From: stephen@sprunk.org   
      
   On 23-Jun-14 09:39, hancock4@bbs.cpcn.com wrote:   
   > On Sunday, June 22, 2014 12:17:07 AM UTC-4, Stephen Sprunk wrote:   
   >> On 21-Jun-14 22:33, hancock:   
   >>> I have no idea what the voltage/currents are on VOIP phone   
   >>> lines.   
   >>   
   >> On the POTS side, both should be the same as a real POTS line.   
   >>   
   >> On the IP side, it's whatever the relevant IP technology uses, but   
   >> in general the longer the wires it's intended to run over, the   
   >> higher the voltage (at least at the source) will be.  Most use   
   >> differential or alternating signaling so the net current is zero.   
   >   
   > At some offices, there are various types of phone landlines.  One is   
   > the traditional analog.  Another is 'digital' (POTS telephone sets   
   > will not work on it).  Still another, the newest type, is VOIP (no   
   > POTS phones either).  I don't know the difference between 'digital'   
   > and 'VOIP'--indeed, I thought the two were the same.   
      
   "Analog" PBX lines are pure POTS; all the signaling is done in-band with   
   DTMF and hookflash.   
      
   "Digital" PBX lines typically use POTS-style analog on the first pair   
   for the voice path plus digital signaling on the second pair.  This   
   enables numerous features not available with analog signaling.   
      
   VoIP is pure IP.  They may look the same as a "digital" phone to the   
   user, but the guts are completely different.  Most importantly, you no   
   longer need separate phone wiring; a VoIP desk phone is typically   
   plugged into the Ethernet wall jack, and the computer plugs into a   
   2-port switch in the back of the phone.  (Cheaper models don't have a   
   switch, which means they're only useful for locations without a computer   
   or where there are multiple wall jacks.)   
      
   > Anyway, as mentioned, plugging in a voltmeter to a traditional   
   > landline yields 48V DC.  I have no idea what voltage will be given   
   > if plugged into a 'digital' [line]   
      
   It varies by vendor, but probably 48VDC as well.   
      
   > or 'VOIP' line.   
      
   There is no such thing as a "VOIP line".   
      
   If you meant Ethernet, that's a low-voltage, differential system, so   
   your voltmeter would probably show nothing at all.   
      
   S   
      
   --   
   Stephen Sprunk         "God does not play dice."  --Albert Einstein   
   CCIE #3723         "God is an inveterate gambler, and He throws the   
   K5SSS        dice at every possible opportunity." --Stephen Hawking   
      
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