Ah,
Sorry for the confusion, the technology I had in mind was actually ADSL over ISDN (which is actually "ADSL with ISDN") where the operators install a special DSLAM which allows you to use ADSL on an ISDN upgraded line. You'll find a lot of these in Germany where many people bought IDSN before ADSL became available and now want to use ADSL, but do not want to get rid of their ISDN line since it has other benifits besides slightly faster internet access.
Technically, this would not really qualify as xDSL though, just as a 128kbps ISDN basic rate leased line. These are quite common overseas and are used by businesses and home users that want only very basic permanent internet access. Remember, in the first world, people who would buy a 64kbps Diginet line here would be using one or more T1s to connect their business to the internet, so an ISDN basic rate leased line would be considered very very basic permanent access.
I doubt Telkom would go for ISDN basic rate leased lines though, since ISDN is not xDSL, but an end to end digital service with terminals and terminating units, permanent ISDN would be a direct competitor to Diginet and everybody would probably move immediately. Remember that Telkom make a lot of money from Diginet and they would not easily do anything that could unsettle Diginet.
Also, only being available as a dial-up service, ISDN is proving to be a great money-maker for Telkom. The monthly rates for basic rate ISDN dial-up lines are high and people usually spend hours downloading, meaning expensive phone calls that feed the money hungry share holders.
At the moment, ADSL is priced to only be used by a niche market, with many limitations like the 3GB cap, due in part to infrastructure limitations and in part to Telkom not wanting businesses to move from the more reliable, but much more expensive Diginet lines.
Diginet and ISDN are priced so that they target two separate markets. The small business that wants reliable internet access, who will buy a Diginet line and the home or small office user who just wants slightly better speed from his dial-up, who will buy ISDN. Thus, Diginet and ISDN are complimenting each other instead of competing with each other and are thus earning handsome profits for Telkom.
Starting to offer ISDN basic rate leased lines would not make sence, unless they were to price them at exactly the same rates at which Diginet is currently priced. Having two services with identical capabilities at the same price would be stupid, even by Telkom's standards.
Getting back to the point of it this kind of connection not being xDSL, I've seen some funny descriptions of a few kinds of permanent connections which have left even me quite confused.
For some reason, it has become fassionable to append the letters DSL to any kind of permanent connection that's in any way faster than dial-up.
My favorite of all time is CDSL, which does not exist, but is referred to by many users. This non-existant acronym supposedly stands for "Cable DSL", meaning permanent broadband over cable TV networks.
That's totally inaccurate, since cable internet works in a completely different way. Cable internet is an "ethernet over point to multipoint broadcast fiber network" connection. It's actually a very bad connection, since a packet for any user in an area must propagate to all users in that area, like ethernet did before somebody invented the switch.
WDSL is another wonderful concontion that I have come across, even mentioned once in this forum. This supposedly stands for "Wireless DSL", meaning a fast, permanent wireless connection. Wireless internet, in any form, be it 802.11 or 802.16 based, GPRS or UTMS TDD (3G) based, is just wireless internet.
One lesser known of these mythical connections is PDSL, or "Power Line DSL", this also does not exist. Power line internet is very real in countries like Germany and Japan, but it uses modulation over power cables and is very definitely not DSL.
For something to qualify is being any kind of DSL, it must actually be DSL. DSL technology is quite simple. It works by splitting an analogue phone line into two bands. Normally analogue voice telephones and voice band modems use only the audible frequencies of sound waves where the human voice would naturally occur. These devices will generally only transmit and receive between 300Hz and 3KHz.
However, analogue phone circuits are not limited in terms of the frequencies of sound waves they can carry. In fact, you could send frequencies into many GHz, way beyond the hearing capability of a human, through any analogue audio device, if your device and your cabling was of sufficient quality.
Generally, phone lines close to the exchange can carry frequencies at least up to 1MHz, although they start to distort badly at very high frequencies. Still, from the 3KHz which is the absolute maximum that voice traffic could ever use, there is a large block of totally unused frequency bandwidth on every phone line, ranging up to 1MHz.
ADSL modems use DMT (Discreet Multi-Tone) modulation, which operates between 20KHz and 1MHz and allows for 256 channels, affording it tremendous bandwidth over standard analogue copper lines.
Splitters and filters are used to filter off the high frequencies, above 3KHz, from the voice devices, so that the voice devices do not pick up noise from the ADSL device operating at the high frequency band on the same line. The DSLAM (DSL Access Multiplexer) unit, filters off frequencies below 20KHz, so that voice traffic can not cause disruptions to the digital traffic.
In the frequency band between 20KHz and 1MHz, the ADSL modem's DMT modulation creates 256 distinct tones which act as channels and offer a maximum possible bandwidth of about 640KBps downstream and about 6.4MBps upstream. This is achieved by modulating digital data into the 256 tone channels, which offer a massive margin for error, so that the distortion generally holding back digital over analogue transmissions does not become a problem.
Other technologies that use similar means of transmission, in other words, using very high frequencies on standard analogue copper wire, are HDSL (High Speed Digital Subscriber Line), SDSL (Synchronous Digital Subscriber Line),VDSL (Very High Speed Digital Subscriber Line) and UDSL, a new DSL technology that can carry up to 200MBps of bandwidth over short distances over standard analogue copper wire.
The other technologies mentioned here use very different ways to transfer data over the line. In all cases, there is no analogue modulation what so ever involved.
Cable internet uses point-to-multipoint packet based networking. Wireless internet uses packet based networking over radio frequencies and power line internet uses harmonics to influence the modulation of the electric alternating current.
None of these can be DSL since they do not use analogue copper wire and do not use sound modulation. Yet, they are still referred to as DSL for some reason, since they are all permanent and fast. It would appear that the three letters DSL, have become synonymous with cheap, fast, permanent internet connections and as such, have become the publically known name for any kind of broadband permanent internet.
Willie Viljoen
Web Developer
Adaptive Web Development
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Especially when you share it with 4 people...that sucks a lot.