The Perils of ADSL (Part 2)

In the first part of this article we discussed some of the challanges associated with using ADSL technology to support an event or business. Despite the government’s sabre rattling and some serious investment from certain parties in newer fibre technologies, ADSL is, and will continue, to be the way much of the UK gets its broadband for some time to come. So until we all enjoy fibre links to the doorstop (don’t hold your breath) it helps to know a little bit about ADSL and how you can get the most out of it…so we’ll continue where we left off:  

From this comes the magic of broadband. Thanks to for the image


  1. It’s just copper – The POTS (plain old telephone service) hasn’t really changed that much since the days of the 1880’s. In the 1970s things started to go digital at the back end but in terms of actual delivery to the home it has remained a pair of copper wires. It was well known even then that the wires carrying all those voice calls could do much more. In fact a copper line being used for a phone call is only using about 0.3% of the theoretical throughput. Early work on using the phone system for data goes back as far as 1948 but it was the mid-1980s where most progress was made, first with ISDN and then DSL. DSL actually covers a number of variations (often called xDSL) of which the most common is ADSL or Asymmetric DSL.
  2. Why do speeds vary so much? – ADSL currently uses a technology called Discrete Multi-Tone (DMT). Essentially it divides your copper line into 247 different 4-kHz channels. You get the equivalent of 247 channels divided across the phone line with which to potentially send data back and forth. Each channel is monitored and, if the quality is not good enough, the signal is shifted to another channel. The system constantly shifts signals between different channels, searching for the best channels for transmission and reception. In addition, some of the lower channels (those starting at about 8 KHz), are used as bidirectional channels for upstream and downstream information. As the system constantly monitors all the channels and stops using those that do not provide a good enough signal it is not unusual to see the overall line speed vary from day to day or hour by hour depending on interference or even weather conditions!
  3. ADSL needs to ‘train’ – In an effort to make sure everyone got the best speed possible and knowing the variation in the condition of the copper which makes up our legacy telephone system, ADSL is designed with an inbuilt sliding scale. Once the modem is installed and the service is live the modem will start negotiating with the equipment at the exchange, initially trying higher speeds and then slowing down until it finds a compromise between speed and quality. Over the first 48 hours or so it will continue to do this, varying the settings it uses automatically.  Some modems are better at doing this than others so it’s worth investing in reasonable kit if you want to see the maximum speed your line will maintain.
  4. It say’s up to 20Mbps, I’m getting 2Mbps! – Distance is the bg problem as the further you are from the exchange the weaker the signal is by the time it reaches you, which means more of those little channels are unusable.  In the real world unfortunately very few people will get the headline speed. There are some steps you can take to help the situation though. Firstly, try to minimise any additional cabling so that you are not adding to the length in you house, ideally an ADSL modem should be connected to the master socket (where the the phone line enters the house) to minimise any further loss or interference. Make sure you use a reasonable quality micro-filter, saving an extra £1 on the cheapest filter may end up losing you some speed. All ADSL modems are not the same, some are more sensitve  and use better quality components leading to a more stable and better performing connection so do your research when purchasing.  If you do have to run additional cable in your house then make sure it is connected correctly as mis-wiring can cause all kinds of problems. It is also worth routing any cable away from power cables and pipes where possible to minimise any interference.
  5. Sharing the service – One of the main challenges with ADSL is the contention within the service. In most cases ADSL is offered as a consumer service which is cheap. Like everything with a price point that means it has a catch which is normally the contention ratio. A contention ratio is managed by the ISP and means you could be sharing your 20 Mbit/s download with up to 50 people at the same time (each ISP has different rules, you’ll see it in the very very very small print). If all users in the same contention group play fair the chances of everyone downloading content (websites, email etc) at the same time is low and therefore everyone enjoys the Internet at a fast speed. However the model falls down when people download much larger files, stream video content or share files with others. It’s not the user’s problem (or, to be fair the ISP since there has to be some money somewhere!) it’s just a reality of offering a competitively priced service.

ADSL is in many ways a great technology for home use but does have significant limitations when it comes to business or critical services, especially when you consider there are no real guarantees on service or performance. You can never predict exactly how a line will behave until it is installed and has been running for a few days – not ideal in our case where we are deploying services for short periods. There are a few other options though:  

  • Bonding and Load Balancing– this is a complex area in itself and often mis-sold but there are cases where it makes sense to ‘bundle’ multiple ADSL lines together to give improved performance. There are many cases though where this approach will not significantly improve things and can lead to all kinds of other problems (VoIP and VPN for example are very intolerant of many bonded solutions).
  • SDSL (Symmetric DSL)  – A variant of DSL which as the name suggests offers the same upload and download performance. The catch? It is limited to 2Mbps per line and many exchanges do not support it. Where it can be used though it is normally offered as a business service with little or no contention so will often outperform a much ‘faster’ ADSL service. SDSL lines can also be bonded or balanced in a similar way to ADSL but again there are catches.
  • Alternative Leased Line technologies – There are several newer technologies being rolled out which although they still use the copper wires can deliver much higher speeds. Presently they tend to only be available in major towns and cities and are more expensive than broadband DSL services but they are an alternative to technologies like fibre which are very expensive unless an existing service is in place.
  • Optic Fibre, Satellite & Wireless MANs – Beyond the copper wire there are several other technologies to deliver high speed connectivity which all have their pros and cons too. The detail of these will have to wait for another day.

The key point though is that if you need connectivity at an event it is very important to talk through with someone the requirements and options available. It is all too easy for an event to be hampered by poor connectivity and there is no ‘one size fits all’ approach. In the last few months alone we have used all of the methods listed above at events, from a 1Mbps ADSL line to 2Gbps of optic fibre. It is also worth noting that in general there are at least 4-6 week lead times on getting phone lines and DSL services installed so good up front planning is also very important!

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