Fix your broadband

In the UK, almost all broadband connections are made through ADSL from the local telephone exchange or over cable using the existing cable TV infrastructure. There are a few remote areas where these are not available, and where the only options are satellite or wide-area wireless networks, but for simplicity we’ll just be looking at ADSL and cable in this feature.

Internet to MODEM
The most obvious performance measure for these broadband connections to the home is the overall speed of the broadband link, as quoted by the supplier in Kbits/sec or Mbits/sec. But that is only a nominal figure which users should not expect to achieve all the time, and some may not achieve at all.

This is partly because of physical circumstances – the distance the user is from the ADSL exchange, for example, or the quality of the cabling into the user’s house – and partly because of technical and marketing decisions taken by the broadband provider.

These decisions involve things such as the bandwidth of the internet connection installed at the provider’s distribution centre, commonly called the backhaul capacity, and the number of simultaneous users expected to share either that connection or the more local link to the distribution centre, usually called the contention ratio.

In an ADSL network, each user is individually connected over the local telephone system to the local exchange, where a unit called a Digital Subscriber Line Access Multiplexer (Dslam) routes the traffic from multiple users into a single asynchronous transfer mode (ATM) link that connects to the wider internet.

In most cases all this equipment and traffic is handled by BT, which sells capacity to ISPs to sell on to users; in some cases, though, local loop unbundling (LLU) has meant ISPs can install their own equipment in BT exchanges to route traffic themselves and, hopefully, provide cheaper and faster broadband connections.

The ‘asymmetric’ in the ADSL term means that the bandwidth of the connection is much higher for data travelling from the Dslam to the user than for data travelling in the opposite direction. A typical link might run at 8Mbits/sec for downloads and only 256Kbits/sec for uploads. This reflects most people’s internet usage, but is not ideal for, say, those trying to run their local machines as web servers.

Contention ratios
It is only on the other side of the Dslam at the exchange that ADSL contention ratios come into play. The internet backhaul capacity from the exchange to the nearest or most convenient internet point of presence (PoP) is set by BT or the provider handling the equipment at the exchange, and is not often quoted to users.

In the early days, the standard BT backhaul bandwidth was 4Mbits/sec for up to 400 home users at 512Kbits/sec, and the company offered a contention ratio of 50:1; in effect, each user was sharing a single 512Kbits/sec connection with up to 49 others.

In practice, though, the wider backhaul pipe meant that most users could get their full bandwidth or close to it for most of the time, as internet use is typically ‘bursty’ and not continuous. It was still possible for greedy users to saturate the connection with large continuous downloads, but they were quickly spotted and warned off.

Today, BT and its resellers do not quote contention ratios. The backhaul capacity provided at each exchange varies depending on the demand and size of the exchange, and can be beefed up if required. However, it’s a fair assumption that home users are still faced with a contention ratio of about 50:1, and business users with about 20:1, unless they pay specifically for something better.

In cable systems, the contention starts straight away. Broadband users in a cable segment – which can be about 2,000 households – connect over a single cable to a Cable Modem Termination System (CMTS) at the supplier’s local distribution centre, and share its bandwidth. That is not such a problem as the cable they share between the local cable hub and the CMTS is a high-capacity fibre-optic one, and the cable between the hub and each house is a high-capacity coaxial type.

The details of these standards, such as MCNS/Docsis in the US and Eurodocsis or DVB/Davic in Europe, are significant for cable companies and their suppliers rather than for users.

But they do fix the maximum speed possible per channel between the CMTS and a user’s cable modem; in the UK, using Eurodocsis 2.0, this is 54Mbits/sec. Of course, this bandwidth is used by all those on the cable segment, so it might be shared between thousands of simultaneous users.

Cable companies manage their systems by capping the connection rate for each cable modem, downloading a configuration file at start-up that fixes the maximum permissible upload and download speeds.

This is why they can offer a variety of speeds and upgrade services without having to change cable modems. In all cases, however, the speeds are asymmetric, so a nominal four or 10Mbits/sec cable connection will have an upload speed fixed at a maximum of 384Kbits/sec.

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