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|A Guide to Painless Networks|
|ADSL Broadband Access|
ADSL ("Asymmetric Digital Subscriber Line") works by splitting your existing telephone line signal into two, one for voice and the other for data. ADSL technology can work at up to 8Mbps download. The most popular services in the UK at the moment are running at speeds of 512Kbps (approx. 9 times faster than a modem), although speeds of up to 2Mbps can be obtained. Upload speeds are 256Kbps on all products and hence this is why it is "asymmetric", because the download speed is different to the upload speed. This is deliberate as typical "end user" use of the internet means most of the data goes in one direction but does mean that ADSL is not really suitable if you want to run a web server.
This is achieved by BT or sometimes your ISP installing a device at the exchange which is capable of mixing two different signals on your telephone line. It splits the signal into two channels, one for voice communications and the other for the high speed data connection which makes use of a frequency range not used in normal voice communications. To be specific the voice channel covers 0 - 20 kHz and the data 25.875 kHz - 1.104 MHz. Provided the exchange you're connected to is has been upgraded to enable Broadband , you can have your phone line "converted". Filters are needed at your end of the line to splits the two signals. Your conventional analogue telephone equipment (telephones, fax machines, modems) continue to work as before (though there are one or two services which are incompatible with ADSL use on a line) but you can also have a permanent digital broadband connection to an ISP via the new ADSL "data" channel on the line. You then connect a modem or router to this, and one or more computers, and you have permanent fast internet access.
Micro-filters and splitters solution: Every phone socket needs to a micro-filter/splitter if you want to connect a telephone, fax or modem to it. The telephone plug on your micro-filter is designed to limit the frequencies your phone is able to use. Although you would never normally use the frequencies outside this range, actions such as picking up the receiver can generate frequencies outside normal voice range. Without the filter, this interference would corrupt data within the ADSL frequency band.
The socket providing the connection to the ADSL Modem or Router needs a Micro Splitter which has both a standard socket for a telephone and a RJ11 socket for the unfiltered signal for ADSL. In practice the ISP normally provides several Micro Splitters as part of the package and extras cost about £6-£8 each.
Faceplate splitter solution: The vast majority of households will have no problems with using multiple micro-filters however BT have started using a centralised filter (NTE5 Faceplate splitter) on high-attenuation lines and systems with poor signal to noise ratio. As well as potentially improving the SNR on your line, this connection method also has the advantage of looking tidier with no microfilters dangling from your phone sockets. A BT modification has to be arranged via your ISP however a number of faceplates are now available for one to fit oneself. Once you have a centralised filter, then your ADSL modem connects to the socket on the faceplate and all your extension wiring is automatically filtered. Firms supply NTE5 faceplates include Broadbandzone (www.broadbandzone.co.uk) ADSLNation (www.adslnation.com) or Search for "NTE5 Faceplate splitter" in Google
An NTE5 faceplate splitter is only suitable for telephone lines that have an NTE5 master socket. This is a white plastic square with a removable faceplate on the lower half. The master socket marks the boundary between the telephone wiring provided by BT (which you should not modify) and your own internal wiring. If you remove the NTE5 faceplate, then you will find a telephone socket behind that is connected directly to the incoming BT line - the action of removing the faceplate disconnects all your internal wiring. This is a good way to find out if any problems you have with your telephone (or ADSL) service are being caused by your internal wiring. Because all your telephone extensions can be wired from the rear of the NTE5 faceplate, this makes it the ideal location to place a centralised filter. To fit a faceplate splitter you need to:
Before BT will conect you for an ADSL service they check the line passes a number of criteria and carry out a number of tests. I understand that the criteria have recently been downgraded (Early September 2004) and this may cause problems where customers are now offered a service which would not have previously been available:
The meaning and importance of most of the above are obvious, however, a few of the terms need some clarification.
ADSL migration is the method BT wholesale (who provide most of the UK underlying ADSL infrastructure) can use to allow 'seamless' ADSL migration between different ISPs - say for instance you have ADSL with ISP_one and which to swap to ISP_another, if both support the BT wholesale migration process you can swap between providers by paying about 30 pounds and the swap over can be done the same day (so little downtime). If the ISP you are leaving does not allow migration they can 'hold' you connection, meaning that in order to swap providers you have to ask them to de-provision your connection. Then to be connected with a new provider you have to have you line checked again and and ADSL have to be re-provided. This process will leave you with no ADSL for upto 2 weeks while the swap takes place. It is also potentially more expensive. Oftel (now ofcom) are trying to get broad agreement to allow migrations between any ISPs, at the moment an ISP can choose not to release your line, effectivly you are then stuck with the ISP unless you are willing to be without ADSL for 2 or more weeks.
