I first learned about BT Infinity last year when a friend I used to work with at BT pointed out that my exchange, Ingleby Barwick had been scheduled to be enabled in June 2011. At the time that sounded a long time away but now it’s getting closer, I decided to do a bit of digging and find out technically how it worked.
BT Infinity is BT’s new Fibre To The Cabinet (FTTC) broadband service which is slowly being rolled out across the country and promises speeds of up to 40Mbps downstream and 10Mbps upstream.
Pretty much all of the country can now get broadband in the form of ADSL (Asymmetric Digital Subscriber Line). ADSL works by utilising the existing copper wiring for the telephone lines already in the majority of homes. Unused frequencies are utilised to send data over the lines and a splitter is placed on the customers phone sockets to split the broadband signals off and allow simultaneous use of the telephone and broadband.
The original ADSL standard gives a theoretical maximum downstream speed of 8Mbps and upstream speed of 1Mbps and the newer ADSL2+ standard gives a theoretical maximum downstream speed of 24Mbps and upstream speed of 3.3Mbps. I say theoretical because it’s practically impossible to ever obtain those speeds unless you are literally next door to the exchange. The majority of people only obtain a fraction of those speeds. I am reasonably lucky to be able to be able to get 5Mbps downstream and just under 1Mbps upstream – most people I know get even less than that. (My modem actually sync’s at around 6000kbps and 1000kbps but only get around 5Mbps and just under 1Mbps real world speed test). The reason for this is that when transmitting signals over long distances of copper wire, noise on the line degrades the signal and the maximum speed reduces. As the exchange can be miles away from the premises and cable ducts do not necessarily even run directly as the crow flies, this loss can be great. It’s also very very sensitive to dodgy wiring – for this reason it’s recommended to plug the modem/router into the master socket and use a filtered faceplate to filter off the ADSL signal before the extension wiring to minimise the risk of interference (which is exactly what I do).
The only other serious option for fast home broadband in the UK is if you are in the coverage area for Virgin Media’s Cable Internet service. I used to have cable internet for several years – however when we moved house, even though it was less than 5 minutes walk round the corner, our new street is not wired for cable. There is a Virgin Media cabinet opposite our road end but they have not cabled down the street. If you can get cable then you are able to obtain speeds of up to 100Mbps downstream and 10Mbps upstream through their latest packages. The good thing about cable internet is when you sign up for a certain package, be it 10Mbs, 20Mbps, 30Mbps, 50Mbps or 100Mbps, you do actually get that speed connection. Of course, with connection in NTL’s network and quite harsh traffic management (throttling) they apply at peak periods, you wont necessarily see those real life download speeds all of the time, but at least you are actually connected at the speed you are paying for. The way this is achieved is, Comcast as they were known when they first started laying cables (they were later sold to NTL and later to Virgin), similar to BT, distributed cabinets around their coverage area to interconnect users. However, unlike BT, Comcast ran fibre optic connections to their cabinets rather than huge quantities of copper wires (one pair per line). What they then do is lay low-loss coaxial cable (coax) between the nearest cabinet and the premises. A single length of coax can provide the subscriber with fast broadband and cable television.
With ADSL, a modem at the customers premises (usually ISP’s supply a combined ADSL modem and router so the single connection can be shared between several machines connected via ethernet or 802.11 wireless using NAT (Network Address Translation) which allows multiple computers on a Local Area Network (LAN) to communicate to the internet with a single external IP address) connects to the ADSL splitter and over the copper wire direct to DSLAM equipment in the telephone exchange.
With BT’s new FTTC network (BT Infinity), as it’s name suggests, the local cabinets are connected via fibre (optics) back to the telephone exchange (which I assume will be connected by fibre to BT’s core network). As fibre travels at the speed of light, there is negligible loss – hence why it’s also used to connect different countries together round the world.
When I first heard about it, I didn’t really think about it and assumed that they would do similar to Virgin’s cable service and lay new cables of some kind between the cabinet and the customers premises. When you actually think about it though, that would be expensive, slow to roll out and would end up like Virgin’s cable network – severely limited to certain areas. i.e it wouldn’t really be practical.
So, how does it work? – well, what they are doing is using a technology called VDSL which is similar to the current ADSL technology already in use.
What this means in reality is the following:
Once the exchange has been enabled for FTTC, they will distribute new and slightly bigger cabinets which will house DSLAM equipment which is similar (but newer) to that currently housed in the telephone exchange, the fibre backbone, patch panels and a cross connect to the existing BT cabinet.
When you order BT Infinity, an engineer will come out to install the product. He will replace your current master socket with a new NTE5 master socket with a built in filter (so the modem/router will need to go into the master socket as the broadband frequencies will be split off before the extensions). He will then hook up a VDSL modem and separate “BT Home Hub” router.
What this modem does is connects using VDSL from your home to the VDSL cabinet using the existing copper wires, on to the DSLAM, back to the exchange (over the fibre) and on to BT’s core network.
Your phone line is then still terminated in the original cabinet for telephony (using the cross connect between the old and new cabinets I mentioned above) and back to the exchange over the original multi pair copper cable as it always has done.
What this means is that the noise induced loss is now only an issue between your premises and the cabinet rather than your premises and the exchange. This is how they are able to provide the quoted maximum speeds of 40Mbps downstream and 10Mbps upstream and you are much more likely to be able to get somewhere near these speeds depending on the distance to your cabinet and the quality of the lines and wiring to it.
Another area worth mentioning is currently, any ISP (Internet Service Provider) can sell you ADSL broadband. They either do this by using BT’s network and renting from their wholesale devision (I believe the product is called IPStream) or renting space in their exchanges and installing their own equipment (known as LLU). I believe this will still be possible with the new FTTC network but there doesn’t yet seem to be a great uptake in this – probably due to the costs involved.
Disclaimer: This is only my own knowledge mixed with snippets i’ve read about FTTC rather than any inside information so please do feel free to comment if you know anything here to be incorrect.
), there is a great video by the author, Ryan Dahl here: