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January 2000

What is DSL?

By: Daniela Engert

If you consider your plain old analog telephone line running into your home as ASL (analog subscriber line), then what's the difference to DSL (digital subscriber line)?

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 (ISDN) equipment

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.

Daniela Engert is a systems engineer at MEDAV GmbH, in Germany. She is the author of the freeware DANIS506 replacement IDE driver for OS/2, which gives busmastering support for non-Intel chipsets, as well as fixes for some bugs IBM didn't. Also DANIADSK, which is a IBMATAPI.FLT replacement with additional support for the ZIP-250 (including media swapping), a patch for OS2LDR which fixes the OS/2 >64MeB memory detection defect found with recent BIOSes and a patched version of the ESS Maestro-2 sound card driver for use with the Maestro-2E .

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.

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