Ringing (telephony)

Ringing is a telecommunication signal that causes a bell or other device to alert a telephone subscriber to an incoming telephone call. Historically, this entailed sending a high-voltage alternating current over the telephone line to a customer station which contained an electromagnetic bell. It is therefore also commonly referred to as power ringing, to distinguish it from another signal, audible ringing, or ringing tone, which is sent to the originating caller to indicate that the destination telephone is in fact ringing. In landline telephones, bells or ringtones are rung by impressing a 60 to 105-volt RMS 20-Hertz sine wave across the tip and ring conductors of the subscriber line, in series with the (typically) −48 VDC loop supply. This signal is produced by a ringing generator at the central office. When the switching system directs a call to a particular subscriber line, a relay on the line card connects the ringing generator to the subscriber line. The exchange also sends a ringing tone to the calling party. When the called party answers by taking the telephone handset off the switchhook, the subscriber's telephone draws direct current from the central office battery. This current is sensed by the line card and the ringing relay is de-energized. On multi-party lines, ringers would typically be connected from one side of the two-wire line to ground; a "tip party" and "ring party" would have bells connected from opposite sides of the line. On a two-party service, each user would not hear ringing for calls to the other party. Some 20th-century independent telephone companies deployed four-party lines which used differing frequencies for selective ringing of individual parties (the four possible combinations were 20 Hz or 30 Hz from tip to ground, 20 Hz or 30 Hz from ring to ground). If additional parties were added to the same line, distinctive ringing patterns would need to be used to identify the called subscriber; these were audible to the multiple users on the line.