|How a Telephone Line Works|
Telephones are essentially very straightforward audio
devices with a send path, receive path and a means of generating dialling.
Any confusion usually
lies with the way the device is interfaced with the line/
exchange or with the many features available on
In this state no calls are in progress and there is no ring signal present. A nominal 50 volts DC can be measured on the line.
Now the exchange has superimposed the ring signal onto the 50v, In its most common format this is an AC waveform 75vRMS 25Hz with a cadence of 0.4 sec on, 0.2 sec off, 0.4 sec on, 2 sec off.
|Other tones are generated at the exchange from time to time, e.g. dial tone, ringback tone. The parameters of these vary with line loss and type of exchange. However, they are not critical to the operation of most equipment.|
|Let's now take a look at some of the signals generated at the customer's end of the line. The most obvious of these is dialling and there are two types:|
|DUAL TONE MULTI-FREQUENCY (DTMF)|
This feature is commonly referred to as Tone Dialling. Each key on the keypad selects a tone from a group of high frequencies and one from a group of low frequencies. The two tones are combined and sent to line (either for a preset time or for as long as the button is held down).
|Whenever a key is pressed the loop formed when the apparatus went "off hook" is pulsed, 5 times for key 5, 10 times for key 0. In this country the pulsing or loop disconnection is done at 10 pulses per second. This is quite slow compared with tone dialling.|
|This function is used on PABXs to transfer an incoming call to another extension. There are two types of recall, Timed Break and Earth Loop. Timed break recall causes the loop to disconnected for approximately 75ms. With earth loop, the recall switch connects line (B) to earth (pin 4 of the modular socket).|