Friday, May 7, 2010

How a Caller ID Feature Works?

With a caller ID feature, a user is able to see caller information on the telephone display or on an attached caller ID box. It is usually available on most digital, VoIP and on some analog phone systems. This feature is a telephone service that transmits caller information like name and number to called party’s device.

The process of sending caller ID information to the other end of the line is quite simple. Telecommunication providers use the FSK (Frequency Shift Keying) technique, similar technique used on baud modems, to send ASCII data over a telephone line to the called party’s device display. FSK sends data by changing tone or frequency (that represents data bit 1 or 0). So, when a caller dials a number the data is sent between the first and second telephone ring. On some telephone or caller display box there is an audible half-a-second bleep sound that, when decoded, contains the caller ID information: type and length of message; time and date of the call; the phone number; and a checksum byte for making sure that the data received is complete.

Other caller ID systems, found on some toll free service, use advanced techniques, similar to FSK, but transmit data as an 8-ASCII character preceded by a start bit 0 and then followed by stop bit 1. With a circuit, a ring signal is detected and the bits are decoded using the modem contained on the telephone or caller ID box. The display is then handled by a simple processor. This technique enables the caller ID to display more caller information such as the caller name, number being called, location, date, time, etc.

Caller ID is a very useful telephony feature for tracking down unwanted callers, telemarketers and prank callers.

1 comment:

  1. Great run-down on a common telephony feature. Never knew how caller ID operated.