The exchange

As far as the user is concerned, ADSL installation is just a matter of a change of wall box and the addition of an ADSL modem or router and perhaps the addition of some micro filters to enable voice lines to work without interference.

However, for it all to work the exchange also has to be upgraded to support incoming ADSL. It also has to have some sort of connection to the Internet so that it can pass the data on to somewhere more useful.

At the exchange the digital channels are split off from the voice channel and fed into a device called a DSLAM - Digital Subscriber Line Access Multiplexer. This acts as a data concentrator and connects all of the ADSL customer data channels to the Internet - eventually. Sometimes there is an Internet connection directly at the exchange but more often there is a special ATM – Asynchronous Transfer Mode – network. The ATM network isn’t the Internet, it’s the telecom providers' way of moving data round the country at high speed.

What the ATM does is to transfer the data from the DSLAM to a gateway hosted by the ISP of your choice and back. It acts like a virtual hardwired connection between the DSLAM and the ISP’s ATM gateway. The details of how the ATM works is interesting in its own right but is not really a mainstream part of the ADSL story.

The only interesting point is that the ATM network servers are responsible for validating you are a user and creating a virtual connection to your ISP who then validates your user name and password for connection to the Internet. As long as your credentials are valid you finally have access to the Internet! Your ISP accepts the data from the gateway, usually via a fast Ethernet link, and then transfers it to the Internet as it would with a dial-up connection. So you can think of the ADSL/ATM part of the connection as a very long wire that connects your building to the ISP of your choice. Of course this is also the reason why IP geolocation often place you somewhere you aren't. With an ATM connection involved it is where your ISP is located you first make a connection to the Internet.

If you think that this is a long route to get to the Internet – you would be right and ADSL certainly isn’t an Internet connection in your local exchange. But it if all works correctly this is what it should look like to the end user. An ADSL connection is always on in the sense that once the ADSL modem is on and your credentials have been validated the data pathway from the DSLAM to your ISP is set up and connected. What this means is that if a data packet has your address on it then it will be delivered to you via your ISP, ATM, DSLAM and finally your ADSL wall socket. Similarly if you want to send a data packet there is no need to dial a number and make a connection – just send the packet down to the DSLAM and it’s routed to the ISP and onto the Internet.

Contention

With all of this data flying about using always on connections you might think that there would be a problem with carrying it all and you would be correct.

Commercial ADSL is built on the principle that not all users will want its full capacity at the same time. The phone system also uses this idea and doesn’t, for example provide, enough lines between New York and London so that everyone could make a call at the same time.

ADSL is a “contended” connection, which means that you are sharing the connection’s capacity with a number of other users. How many users depends on the price you pay for it. A home user pays less than a business user and typically receives a 50:1 contention ratio, compared to a 20:1 ratio for a business user. This sounds like an unworkable system because if 2 of the 50 people sharing the capacity are on at the same time they only get half what is promised and so on. If they all try to download at the same time then they each get 10Kbps – much less than a modem offers!

The trick to making it work lies in the statistics. The number of people you share with is much larger than the contention ratio suggests. If you share twice the capacity with 100 people then two of you can be on at the same time and get full speed. If you share 32 times the capacity with 1600 then 32 users can be on at the same time at full speed.

Sharing bigger capacities among more people reduces the likelihood that contention will be noticed but variations in speed will occur according to the overall level of use.

The future

While the world is finding alternatives in fibre optics and Wimax style radio connections it will take a very long time for the local loop to be updated. You can bet that ADSL or something very like it is going to be around for a while yet. But are there any technologies on the horizon which might increase the speeds we currently get via the local loop? 

There are two possible new technologies to look out for - GDSL and VDSL. VDSL Very high speed DSL, simply makes use of the fact that the local loop works better the closer to the exchange. Currently VDSL manages to provide 250Mbps just outside the exchange falling to 100Mbps at 1 Km distance and equals ADSL2+ at 1.6Km. VDSL is mostly used in Japan and Korea at the moment. What VDSL allows is for the telephone companies to provide fibre optic connections to the end of your street and then to supply a high speed connection via the local loop. In this sense it moves the exchange closer to your building so that you can get VDSL.

The alternative is GDSL (Gigabit DSL) makes use of multiple cables to achieve a higher data rate. That is, you order a second, third or fourth phone line to get the speed you need over the bonded path. At the moment GDSL is a rare service and might never catch on.

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