What is ADSL?


The increasing use of the Internet is driving the use of ADSL among businesses and homes, more and more people are getting hooked up and using ADSL and the Internet in new and exciting ways, running more services and using their internet connection to links networks through VPN and running their own web servers.

Getting ADSL can be a bit confusing, so this guide should give you some help when deciding what ISP and also how you go about getting wired up with a super fast Internet connection.

What is ADSL?

ADSL is an Asymmetric Digital Subscriber Line, well basically its a way to get high bandwidth data services down some old twisted pair copper wire. Basically ADSL is operated by BT (British Telecom) and you sign up to an ISP (Internet Service Provider) who provides your Internet connection via BT ADSL ATM (Asynchronous Transfer Mode) network. The ISP could be BT but there are over 100 ISP offering ADSL services via BT lines, check www.adslguide.org.uk to see a list of ISPs and reviews of them.

How does it Work?

Information about how BT’s ATM network works is near impossible to find, but ive come up with a diagram which is kind of how it works.

Below is a diagram of how the whole thing works then underneath that each specific section explained.

Basically when you make a request of something from the Internet the request is passed to your modem or router which then goes through your line splitter to the phone line to the exchange, this is then routed through BTs ATM network to the home gateway where it connects to your specific ISP and thus the Internet.

Other ISPs also offer their services in the same way across the same system, that’s why switching ISP when on ADSL is as easy as changing settings in your router or modem and within a few seconds it can be changed to use the new ISP.

Listed below is the various parts and a more details explanation of what they are.


This is some hardware that converts the ADSL line signals into computer signals and back a again. You need one of these, whether it be a router or a modem its up to you.


Its the thing that filters out the telephoen voice calls from the ADSL Internet signals, see the section below marked “What is a Microfilter?” for more details.

Local Loop

The lines between a customer and the telephone company’s central office, often called the “last mile.” Local loops use copper-based telephone wire.

Local Exchange

This is the local exchange where your telephone supplier connects your local line to its main telephone network. Basically its a load of switches.

BTs ATM Network

Asynchronous Transfer Mode A network technology for both local and wide area networks (LANs and WANs) that supports realtime voice and video as well as data. The topology uses switches that establish a logical circuit from end to end, which guarantees quality of service (QoS). However, unlike telephone switches that dedicate circuits end to end, unused bandwidth in ATM’s logical circuits can be appropriated when needed. For example, idle bandwidth in a videoconference circuit can be used to transfer data.

ATM is used so BT can supply its data services through it a scalable speed of of 1.5, 25, 100, 155, 622, 2488 and 9953 Mbps (see OC). ATM is also running as slow as 9.6 Kbps between ships at sea. An ATM switch can be added into the middle of a switch fabric to enhance total capacity, and the new switch is automatically updated using ATM’s PNNI routing protocol.

Home Gateway

Not totally sure what this is but is seems to connect you to your specific ISP through the ATM network, it is changed if you change ISP, the connection switch is made logically, i.e. not moving wires done on a computer.

Your ISP

An ISP or Internet Service Provider is an organization that provides access to the Internet. Small Internet service providers (ISPs) provide service via modem and ISDN and ADSL while the larger ones also offer private line hookups (T1, fractional T1, etc.). Customers are generally billed a fixed rate per month, but other charges may apply. 

Large Internet services, such as America Online (AOL) and Microsoft Network (MSN), also provide proprietary databases, forums and services in addition to Internet access. 

What is RADSL?

It is essentially ADSL except its Rate Adjusted Asymmetric Digital Subscriber Line, what this means is that the upstream channel is rate adjusted meaning its speed can vary depending on line conditions, this makes the theoretical maximum distance from the exchange that a subscriber can be increase from 3.3km to around 5.5km.

I want ADSL, how do I get it?

Firstly check on the BT website or check on the ADSL Guide website (and go for “Availability Check”) to see if your local exchange has been ADSL enabled.

If it hasn’t all is not lost your can check the demand tracker to see if your exchange will be cabled soon, and you can add your interest on the BT site or on the ADSL Guide site.

If your local exchange is enabled start looking for an ISP, you can find one from the ADSL Guide site. Once you have found one that you like the look of go to their site and register.

Once registered and normally you will need to pay a connection fee the ADSL will be set to be installed by BT. BT will conduct line checks to see that your line is ADSL capable. The “lead time” as they call it is around 2 weeks but can be shorter.

Assuming all is well with the line you should be notified by your ISP and/or BT when your line is ready. Now you can buy the ADSL equipment (Router, Microfilter) or if you selected a ISP that provides a modem and microfilter you can now install it.

You should have been supplied with all the information you need to install your ADSL and get it working. This should include things like usernames and passwords, connection settings and DNS server IP addresses and the like.

You should be ready to start surfing.

Choosing an ISP

There are loads of ISPs out there that can provide your Internet connection, they all offer something slightly different and for a different price. Check out the ADSL guide website for a ISP summary http://www.adslguide.org.uk/isps/summarylist.asp.

What’s Best a Router? or a Modem?

It depends. If you have only one computer in your house and no network a modem is the cheapest way to go, also if you have little computer experience this is the most simple way to connect.

However if you have more than one computer and probably a network, a router is best, although it is more expensive it will allow you to use your ADSL in a more flexible way. This is what I have got, I have a Draytek Vigor2600 ADSL router, its only about £120 ($150) and is packed with features.

What’s a Microfilter?

A microfilter splits your normal voice telephone signals from the broadband Internet signals on your phone line. Without one you will be able to hear interference when you use your telephone. It also makes sure the ADSL line is terminated properly it stops reflections down the line that could cause errors on the ADSL line.

I live in the middle of nowhere and can’t get ADSL what do I do?

There are some alternatives, these include things like Satellite Internet, ISDN or joining a wireless or broadband community scheme. Some links are shown below:

http://www.abcampaign.org.uk/ – Access to Broadband Campaign.

http://www.communitywireless.org/ – Community Wireless.

Can I use an analog modem or fax machine on the same line as the ADSL?

As far as I know yes, you can. However I have read somewhere that the performance of the analog modem or fax machine will be reduced because of ADSL on the same line. This seems to make sense but also doesn’t make sense, the modem should only use the range of frequencies used by a normal phone call so should not be affected by the ADSL. But the simple answer is yes.

Whats the difference between PPPoE and PPPoA?

Both of these protocols are layers between the ATM and Point to Point Protocol (PPP). A PPP over ATM (PPPoA) session indicates that PPP protocol is running on top of the ATM layers. A PPP over Ethernet (PPPoE), indicates that the PPP session is running over an Ethernet protocol that in turn is running over ATM. Technically PPPoE should be called

PPPoEoA in ADSL networks.

Also possibly the main difference is that PPPoA is for the UK/Europe and PPPoE is for the US. Though always check with your ISP and equipment supplier before purchase.

Related Links

http://www.adslguide.org.uk/ – ADSLGuide, one of the best for UK ADSL services.

http://www.multithread.co.uk/adsl/ – Another good ADSL site.http://www.wown.com/articles_tutorials/adslinfo.html – More good information.

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