Contention Most of the lower rate home services (512 Kbps or lower) have a contention ratio of 50:1 meaning that there is more potential demand from users than the band width available if they all called on it simultaniously and there is a risk that you will not achieve the full bandwidth at a peak time. This is similar to a dial-up conection where modem lines are often limited to 1 line for every 20 users as well as a limit on the ISPs overall connection to the Internet backbone. Contention or limited bandwidth is inherent in the Internet and is unlikely to be a problem other than at times of peak demand, for example, when a major upgrade has come out for Windows and, even then, other bottlenecks in the Internet are may be the major factor causing a slowing of Internet response. Without contention at the various points of the Internet it would be vastly more expensive to run. It is possible to obtain business services with a lower contention ratio of 20:1 and even 10: but at a price.
It is difficult to get definitive statements on how contention is implemented. It seems that within BT a common implementation used in order to prevent one user hogging all an exchanges bandwidth is to have a larger input bandwidth 'pipe' than a single users maximum allowed rate feeding a large number of users. For example a normal BT basic pipe for a 50:1 service is 4Mbps and this pipe will provide for an absolute maximum of 400 x 512 Kbps users, in practice it is more usual to supply 200 to 300 users from each 4Mbps pipe. The fact that each user is capped by their connection over the ADSL to the exchange means that no one user can saturate the pipe, and therefore the chances of you receiving your full 512Kbps is more likely. Lower rates than 512Kbps are obviously allow more users on the same pipe.
Some low price products may use a different system for contention controlled by the individual service provider who rent the minimum amount of bandwidth needed from BT Datastream or using their own local loop unbundling - such users are alleged to see more restriction of bandwidth in practice, a penalty for the lower price of the services. Unfortunately there is no simple way of telling if any contention seen in the BT or ISP controlled parts of the network.
Performance Record A number of sites such as ADSlGuide maintain performance records of downstream and upstream data rates and user satisfaction compiled from users tests. They also have useful information on the major service providers - note however I have found that costing and package information etc may be quite out of date as the field changes so fast.
Fixed IP addresses are important if you want to run a web site server etc or be able to access your network from the internet to view video cameras, burgler alarms etc. They are rare on cheaper packages and may have an additional cost. Not having a fixed IP address improves your security from hacking as your address is different every time you log on with packages where the ISP allocates an IP address from a pool.
Limits on Download data and connection time should be avoided - the remove the major advantages of Broadband. Having instant access without worries of any limits is even more important than speed and you will find you always have a machine turned on for instant access to any data you require.
I used Internet Connection sharing successfully for many months but it has the disadvantage that the machine used for access must be on whenever any other machine needs Internet access and it also needs a good software firewall capable of supporting ICS. This meant I had to use Zone Alarm Pro instead of the free Zone Alarm on the master machine which costs $39 for a year. The firewall places a considerable load on the machine and there were a large number of attempted intrusions. ICS can be difficult to set up although my article should reduce the pain.
A hardware solution using a combined ADSL Firewall Router with Wi-Fi access is a better solution as only the unit has to be powered up all the time for any machine to get access through ethernet cables or a Wi-Fi link. Most units have a powerful hardware firewall with NAT (Network Address Translation) virtually isolating one's network from the outside world. I have seen no intrusions get past the hardware firewall to the individual firewalls on my machines which now only serve to protect outgoing traffic by monitoring programs. It also replaces ones existing network router making it even better if one is starting from scratch. The units are easy to set up and configuration of the machines requiring Internet access has also been simple. A number of manufacturers offer such combined units and I have installed a 3Com OfficeConnect ADSL Wireless 11g Firewall Router model 3CRWE754G72-A which came with an ethernet and an ADSL cable and fwas on offer with a free wireless network PC card for a laptop at under £80 including VAT and carriage from Simply. Overall I found this hardware solution significantly easier than installing ICS, potentially more secure, and has the bonus of integrated Wi-Fi for laptops.
Copyright © Peter and Pauline Curtis
29th September 2004