ADSL - Questions & Answers

Welcome to SEG's ADSL Questions & Answers !

  1. What are the different types of ADSL service ?

    Originally, BT provided 3 types of line at the network level; Videostream (for video on demand, providing guarantee bandwidth), DataStream (An ATM product provided without an IP layer, used for corporate linking typically) and IPStream which is the service that most domestic and business users will use in the UK. As such, when people refer to an 'ADSL line' in the UK, it will be based on BT's IPStream service. Now that several other companies provide ADSL, these distinctions become less important - this section remains for 'historical' interest (although BT do still provide all three services).

  2. What is SDSL ?

    SDSL is delivered in a similar way to ADSL but the difference is that the upstream and downstream speeds at the same - i.e. Symmetric (the 'S' in SDSL), for example up to 2.3Mb/s upload and 2.3Mb/s download). This is as opposed to ADSL where the upstream speed is always 256Kb/s regardless of the download speed (i.e. the speeds are Asymmetric - the 'A' in ADSL). The official standard now used for SDSL is known as G.SHDSL. (Many people get confused by the two terms G.SHDSL and SDSL, so we're pleased to clarify it here!). SDSL services are considerably more expensive than ADSL, intended for corporate applications where a greater upstream speed is necessary.

  3. What happens to my existing line when I upgrade to ADSL ?

    ADSL is an upgrade to your existing analogue phone line; it can also technically be an upgrade to an ISDN line (known as 'Annex B'), but that is not currently offered by BT or ISPs in the UK. Your existing analogue (voice) line will operate as before, including BT 'Select Services' such as Call Waiting, Caller Display and Call Minder. You will not see any physical change to the line for a new ADSL conversion.

  4. What is 'Wires Only' ADSL ?

    As you will see later, when you order ADSL from your chosen ISP, BT used to visit your premises and fit a 'splitter' to the line and install an ADSL modem or router. From early 2002, a BT engineers no longer visited. Instead, you purchase your own modem/router and also your own 'splitter' or 'microfilter'. This change gave greater flexibility as you can connect your ADSL modem into any phone socket on the same line, in your home/office. You require a splitter for each of your existing analogue instruments in order to 'remove' the ADSL data signal so that you don't hear interference on your telephone calls.

  5. How does ADSL differ from ISDN in functionality ?

    Both ADSL and ISDN use a copper pair of wires between you and the local exchange (sometimes known as 'the last mile' or 'the local loop'). An ADSL line provides a normal analogue phone service, plus the digital service. An ISDN line provides only a digital service, but that can be 'converted' to analogue by your terminal equipment or Highway wallbox for voice calls.

    ISDN / Home/Bus. HighwayADSL
    64Kbps or 128Kbps Full Duplex0.5 - 24 Mbit/s Downstream
    250-800 Kb/s Upstream
    Guaranteed Bandwidth to host (ISP)Contended Bandwidth to host
    Data calls to any locationISP Data only
    Use any ISPPermanently tied to one ISP
    Dial-on-DemandAlways-On (AO)
    Online time meteredUnmetered Connection
    Two voice calls at onceOne voice call only
    Multiple Phone numbers (& MSN)Single Phone number
    NTE (wallbox) consists of a single wallbox plus your own choice of equipment (TA, router etc).NTE connects into Service provider supplied router/device.

  6. How does ADSL compare to 'Cable modems' ?

    Various phone companies in the UK are rolling out 'cable modem' services. If you have Cable TV or phone services, a cable modem will give you high speed data access to the Internet. Cable modems run at downstream rates of between 500Kbp/sec and 15Mbit/sec, though that varies on your supplier and chosen service level. One major physical difference between ADSL and Cable is that with ADSL, your data is carried down your own copper pair (wire) all the way to the BT exchange. In the case of cable modems, you share a common 'bus' with your neighbours back to the head-end router in the cabinet at the end of your street.

    Another difference between Cable and ADSL service is that the termination may be different - i.e. the interface to your own equipment. Cable service from Telewest and NTL presents itself as an Ethernet (10BaseT) interface - suitable for connection to a network card in your PC, or an Ethernet router (e.g. Vigor2900). ADSL on the other hand is available as either an ADSL modem with USB connection, or via an Ethernet router, depending on the service selected.

