Test & Measurement


Equipment to test ADSL

25 August 2004 Test & Measurement

Test telephones can create line disturbances that affect DSL. Many telephone companies have thus adopted the use of lock-out sets that alert users to the presence of a digital line. A further refinement to these sets is the ADSL-compatible model, which, like Tempo's Data Mate 7780 tone generator, is designed to avoid creating disturbances even when active on an ADSL service line.

The D361 is an ADSL compatible D360 targeted at telcos and ISPs who provide an ADSL network connection to their customer base.

With all the functionality of the company's D360: high impedance monitor, full duplex hands-free, data voltage alarm and lock-out, it also incorporates an ADSL type filter that allows it to work on an ADSL line without affecting the data transmission, and without the need of an external ADSL splitter. This is particularly useful when trying to test and qualify the analog part of an ADSL line, as it allows testing without having to disconnect the ADSL link (especially if the ADSL is supplied by a different service provider to the one supplying the copper pair).

When connected to an ADSL line, the data alarm is triggered (beeps), and the unit locks out. Because it is an ADSL line, the DC voltage present is of a 'normal' POTS voltage amplitude, and it is therefore possible to override the alarm and force an off-hook on the line. Once overridden, the alarms are disabled until the unit is disconnected from the line or switched off, so the unit can be left in monitor mode on that line. In the event of an incoming call, there will be no need to override the alarm, as the unit will go off-hook as soon as the talk key is pressed and released. This is a very unique feature. If at any time the unit is disconnected and then reconnected to another line, the alarms will all be reset, ready for detection again.

ADSL tone generator

The Datamate 7780 ADSL Tone Tone generator avoids problems in testing ADSL installations. It sends out a pure 900 Hz sine wave signal, and also limits the harmonic effects (interference) thus enabling ADSL services to continue working whilst tracing the tone. The Datamate is ideal for tone tracing in ADSL and other networks as it does not produce signals that interfere with ADSL signalling nor disturb the line in a way that degrades ADSL transmission. The main purpose of the Datamate is to prevent disruption to the line when accidentally connecting the tone generator whilst active. The 7780 features include: tone generator, polarity check, continuity check, talk battery, incoming ring and low battery indicator.

The PE7780 Data Mate Tone Generator is designed to operate around communication services that are especially sensitive to high frequency noise. Such services include Telecom DSL and voice/data cable services, among others. The PE7780 produces pair tracing tones that are both stronger and more pure than conventional models, and the unit has special filtering that limits disturbances to line impedance. Conventional trace tone generators work well for most applications, but their output signal can contain low-level, high frequency noise that will disturb certain kinds of services.

Example

An example of the kind of problems the PE7780 solves: A cable service provider added voice telephony and cable modem services to its HFC network, using T channels for the subscriber's upstream transmissions. Occasionally a single voice node would go out of service because of a carrier-to-noise ratio problem. In tracing the source of ingress technicians discovered that a trace tone generator had been left on a subscriber's coax distribution cabling during routine maintenance work. The small, but significantly high frequency part of the signal from the generator was providing intolerable ingress in their two-way system. This had not been a problem with ordinary cable service.



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