Broadband Internet Access through ADSL

Finally I succumbed to the temptation of high(er) speed access to the Internet. It's not that I am unhappy with the Melbourne PC User Group (delighted in fact, and intend to stay a member), just had the "need for speed".

Unfortunately OPTUS Cable was not available in my street and I was not entirely happy with the Telstra offer. After spending quite a while researching all the options I finally settled on ADSL supplied by Netspace because their price and packaging suited my needs better than the other ADSL offers.

Getting set up was a breeze. Just attach the turquoise coloured, stingray shaped Alcatel USB ADSL modem to the phone line and the computer, then attach low pass filters to all other devices on the phone line. Follow the software set up instructions and "Bob became my uncle". Very quickly I had Internet connection sharing working on my small home network and was able to set up access to Melb PC and Netspace services (e-mail, browsing, newsgroups and FTP).

At his point I discovered that my Panasonic analogue cordless telephone was suffering a major signal distortion problem. The sound was very fuzzy, the sound level was dramatically reduced and it was unrelated to proximity to the ADSL modem. Disconnecting the ADSL modem from the phone line instantly cured the interference. I e-mailed the Panasonic technical support team and got a non-helpful response.

Searching on the Web for information on this problem was a great education and made me aware of the many varieties of DSL (Digital Subscriber Line) services, of which ADSL (Asymmetric Digital Subscriber Line) is just one. There are also several types of ADSL. I found many papers discussing the potential for various types of "cross-talk" and interference with ADSL services but none exactly matched the problem I was having.

At this point I discussed the problem with another Melb PC member who had joined the same ADSL service at the same time as I had. His cordless telephone (digital) had not experienced interference problems, even without fitting filters to all line devices. To see if my telephone was the problem I installed it at his residence and sure enough, the interference was present again. So it seemed that my analogue cordless telephone (about 6 years old) was not going to work properly with ADSL connected.

I bought a 900 MHz digital cordless telephone and have had no interference problems since.

The message is this. If you are contemplating connecting to ADSL and you have an older cordless phone unit, you may need to upgrade. A digital cordless unit operating at 900 MHz did the trick for me. Don't be fooled by cordless telephones labelled as digital, which only have a digital answering machine.

What follows is some (non-expert) background to what seems to be happening.

ADSL works by using specific frequencies for upload and download, as far as I can tell in the range of 40 KHz to 1.1 MHz, whilst other devices attached to your phone line such as handsets, dial up modems, fax machines etc use frequencies below 40 KHz; all of which are isolated from the ADSL system with the filters mentioned above.

A cordless phone however uses certain frequencies to transmit between the handset and the base station, attached to the phone wall outlet. One can find product specifications for analogue cordless telephones that use 30-39 MHz and digital cordless units that use 900 MHz. There are also cordless units that operate at 2.4 GHz. I'm unsure if they are digital or not.

So it seems that some combination of the analogue nature of my old cordless telephone and the frequencies used, result in degradation of the phone's performance when connected to the same line as an ADSL modem.

Reprinted from the February 2002 issue of PC Update, the magazine of Melbourne PC User Group, Australia