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This ADSL FAQs guide aims to answer the more detailed but
important questions which you might have about ADSL.
What are the different types of ADSL service ?
Although the version of ADSL you're most likely to come across will be used for Internet access, BT do provide three types of ADSL. In these pages, we're generally referring to the BT IPStream variant of ADSL, which is what an ISP would (most likely) use to provide user access to the Internet, via ADSL. The three types of ADSL are :
- BT Videostream - Intended for companies who wish to sell 'Video on Demand' services (see later). Videostream provides a constant bit rate (CBR) to ensure that picture quality is not affected by network traffic. The Downstream rate is 2.3Mb/sec, plus an extra 160Kbps bidirectional channel for control. Although VideoStream is primarily intended for VOD (Video on Demand) services some companies are providing Internet access with some of the spare bandwidth. Such services typically provide a 115200bps Internet feed, with the main bandwidth being permanently reserved for video.
- BT DataStream - This service would be typically used to connect head offices or services providers to remote users or teleworkers. An ATM protocol is supported, so the service provider would run their own choice of protocol on Layer 3, e.g. TCP/IP. DataStream runs at a variable bit rate (see 'contention ratios' later).
- BT IPStream - Th is similar to Datastream but provide a TCP/IP transport layer, on a private network address range - it's not part of the Internet unless you link at the head office. IP Stream is be used by Internet Service Providers for providing Internet access to users. If you sign up for typical ADSL with an ISP, this is the BT service which they will use.
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 1Mb/s upload and 1mb/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.
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, 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.
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 engineer no longer visits. Instead, you purchase your own modem/router and also your own 'splitter'. This gives greater flexibility as you will be able to 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.
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. Highway |
ADSL |
| 64Kbps or 128Kbps Full Duplex |
500 / 1000 / 2000 Kbit/s Downstream
250Kb/s Upstream |
| Guaranteed Bandwidth to host (ISP) |
Contended Bandwidth to host |
| Data calls to any location |
ISP Data only |
| Use any ISP |
Permanently tied to one ISP |
| Dial-on-Demand |
Always-On (AO) |
| Online time metered |
Unmetered Connection |
| Two voice calls at once |
One 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. |
How does ADSL compare with 'Cable modem' services ?
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.
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.
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The originally 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.
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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.
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.
- 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.
- 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.
- 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 tis facility is the Vigor2200USB from DrayTek, but that product is now discontinued (June 2004).
What is 'NAT' ?
When you connect a single computer to the Internet, your ISP allocates you an IP address - a single, unique address which identies your address on the Internet for the duration of your call/connection. All TCP/IP packets sent to/from your computer contain this IP address. Every computer with Internet connectivity must have a unique IP address, so if you have many computers on your LAN, each must have its own local IP address. NAT (Network Address Translation) translates from one IP address to another. In the case of sharing a single public IP address, it is used in a "one-to-many" mode for sharing a single Internet IP address amongst many local PCs which are given IP addresses in a private subnet (IP address range). The NAT system automatically translate the PC's local IP private address into the public IP address being used for Internet access, and back again. Routers and Software-Routers typically provide an NAT facility operating on a "many-to-one IP address" method (i.e. many private IP addresses map to a single public IP address). Despite being a complex operation, NAT generally works well, though not every application will work through a NAT router, so if you have particular requirements, check with your supplier.
What is 'Port Forwarding' ?
This will generally only apply if you are intending to run some sort of server on your network, or where a particular application needs port forwarding.
When operating a NAT system to share an ADSL feed (see above), all of the computers on your local network (LAN) appear on the Internet under the same IP address. As such, if an incoming packet arrives at your router which is not a response to an outgoing packet, the router will be unable to tell which local PC the data packet is intended for. Port Forwarding enables you to specify a default local PC which is to receive such packets, either always, or based on the 'port number' - the port number identifies the service type. For example SMTP email is always on port 25, so you might choose to forward all incoming packets on Port 25 to your local mail server. Most routers will allow either specific ports or ranges of ports to be forwarded; check with your supplier. It is recommended that you do not open all ports through to a PC as this leaves you more vulnerable to hacking.
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.
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.
So who would I call if it stops working ?
If your phone stops working, you will phone BT fault reports. 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. You cannot report it to BT; you're not their customer (unless you're the ISP!).
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.
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.
How fast is ADSL ?
BT Highway ADSL (Entry Level Service) will be available in three sizes. The upstream rate (send speed from you to your BT exchange) is 250 kilobit per second (Kbit/s) on all BT Highway ADSL lines. The downstream rate (from BT exchange to you) is either 500Kbit/s, 1000Kbit/s (1Mbit/s) or 2000Kbit/s (2Mbit/s). Note that this refers to speed from/to the BT exchange, not from/to your ISP - see contention ratios, later.
Great, so I can download at a permanent 2Mbit/s ?
