Ringing signal
From Wikipedia, the free encyclopedia
A ringing signal is an electronic telephony signal that causes a telephone to alert the user to an incoming call.
On a POTS telephone system, this is created by sending an alternating current signal of about 100 volts into the line. Today this signal is transmitted digitally for most of the journey, converted into an alternating current only if the line is not digital end-to-end.
On old, rotary phones, this voltage was used to trigger a high-impedance electromagnet to ring a bell on the phone. Modern Fixed phones detect this AC voltage and trigger a ring tone electronically. Mobile phones are fully digital, hence are signalled to ring as part of the protocol they use to communicate with the cell base stations.
In fixed POTS phones,ringing is said to be "tripped" when the impedance of the line reduces to about 600 ohms when the telephone handset is lifted off the switch-hook. This causes the telephone call to be answered, and the telephone exchange immediately removes the ringing signal from the line and connects the call. This is the source of the name of the problem called "ring-trip", which occurs when the ringing signal on the line causes a low-resistance short between the conductors, which trips the ring out before the subscriber's actual telephone has a chance to ring (for more than a very short time); this is common with wet weather and improperly installed lines.
Early research showed that people would wait until the phone stopped ringing before picking it up. Breaks were introduced into the signal to avoid this problem, resulting in the common ring-pause-ring pattern used today. In early party line systems this pattern was a Morse code letter indicating who should pick up the phone, but today, with individual lines, the only surviving patterns are a single ring (in the USA) and double-ring (in the UK), originally Morse code letters T and M respectively.
The ringing pattern is known as ring cadence. This only applies to POTS fixed phones, where the high voltage ring signal is switched on and off to create the ringing pattern. In North America, the standard ring cadence is "2-4", or two seconds of ringing followed by four seconds of silence. In the UK, the standard ring cadence is 400 ms on, 200 ms off, 400 ms on, 2000 ms off. These patterns may vary from region to region, and other patterns are used in different countries around the world.
A service akin to party line ringing is making a comeback in some small office and home office situations allowing facsimile machines and telephones to share the same line but have different telephone numbers; this CLASS feature is usually called distinctive ringing generically, though carriers assign it trademarked names such as "Smart Ring". This feature is also used for a second phone number assigned to the same physical line for roommates or teenagers, in which case it is sometimes marketed under the name teen line.
Caller ID signals are sent during the silent interval between the first and second bursts of the ringing signals.
The interrupted ring signal was designed to attract attention and studies showed that an intermittent two tone ring was the easiest to hear. This had nothing to do with the coded ringing that was used on party line.
