just works, and short of someone putting their JCB through it, it will stay working pretty much forever.

So it matters what is in the last bit - the link to your home - and whether it is fibre or not.


  1. Agreed on all points.

    However, for some folk (particularly gamers), it isn't so much about the bandwidth but more about the latency of the connection.

    The speed of light travelling down fibre will always be faster than electrical signals travelling down a copper pair.

    You also need to consider how many times the traffic is converted from optical to electrical and back again as each conversion adds to the overall latency of the link.

    1. I was trying to keep the post "Pauling proof", so left some of those minor details out. The speed of light difference in the last bit of copper is insignificant even for gamers, and yes, the conversion matters but more importantly is time within routers and switches. FEC on protocols like ADSL matters too, and that will be massive compared to anything on a fibre.

  2. The thing that appeals most to me about fibre is being able to buy services by the wavelength. I imagine getting a direct netflix service (no contention!), regular internet service, and maybe one or two tv services. Of course we would need a dutch or singapore style infrastructure where the plant is separated from the service provider.

  3. Doesn't the speed of electrical wave propergation in copper closely match the speed of light? Wikipedia seems to agree with me.

    1. I stand corrected again - I'll update. This was what I had always known from the past, but sources on the internet (which must be right :-) ) suggest it is only a little slower.

    2. You're probably thinking of coax, where the signal does indeed propagate more slowly, depending on the dielectric - about two thirds of the speed of light for solid polyethylene. Much faster for non-coax, which is presumably what you found.

      Alex does have a point about short runs of coax in HFC being quite different from copper (or even aluminium!) pairs, though I still think calling HFC "fibre" for marketing purposes is misguided.

  4. I realise this is a hobby horse topic on which you're ultra-dogmatic, but IMHO it's no better to conflate twisted pair and coax under the 'copper' banner than it is to abbreviate FTTC as 'fibre' in communications to the general public.

    Coax can carry Gigabits over the distances used by providers like Virgin, which is how it successfully carries multiple TV channels as well as broadband, and why it's been used for decades in satellite TV systems (although it's now being slowly phased out in favour of fibre).

    It makes perfect sense to hybridise coax with fibre, and if you're talking to the public it makes sense to call this fibre because it can achieve the same performance as FTTH.

    I can't say the same for BT-style FTTC, but to me that's another subject.

    1. I think the reliabiltiy characteristics of fibre and coax are quite a bit different, though, when used as a shared medium covering lots of customers?

      Virgin's product seems to be pretty sensitive to your neighbours fiddling with their wiring and allowing ingress noise in, water seeping into joints, etc. I haven't got any personal experience with BT's FTTH yet, but I'd have thought it would avoid a lot of that.

    2. RevK does bundle coax and twisted pair together in his description, but I suspect that's largely down to Virgin themselves simply referring to "copper" rather than the specific technologies.

      From Virgin's website: http://store.virginmedia.com/broadband/speeds-explained/index.html
      "The mega speeds of up to 152Mb that you can get with Virgin Broadband are thanks to fibre optic cable. It’s made from strands of glass as thin as hair, which carry information by light. This is much, much faster than the copper telephone wire used by other providers."
      "Our broadband comes to you via fibre optic cable rather than regular copper telephone wires. This gives you more bandwidth, so everyone in the house can do their thing online at the same time."

      Ok, so "regular copper telephone wires" possibly implies twisted pair, but nowhere do they actually say that - they explicitly state that their technology is faster because it uses fibre optic cable instead of copper cable. This is patently untrue - both BT and Virgin (and anyone else selling unbundled services) use fibre backhauls and the sync rate is limited entirely by whatever is connecting the fibre to the CPE. In Virgin's case, the speed is limited by the *copper* coax, in BT's case the speed is limited by the *copper* twisted pair.

      If Virgin had said "we're faster because we use coax instead of twisted pair" then that would be a valid comparison. But they didn't - they said "we're faster because we use fibre instead of copper" which is, frankly, a lie.

      Now I'm going to reiterate what I've said in the past: for the most part, the end user really shouldn't need to know or care about the underlying technology; the end user should just care about the capabilities of the *product* they are buying - if an ISP says "50Mbps" then that's what I expect to get - I don't care how they achieve that 50Mbps, all I care about is that it does what I've been sold.

      Part of the problem is probably the nutty idea of advertising "up to" speeds instead of the actual speed I can expect (or at least a good estimate of it). When I buy an internet connection, I fundamentally don't care that someone who lives right next to the cabinet can get 80Mbps - what I care about is what *I* can expect to get.

      Put another way, if I buy my groceries from Tesco and ask them to deliver them tomorrow, I don't care whether the delivery turns up in a van, on a bike or in a helicopter, all I care about is that it occurs tomorrow - the technologies they use to achieve their promise don't concern me. I similarly don't expect them to offer me a delivery that is "up to" as fast as 1 day, only to later tell me that this only applies to people who live within 50 metres of the store and that I can expect to wait a month for the delivery instead.

      *However*, whether or not the customer really has reason to care what the underlying technology is, if Virgin (or anyone else) chooses to use it as a selling point then that selling point should be truthful. "We're lieing but that's ok because the customer shouldn't have any reason to care" isn't really good enough.

    3. I honestly can't understand why they're allowed to get away with such a lie (except that the ASA has no teeth) but indeed, if they form a contract based on such a lie, they're going to be in deeeep trouble when someone pulls them up on it.

      Interestingly the core of most RG6 style co-ax is steel with a copper plating, rather than pure copper. For the frequencies in use it doesn't really matter, but what DOES worry me re: the steel core is the issue the GPO had with copper plated steel dropwire in the 60s and 70s, namely the copper getting damaged, damp rusting out the core, and thus causing HR or DIS faults on the line. I'd imagine with cable, that would not cause an actual "failure" but a massive drop in signal strength which can play hell with cable modems.

      Just another negative toward cable for internet.

    4. You're right about "Virgin Coax copper fibre" being very different from "BT VDSL copper fibre", and coax can (and indeed does) allow a faster data rate than TP.

      But it's not just about the physical layer - the network layer's important too. DOCSIS is (as implied by another commentard) a shared medium, whereas VDSL is point-to-point back to the "hub" (for lack of a better word)

      Then again, FTTP (as far as I understand it) uses PON, so again it's not quite as much fibre to you as you might think.....

      Simple question, no simple answers.

  5. I haven't read the whole article because it's boringly educational and not ranty enough. However I understand you think fibre optic broadband should be provisioned using fibre optic.

    Virgin Media's site says: "Our broadband comes to you via fibre optic cable rather than regular copper telephone wires."

    This tells me that a fibre optic cable comes to the property.

    Bollocks to the ASA - it's the advertising industry pretending to police itself. Misrepresenting a product in this way may be an offence under CPUTR and Trading Standards should be informed.