TUCoPS :: Phreaking Technical System Info :: metrpuls.txt

Metering pulses in Europe

(204)   Sun 3 Nov 91 15:03
By: Peter Knoppers
To: All
Re: Re: Need Information on European Metering Pulses
@PTH 1:340/201.0@Fidonet
From: knop@duteca4.et.tudelft.nl (Peter Knoppers)
Newsgroups: comp.dcom.telecom
Subject: Re: Need Information on European Metering Pulses
Organization: Delft University of Technology, Dept. of Electrical Engineering

moon@gdc.com writes:

> I am looking for information on the "metering pulses" that are used in
> many European countries.

In the Netherlands metering pulses are available at a price if you are
connected to a computerized telephone exchange, or free if you are
connected to an old-style exchange. (The telco cannot suppress the
signal in old-style exchanges.)

The signal is a 50Hz common mode signal of approximately 60 Volts.
The signal lasts about 400 ms (but this varies a lot). Each pulse
corresponds to DFL 0.15 on your phone bill (about US $ 0.08).

To count the pulses a special (rather expensive) counter is used
that contains the following circuit:

  a     b     a and b are connected parallel to the phone(s)
  |     |     (These wires are called tip and ring in the US)
  |     |
* > ||| <     This is a kind of transformer that only passes
  > ||| <     through common mode AC and DC current.
  > ||| < *
  |     |
  |     |          These capacitors block DC current, preventing
 ===   === 0.3 uF  an off-hook condition that would otherwise occur.
  |     |          Only common mode AC current gets beyond this point.
    === 0.5 uF     Don't know why this capacitor is needed.
     \  1.2 .. 2.2 kOhm Resistor.
  |__/__| 2400 Ohm  Coil that controls the mechanical counter.
     e    This wire must be connected to ground.

In computerized exchanges additional equipment must be connected to
the subscriber circuits that are to receive metering pulses.  This
equipment is very expensive and takes up a similar amount of space as
4 subscriber line circuits.

It is possible to replace the resistor in the counter with a bridge
rectifier and a small relay. This relay closes shortly when a metering
pulse is received and can be used to drive other circuits. Such
modifications are probably not appreciated by the phone company ...

> My particular problem is determining the requirements for a modem in
> this scenario, in particular, I need to know the maximum signal level
> of the metering pulses at the modem's input.

As you can see, the metering pulse signal reaches the phone or modem
completely intact. The modem should therefore be capable of normal
operation with a 50 Hz 60 Volt common mode signal superimposed on the
normal line voltage. (The audio signal is differential mode.)  Test
signals used by the phone company to test subscriber circuits use much
higher voltages (a few 100 Volts). While this does not happen while
the line is off-hook, a modem should survive such signals when

Hope this helps,

Peter Knoppers - knop@duteca.et.tudelft.nl

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@Date: 3 Nov 91 15:03:40 GMT
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