Com2u Computer Home Service

Why is my Internet Slow? Part II:ADSL and The Last Mile

December 29, 2009
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To be fair this article has taken more than a week to appear, but we plead Christmas.

In our last article we talked about factors that can cause websites to load slowly, but the biggest reason internet access gets sluggish is between you and your telephone company.  In industry jargon it’s called The Last Mile, and it refers to the last mile of (usually) copper cable that connects your computer to the local telephone exchange. 

Telephone cabling hasn’t changed much in the last 100 years, meanwhile the connection from your exchange is now lightning-fast optical fibre, to pretty much anywhere in the world.  The cost of replacing all that copper in the ground is what keeps it there, and leaves us with few choices for broadband internet in the home.  Luckily we have ADSL.  Unluckily this technology comes with a lot of compromises.

ADSL stands for Asynchronous Digital Subscriber Line.  The Subscriber Line bit refers to your copper telephone cable.  Digital means, well, it’s a digital connection.  Asynchronous differentiates it from DSL or Synchronous DSL.  With plain DSL the amount of data you can upload is the same as the amount of data you can download, but with ADSL, the amount of data you can upload is usually much lower than the amount you can download.  For example, your line may be about to download 4Mbps (millions of bits per second), but will upload no more than 512Kbps or 1Mbps.

The reason Asynchronous connections are used is because you don’t need to upload much to surf the web, your computer requests web pages and those requests are very small compared to the size of the pages.

When you connect to the internet via ADSL, a couple of things are going on.  First up, when you plug in your ADSL modem (it’s actually not a ‘modem’ in the strict sense but the difference is only interesting to techies and pedants) and switch it on, it will try to establish a data connection to your local telephone exchange — to a piece of equipment called a DSLAM, short for Digital Subscriber Line Access Multiplexer.  This data connection is not yet an internet connection, it’s a DSL connection and when it is established the DSL indicator will light up on your modem to indicated that you can connect to the exchange.

What happens next is that you ‘connect’ to the internet.  What this means is that you send your username and password over the DSL connection to your ISP and they’ll check it against their (billing) records (usually using a RADIUS server, if you want to look that up).  If all is well they’ll pass on requests and let you download data through their internet gateway.  If not you’ll find yourself calling their support line.

The trouble comes in two phases.  Firstly, you can set up and pay for a ’10Mbps’ connection, but unless you’re close to the exchange and have a very clean phone connection, you’re never going to get anything like that.  You see, the published maximum rate for ADSL (like 10Mbps) is a theoretical maximum, which means it’s only achievable under laboratory conditions.  What actually happens is that speed drops the further you are from the exchange and, well, that twisted pair of copper cables was just never meant to carry too much data.  Every poor solder or bit of corrosion will lower the fidelity of the line and thus slow down your connection.

So, the first lesson is to work out what the maximum speed your local telephone line is and don’t pay for more than that.  That said, telephone companies will replace or repair particularly troublesome lines.  For any other problems, you can give us a call to fix your internet connection.

The next place where things get slow is at your ISP’s connection to the rest of the world.  But more on that, next week…


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Com2u Computer Home Service can help get rid of your computer blues, in Bangkok, Pattaya, Phuket and Samui.