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SOME FACTS ABOUT THE STANDARD TELEPHONE LINE

 

 

Did you know?

There is a maximum amount of electrical current available from the public telephone network. Overloading a single exchange line means the network is unable to ring connected customer equipment. The carrier (ie Telstra, Optus) sets a maximum load limit to prevent this situation occurring. The public network can support customer equipment having a combine REN not exceeding 3. This is known as the Ring Equivalence Number (REN)

 

 

 

Customer equipment generally displayed on the back of the phone must show the  REN. All older style (bell) phones have an REN of 1.  This means the network will support 3 of these phones on the line (1 + 1 + 1 = 3).

Some other facts about the standard telephone line:-

  • Voltage in normally 50v DC (positive earth),
  • Ring voltage is approximately 70 to 90 V rms (see note #1 below)
  • Impedance is 600 ohms,
  • Analogue service,
  • Suitable for decadic dialling (see note #2 below) or DTMF (see note #3 below)  signalling,
  • The line is powered from the exchange, as you can appreciate the telephone service in your house does not need to be plugged into the power, and it works during black outs.
  • This line is also referred to a plain old telephone service (POTS) or both way trunk, as they can handle incoming or outgoing calls,
  • The line is allocated one unique telephone number and other numbers can be diverted to it. Like a house address it identifies a unique location for mail, and other mail can be redirected to it.
Note 1 rms = "Root Means Square".  This is a method used to determine the power value of an AC wave form. For example if the zero to peak value of the signal is 127v, then the rms is 90v AC.
Note 2 “decadic” this is the original method used to transfer the digits dialled on a telephone to the exchange, this method is synonymous with rotary dial telephones which are now becoming hard to find.  By breaking the DC loop it creates a pulse, by counting the pulse per second this then relates directly to number dialled. For example, the number 2 has 2 pulses, the number 0 is 10 pulses.  The pulsing rate is 10 per second.
Note 3 “DTMF” or other wise known as touch tone system.  It is a signalling method utilising tones to transfer the digits dialled by superimposing a succession of VF (voice frequency) signals on a line.  Each signal comprises of two simultaneously transmitted tones of different frequency DTMF signalling.

 

 

 

 

 

 

 
This page was last modified on 30 December 2009 and maybe out of date with regards to newer practices at the time of reading this article

 

 
 
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