  7. Okay, so if I have ADSL installed, what exactly do I get ?

    The main provider for ADSL in the UK is still BT, so we'll assume that you have a BT ADSL line installed. This may be ordered and billed from any number of ISPs (Demon, BTOpenworld, Freeserve etc.) but the physical line is installed by BT.

    On original ADSL, the engineer will firstly upgrade your linebox to the splitter as shown earlier. Then, into the RJ11 'data' socket, he/she will connect your ADSL modem. Depending on the service selected, this will either be an ADSL modem with a USB connection for your computer, or if you have selected a premium 'business' service, your ISP may supply an ADSL Router with Ethernet interface (combining an ADSL router and modem in one box). On wires-only ADSL, you select your own ADSL modem or router.

    The original most common ADSL service provided a USB modem, such as the one pictured here on the right - though many different ones are available (an ADSL modem which connects to your PC's USB socket). You can see the two connectors - RJ11 to the ADSL line and USB to your computer. There is no power supply - it is powered by your computer's USB port.

  8. What is a Microfilter / Splitter ?

    When you have your ADSL line installed, your existing analogue telephone service continue to work, and you can make voice calls as normal, with the ADSL data feed operating at the same time. The ADSL 'data' signal and the regular voice signal are carried down the same line, each operating in a different part of the 'spectum' (a bit like different radio stations).

    In order that the ADSL data signal does not interfere with your regular telephones, fax machines and answerphones, you will need to fit a microfilter. The Microfilter 'strips out' the data signal so that your phones receive the normal voice signal without interference.

    Originally, the Microfilter was fitted by BT at your 'linebox' to serve all extensions in the house on that line; this was standard on British ADSL lines until the end of 2001. One of the disadvantages of this method is that the ADSL data signal is then only available at the primary linebox, so you cannot install your ADSL data equipment (e.g. modem/router) anywhere you like, without running a data-specific RJ11 extension.

    With the advent of 'wires-only' or 'self-install' ADSL, you do not get a modified linebox, so the combined Data/Voice signal is carried to all extension sockets on your line. This means that you can install your ADSL modem or router anywhere where you have a phone extension. However, as this combined signal will interfere with your regular telephones/faxes/answerphones, you need to fit each extension socket with an individual microfilter to strip out the data signal.


    The terms 'microfilter' and 'splitter' are generally interchangeable and usually mean the same thing. Above on the left is an "old-style" linebox with built-in microfilter. To the right is a regular linebox with a plug-in microfilter.

  9. What if I have several computers ? Can I 'share' ADSL ?

    If you choose a single user service, or wires-only then your USB modem will typically connects directly to the USB port on a single computer. The bandwidth of ADSL is large enough to support many users, so if you have a network (LAN) it is quite likely that you will want to share the Internet connection between all users.

    There are basically three ways to share a USB modem. In all cases, it is assumed that your PCs are already networked together.

    1. Software. Proxy-servers or Software-Routers are programs which you can install on the host computer (the one with the modem attached). This method does require that the host PC be left on and it does have a processing overhead on that PC. The fall in router prices nowadays makes this choice rare except for temporary setups.

    2. Multi-User service from your ISP. You can select a premium service from your ISP which includes an ADSL router instead of the USB modem, but it costs more - typically £100 per month instead of £30/month, and even then you may be restricted to a certain number of users or have no facility for 'Port Forwarding' (see later). These mult-user services do usually have a lower (better) contention ratio of 20:1 compared to 50:1 but check with your ISP.

      On a standard 'single user' ADSL service, you can purchase your own ADSL router which then provides shared internet access to your whole LAN on a standard 'single user' ADSL line. An ADSL router supporting NAT will support unlimited local users (though some do have licence limits and you wouldn't really want too many peoiple sharing one line for practical reasons). note that in some cases, it may be a requirement or contractual term with your ISP or ADSL provider that you use only the ADSL hardware which they provide.

    3. Router with USB connection. A 'USB Router' is a device which share an internet connection but it connects directly to your existing USB Modem - there is a 'USB Host socket' on the back of the router just like you would normally find on the back of your PC. The only router with this facility was the Vigor2200USB from DrayTek and that product is now discontinued (June 2004).

