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On this page you will the find the text of an article written for the Financial Times Newsletter "Telecoms and Standards Monitor".

Using the Copper Fully

Developed by AT&T as a means of providing telcos the means of competing with cable television companies, xDSL technologies have reached a point where Asymmetric Digital Subscriber Line (ADSL) permits transportation of high quality MPEG-2 video to subscribers of ordinary telephone company services.

ADSL operates over the same twisted pair copper loop used to provide voice telephony but utilises the frequency band from 20kHz to 1.1MHz.

The scheme implemented by AT&T, is called Carrierless Amplitude Phase Modulation (CAP) and is an extension of the QAM technique used for ordinary dial up modems.

However, before AT&T (or Globespan Technologies) as the company is now known could deliver significant quantities of chipsets the American National Standards Institution (ANSI) created a standard (T1.413) for ADSL based on a technique known as Discrete Multitone (DMT).

DMT operates by dividing the available bandwidth into 32kHz wide "bins" each capable of carrying up to 64kbits of information.

This allows a theoretical maximum transmission rate of approximately 16Mbps, but current chipsets have a ceiling of 12Mbps.

This is split into two parts, "upstream" to the telephone exchange and "downstream" from the exchange.

Because of the way bins are allocated, the maximum upstream data rate is approximately 640kbps, while downstream performance is approximately 8Mbps.

This performance decreases the farther away the remote modem is situated from the exchange, owing to losses inherent in the telephone line cabling, but at the maximum expected line length (7.5km) downstream performance (1.5Mbps) is more than adequate for MPEG-1 video and more importantly fast internet access.

The ANSI standard is concerned only with two aspects of the modem:

The second aspect is of principal concern to European Approvals Authorities.

The European Telecommunications Standards Institute (ETSI) commissioned working group TM6 to examine the situation.

This resulted in a technical report (ETR 328) which acts as an appendix to ANSI T1.413, amending the CSA loops to be in line with those expected to be found in European countries.

No authority has, as yet published a document which describes the filtering arrangement necessary to separate the voice band signals from the ADSL band frequencies.

The filtering arrangement, known as a POTS splitter will need to be addressed shortly if a delay in the commercial roll-out of ADSL in Europe is to be avoided.

The reasons for this are quite simple.

Firstly, in the US and Canada, a 600 ohm termination of the telephone line is the norm. This permits the use of a very simple, passive splitter, which because of the complex termination arrangements used in prTBR21 cannot be implemented in splitter designs for Europe.

Secondly, the European standards for telephone line use do not anticipate use of the frequency bandwidth above 100kHz. Add the complication of inserting another device into the telephone network in series with the customers branch system and the situation becomes even less clear.

Because of this, new standards will need to be created or existing ones amended to take account of this situation.

Finally, there is an aspect which is outside the auspices of the regulation makers.

Interoperability. The DMT and CAP schemes are incompatible. Effort needs to be made to find a common ground in a similar way to the problems encountered by the analogue modem industry in their efforts to introduce a common scheme for 56kbps dial up modems.

The ADSL Forum is taking steps to address this issue, but no hard and fast measures have so far been taken to remove the obstacle of confusion which must confound the system engineers in companies such as BT and Kingston Communications.

If a decision were to be taken on the basis of technical evidence, then DMT would be the obvious choice for us all to follow.

Purely on the basis of performance, the upcoming DMT chipsets are clearly superior to their CAP counterparts and the future roadmap of increasing performance and flexibility of implementation makes DMT an appealing option if you need ADSL to coexist with ISDN.

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