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ASL offers (and sometimes guarantees) a quality of service 'acceptable to human
listeners', which is a bandwidth of 300Hz-3300Hz, a SNR of up to about 30dB, a frequency
shift of less than say 10Hz, a nearly constant signal delay, hardly noticeable crosstalk
and echoes, and so on. This applies to the switched connection from endpoint to
endpoint. Taking all of these into account, this translates into a maximum digital
data rate of about 33kBit/s achievable with the most sophisticated modulation schemes,
and is near the theoretical limit.
In contrast, DSL is not an endpoint-to-endpoint technology developed for human
speakers, but an technology for the 'last mile' from the home to the next exchange
point carrying digital data. DSL offers a quality of service described by guaranteed
isochronous up- and downstream bitrates with a bit error rate less than a guaranteed
limit. In terms of technology, this is the same as digital data transmission over
an ASL channel using modems on each endpoint; only the names are different. One
modem sits in your home (call it 'terminal equipment'), the other no more than 2-3
kilometers away at the exchange point. Because of the short distance and the 'known'
medium, the channel capacity is no longer restricted to about 30-40kBit/s, but some
MBit/s are achievable.
There are many different variants of DSL in use today or proposed: the well known
HDSL (for T1/E1/J1 lines), the widespread (in Europe) ISDN with all of its services,
ADSL (not so common), ADSL-lite (the splitterless version of ADSL pushed in the
US), RDSL, VDSL, and the upcoming ETSI standard SDSL.
Each of these has its benefits and disadvantages. If you compare ADSL-lite with
SDSL you'll see:
1) ADSL-lite offers 1MBit/s max., SDSL 2MBit/s
2) ADSL-lite is less expensive than SDSL
3) ADSL-lite is compatible to existing analog equipment, SDSL to existing digital
4) ADSL-lite is more susceptible to/generates more crosstalk than SDSL.
5) ADSL-lite basically offers an IP channel, SDSL is something like ISDN++
This is why SDSL is better suited to Europe, ADSL-lite to the US. An example:
here in Germany, a digital channel (one ISDN B-Channel) is cheaper than an analog
line. This is why ISDN is very common (not only) in Germany today. Germany is densely
populated, and the lines run in big trunks in parallel almost from endpoint-to-endpoint,
but usually are no longer than 2 kilometers. ADSL-lite over these lines offers little
benefit over ISDN and requires a considerable amount of hardware. on the other hand,
SDSL offers the same services as ISDN, is compatible to ISDN (ISDN is just a multiplexed
subservice), and works on the existing infrastructure.
Editor note: For more on ADSL and other internet access methods
and OS/2 see Dan Casey's article in the May 1998 issue of the VOICE Newsletter http://www.os2voice.org/VNL/past_issues/VNL0598H/vnewsf2.htm.
For more on DSL in general see http://www.paradyne.com/sourcebook_offer/sb_html.html.