  10. What is VPN or Tunnelling ?

    If you have, say, two LANs, say an office in London and another in Manchester, linking them would traditionally involve installing a leased line. This is a permanently connected dedicated data connection. It is expensive and not necessarily fast (the more you pay, the faster it is but this is thousands of pounds per year even for a 64K connection !). You could also use a dialup connection (modems or ISDN) but that's slow too and you are then also paying call charges.

    If two computers or networks in different locations are both connected to the Internet, then it is theoretically possible to network them to each other directly via the Internet, using their Internet/Public IP addresses, but it's a terrible idea - your private data, and access to both of the PCs is then passing over the internet in clear form - a very public and insecure network.

    VPN is 'Virtual Private Networking' and is a facility where you have software or hardware at each end which sets up a tunnel across the Internet between the two sites. This tunnel is secure and encypted (there are various encryption methods, some stronger, some less strong). This means that once the data tunnel is operating, you can pass any data you like through it, securely. Even though the tunnel itself is running through the public Internet, the contents of the tunnel are secure, which means that you can use it for Wide Area Networking (connecting LANs). Consider it like the difference between walking along, carrying a bag full of cash along the pavement (a very public place) compared to driving it along in an armoured van with armed guards. In the case of a VPN, the armoured van is our tunnel, and the armed guards are the secure encryption (e.g. IPSec). You can have several VPNs running simultaneously so multiple offices can all be linked. As this is all using the Internet, costs are very low compared to leased lines.

    That's the good news. The bad news is that VPN does not always work through NAT links, and if you are using a router or software-router to share a 'single user' ADSL USB connection, then you are probably using NAT and therefore your VPN server will not have a real IP address on the Internet or have access to all incoming data ports. Ideally then, your router will either support VPN passthrough or, provide VPN facilities itself so that it can operate the VPN tunnel endpoint directly.

    For much more detailed explanations of VPNs, you can read about DrayTek's VPN Solutions and we also recommend How Stuff Works : VPN

  11. Who do I get my ADSL bill from ?

    You will get two bills. For the voice service, i.e. your normal analogue voice calls, you will get a BT bill for your analogue line rental and calls. For the data service, your supplier and contract is with your ISP - they are in turn BT's customers. If your ISP and Telco happen to be the same company (e.g. BT), you may get only one bill.

  12. Why do some ISPS offer a phone line too?

  13. So who would I call if it stops working ?

    If your phone stops working, you will phone your telephone company. If the ADSL data service appears to have a fault, then you speak to your ISP. If they check their network and find no problem, they can then report it to BT if BT are operating the line for them. You cannot report it to BT; you're not their customer (unless they are actually your ISP too!).

  14. What does my ADSL line connect to ?

    The following graphic should be used in conjunction with the information in this document. The diagram has been simplifed in some areas so should be taken as a rough guide only.

    Schematic layout of ADSL line

    Notes : (a) The Pink area at the subscriber's premises may vary depending on the service and equipment required. In this example, we have an ADSL modem with combined NAT Routing facilities so that several LAN users can use the ADSL access. (b) The green pipes at the ISP's premises are is their local Ethernet backbone which is shared by all of their local servers and customers coming in on any other means (ISDN, analogue, leased lines etc.). (c) The ISDN PRI (ISDN30) line into the ISP and their Remote Access Server serves their dial-up users on analoge and ISDN/Highway lines. (d) The connectivity onto the Internet (shaded blue) is normally a very high bandwidth leased line, such as a T1 or T2 MegaStream.

  15. What's a fat pipe ?

    A fat pipe is a large capacity data line used to connect an ISP onto the BT network, or their own network directly if they have the infrastructure (most ISPS still rely on BT for at least part of their infrastructure). Fat pipes for DSL come in various sizes from 512Kbit/s up to 34Mbit/s; the latter can support up to 1600 simultaneous client sessions.