No. On ISDN when you make a 128Kbp/s data call, you have that 128Kb/s dedicated for your own use for the duration of that call - it's all yours, end to end between you and the ISP, guaranteed once the call is set up. On Dial-up (analogue/ISDN) a call can of course be engaged, and onward Internet thoughput is unpredictable). On ADSL your bandwidth from the BT exchange onwards is contended.
Bandwidth is not infinite across BT's network. Whilst BT can carry vast numbers of 64Kbps ISDN and analogue calls, when you get into ADSL speeds, it starts to gobble up considerable bandwidth, so you have to share it.
The contention ratio of an Entry Level BT Highway ADSL service is 50:1 (20:1 on the business service). These are 'worst case' figures - i.e. the minimum guaranteed contention, so don't panic just yet. This means that your data path between you and your ISP is shared with anyone else with an ADSL line en-route.
What's a fat pipe ?
A fat pipe is a large capacity data line used to connect an ISP onto the BT network. Fat pipes for ADSL come in various sizes from 512Kbit/s up to 34Mbit/s; the latter can support up to 1600 simultaneous client sessions.
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.
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.
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.
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.
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 BT contention ratio (50:1 or 20:1) is introduced.
What about security ?
There is a two step authentication system on ADSL. when first installed, your router talks to the DSLAM in the BT exchange. This then talks to a 'Remote Access Server' in the exchange which identifies you via a Radius Server Database. This is a 'one time' procedure; once you're authenticated the line is always on (AO). Your ISP will, in turn, authenticate you at their end with their own Radius server. All of these processes should be seamless and automatic. However, there are intrinsic security risks from an Always-On service, so you should make sure you understand the implications and take suitable precautions against unauthorised access.
I play online games ! What's the ping speed on ADSL ?
Get a life... ( Stoppit - ed !) er, sorry, I mean... Okay, if you play online games such as Quake (and derivatives) then your 'ping speed' is the time it takes for your bytes to get from your PC's processor to the Quake server - it's not to do with bandwidth, but end-to-end speed. Latency is the delay between the byte leaving you and getting to somewhere else. On a games like Quake, the ping latency is typically between 30 and 250ms (where 250ms is considered bad) however there are many factors involves, notably the onward connectivity of the Internet, so real-world experience is your only reliable gauge..
Regardless of whether you are using V90 analogue, ISDN or ADSL, the quicker your computer can get the message to the server to 'kill/fire/blast' the more chance you have of winning a shootout. Typically, ISDN users have always had the ping advantage over analogue users as ISDN systems have less to do (e.g. they don't have to waste time turning your bytes into asynchronous bits, then squarky audio tones and back again). So what about ADSL ? There are actually more steps in getting your bytes to the ISP than there are with ISDN, but the line speed is higher. So what's the answer ? We currently only have anecdotal non-scientific answers to that, sorry.
What is BT Videostream ?
As mentioned earlier, BT VideoStream is another variant ADSL package; it is used by companies wishing to offer 'Video on Demand' services. This is like 'pay per view' on Satellite TV, but you select the film yourself from the service provider's library and play it when you want - you can pause, rewind, fast-forward as required. An important technical difference between Videostream and other ADSL services is that there is no contention ratio - digital video requires the full bandwidth to be available continuously. Instead, there is an overbooking ratio of 1:3, which works on the notion that only one out of every three Videostream customers will want to watch a film/event at the same time. Some Videostream providers include an Internet feed with the line, though at restricted speed. Some VideoStream providers do also provide an Internet feed too as part of the service too, but only a restricted speed service (e.g. 115200bps), not full ADSL speed.
What service level guarantees do I get?
On end-user lines, only BT Standard care is available. You cannot have Total Care on ADSL Highway lines. Total Care is standard on 'Fat Pipes'.
How do I know if I can have ADSL ?
To support ADSL, the BT exchange has to be upgraded; a DSLAM unit has to be installed to serve ADSL users. You can check availability in your area at BT's ADSL Web Site, however that indicates availability on your exchange, not that you necessarily comply with the line length restrictions.
Why is ADSL faster in some other countries ?
Technically, ADSL can run at up to 8Mb/sec, but that rate is effected considerably by line length - the further you are from the exchange, the less speed you can get from a copper pair. If you are very close to the exchange (<1Km) then you could run the line at 8Mb/sec downstream. However, in the interests of simplicity, BT have compromised to a fixed maximum downstream rate of 2Mb/sec which can reliably run at distances of up to 3.5Km from the exchange. That should enable 85% of subscribers to connect to ADSL, once available in their area. RADSL extends that further (see below).
What is RADSL ?
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. However, with RADSL (Rate Adaptive DSL), your modem will try to connect at the best speed it can up to the subscribed speed, but that may be some way short of the 1Mb/sec...but it's better than no ADSL
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through its partner Nildram.
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