Saturday, 8 November 2014

Some thoughts on broadband speeds

Some time ago FTTC (Fibre To The Cabinet) broadband was introduced locally, and we upgraded to it. So what is FTTC?

Before asking that, lets go back a bit to how we connected to the internet before broadband came along. Remember dial-up modems? They used the telephone line in your home to send data to and from the telephone exchange, where it connected to the Internet. The modem converted the digital data (ones and zeros, the language of computers) into analogue audible tones which it sent down the telephone line. As the modem 'synced up'  (established the data connection) you could first hear the dial tones of the modem dialing, then the tones being sent down the line as the two modems established the maximum speed they could use to communicate over the phone line, the speed being dependent on the distance to the exchange and the electrical condition of the line. Modern modems with data compression, error checking and correction functions could typically achieve a maximum speed of around 9,600 bps (bits per second), a bit being a one or a zero in the data stream.

Back in the early 1970s when I was first involved in working with data communications in the computer industry (with Burroughs - remember them?), even 9,600 bps over telephone lines was undreamed off. For instance, the UK banks had computer terminals in their branches connected to computer centres by dedicated 4-wire lines. 'Dedicated' means these lines did not route through any telephone equipment in exchanges - they were direct point-to-point connections from the bank branch to the computer centre. And '4-wire' meant that they comprised double the number of conductors used in standard 2-wire telephone lines, so could carry twice the data. These state-of-the-art communication links ran at the then dizzy speed of 1,200 bps! If the dedicated 4-wire line failed, there was a fall back of a dial-up connection through the telephone network using ordinary 2-wire phone lines. These ran at 300 bps. And the modems that achieved this amazing performance were big tin boxes rented from the Post Office (GPO) who back then ran the telephone network. In those pre-privatised times one was only allowed to connect GPO equipment to GPO lines, so even where our Burroughs computers incorporated internal modems neatly implemented on a circuit board within the machine, we were not allowed to use them and had to rent these massive boxes from the GPO instead.

A 1970s GPO Modem on a filing cabinet to indicate its size

So home internet connection at 9,600 bps in the 1990s over a standard telephone line using a modem the size of a match box that cost a few pounds seemed a massive advance on what had gone before. Then we got broadband, and that brought a step change in home internet connection speeds.

Communications engineers had developed a technology to send data in digital form down a standard telephone line instead of converting it to analogue tones as modems did. By using very high frequencies for the data, the line's simultaneous use for the voice telephone was not compromised. This new technology was ADSL, or Asymmetric Digital Subscriber Line. Asymmetric because the available capacity, or bandwidth, was asymmetrically divided to give a download speed (data from the Internet to the home computer) several times the upload speed (data from the home computer to the Internet). ADSL gave download speeds of typically 3 or 4 Mbps (Mega bits per second, or million bits per second). So the same copper pair of telephone wires which in the early 1970s could carry 300 bits per second could now carry 4,000,000 bits per second!

This is the speed between the home and the telephone exchange, over the copper wires of the telephone line. The connection from the Internet to the telephone exchange is fibre optic. Fibre optic technology uses glass fibres to carry the information in the form of light, whereas conventional communications cables are made of copper down which electrical signals flow. The advantage of fibre optic over copper cable is that the former can carry far more information at much higher speeds, and the speed does not degrade with distance as it does with copper cables.

So the bottleneck between the home and the very high speeds of fibre optic technology available in the exchange (about 100 Mbps) is the copper telephone line. Although ADSL allows formerly unimaginable 4 Mbps speeds over this copper link, it's still pretty slow compared to fibre speeds. The obvious answer is to replace the old copper telephone line with a fibre optic link from the home computer to the exchange. This is possible, and is known as FTTP (Fibre To The Premises) and is sometimes supplied to business customers, but it is very expensive to install such a link to each house. A compromise system has therefore been developed, FTTC (Fibre To The Cabinet).

FTTC diagram. This diagram doesn't show that the fibre optic link is in addition to the original copper links between the exchange and the street cabinet.

Telephone lines utilise copper cables from the exchange to the street cabinet from which copper telephone lines radiate to each house. FTTC implements an additional fibre optic link from the exchange to a second street cabinet adjacent to the first, to carry 100 Mbps internet traffic. Within the second cabinet the internet connection is transferred to copper wires which link into the original street cabinet for distribution over the existing telephone infrastructure to each house. The technology used to transmit the data at high speed over the copper links between the street cabinet and the home is VDSL 2. Note that the original copper connection from the street cabinet to the exchange is retained for use by the voice telephone system (and ADSL broadband). Schematically, it looks like this:

Schematic showing how the original copper telephone lines from the exchange to the home are supplemented in FTTC by a fibre link to a second street cabinet which originates VDSL copper broadband connections to the original cabinet. Note that there is still a copper path from the home to the exchange for use by voice telephone and ADSL broadband users.  

In the foreground is the FTTC street cabinet where the 100 Mbps fibre link from the exchange is converted to copper VDSL 2 links, one for each FTTC-connected house. These connect to the adjacent original street cabinet (in the background) which uses the existing telephone infrastructure to provide a VDSL link to each house as well as carrying voice traffic for the telephone.

 The speed of VDSL and therefore the speed of one's broadband link depends on the distance between the cabinet and the home, as this graph shows (click on it to enlarge it).

The line on the graph that we are interested in is the VDSL 2 line. It can be seen that the speed falls off rapidly with distance, down to about a quarter of the potential 100 Mbps for premises 300m or more from the cabinet. Engineers are working on improved technologies to give even faster data speeds over copper wires, so hopefully those of use who live some distance from out street cabinet might enjoy speeds closer to the theoretical maximum than we now do.

Nevertheless, even if one only enjoys a broadband speed of 18 Mbps or so, that's amazing when one compares that to the technology on the early '70s when I first became involved with data communications. A mere 300 bits per second over a telephone line back then; up to 100 million bits per second today!

By the way, fibre broadband is an oxymoron. Broadband is a radio-originated term to describe carrying information over a broad spectrum of frequencies, which is the basis of ADSL and VDSL. Fibre optic links use a single-frequency laser light source to carry information down a glass fibre at very high speeds. So fibre optic technology is not broadband, it is narrow band; uni-band, in fact. Therefore, strictly speaking, broadband is a term that should only be applied to the copper part of your fast internet link, the ADSL or VDSL part.

What has happened of course is that 'broadband' has become a term describing fast internet access regardless of the underlying technology, rather than a descriptor for a multi-frequency information carrying service.



  1. Interesting post Vince. You might be interested in our museum plans to cover aircraft communications and railway signalling at some point in the future too!