  16. How does the contention ratio relate to the fat pipe ?

    We talked about contention ratios across the BT network earlier. There's a further factor which can affect the speed of connection between you and your ISP. To take the earlier example, imagine an ISP had a 34Mbit/s fat pipe and had 1600 clients with 2Mb/s lines all trying to download at the same time. Well it just doesn't fit. You end up with a bottleneck which in practice means that each user gets only a part of that 34Mb/sec fat pipe - which isn't very much relatively.

    Don't panic though - this assumes that the ISP is filling the line with 1600 clients (the maximum user capacity of the fat pipe) and that all of those users are all trying to download continuously and at the same time. In reality this is unlikely, but the bottleneck will almost certain affect your transfer speed.

  17. Is the bandwidth share proportional to the line size I have ?

    Take the 'worst case' scenario from above, of 1600 users on a 34Mbit/s fat pipe to your ISP. 34 divided by 1600 is approximately 20Kbp/s - a sixth the speed of ISDN! If you have a 2Mb/s line and your neighbour has a 256Kb/sec line, then you might say that it's unfair to divide the 34Mb by 1600 equal pieces - you've paid for eight times the bandwidth than your neighbour, you you should get eight times as much of the available bandwidth as him. Well, you don't.

    Here's a useful analogy :

    There's one big motorway, lots of cars, vans and juggernauts all trying to use it. Your ADSL line is your sliproad onto that motorway. If you have a bigger sliproad than your neighbour, you can release more cars at once, but once you're on the motorway, you're all using the same lanes. If there's not much traffic, then your bigger sliproad will enable you to get more traffic through, but in heavy traffic, your big sliproad won't help you go any faster than your neighbour.

  18. Contention ratios aside, I can download from the internet at 2Mb/s ?

    Well, sort of. Even with ADSL, the Internet is still the Internet. At the moment, if you're using analogue or ISDN access, even though you might have ~56Kbps or 128Kbps guranteed to your ISP, you're sharing the onward Internet bandwidth with every other Internet user. This is no different with BT Highway ADSL. Downloading from a site fairly 'local' to your ISP should give you good speeds, but connection to a web site the other side of the world will still be subject to the normal Internet bandwidth bottlenecks as before, and it will be worse at peak times.

  19. Contention ratios, bottlenecks, sliproads....Any more bad news ?

    Don't worry too much! It might sounds like your ADSL line will be throttled until it's no faster than ISDN, but in practice, the ADSL line will run much faster than your 'onward' connectivity.

    What does this mean ? An end-to-end data transfer will only ever go as fast as the slowest part of the link, and if you're talking about Internet connectivity, the slowest part of the link will be the Internet because that's where the highest number of users are sharing limited bandwidth. So, although there aren't any contention ratios or bottlenecks on ISDN or analogue lines in the same way as ADSL, once you get to your ISP all users share the same Internet backbone, including even the local Ethernet at the ISP.

  20. What's a DSLAM ?

    The DSLAM is a Digital Subscriber Line Access Multiplexor - it's the box that your ADSL line terminates in at the BT exchange. It takes the ADSL lines and combines their traffic to send it across the BT network. It's at this point that the ISP's contention ratio (50:1 or 20:1) is introduced.

  21. What service level guarantees do I get?

    On end-user lines, only BT Standard care is available. You cannot have Total Care on ADSL lines. BT and most other ISPs do provide a SLA on SDSL lines, so they are often used where reliability is more important.

  22. How do I know if I can have ADSL ?

    To support ADSL, the BT exchange has to have the appropriate equipment already installed (DSLAMs) and you have to be within 'reach' of the exchange - there are distance limits for ADSL lines. All ISPs can check this for you before ordering.

  23. How long can my line be?

  24. What is RADSL and ADSLMax ?

    As mentioned previously, the length of the line, i.e. between you and the telephone company's exchange will have a bearing on the speed possible on an ADSL line. If you order a 1Mb downstream line but are outside the normal distance limitations, you can't have regular ADSL and your order will be rejected. With RADSL (Rate Adaptive DSL), your modem will try to connect at the best speed it can up to the subscribed speed. ADSLMax is a new variation, introduced in 2006 in which the top speed of the connection is extended up to a teortical maximum of 8Mb/s and your equipment will assess the line each time it connects and try to get the highest possible speed connection. This will depend entirely on your line length and quality.

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