For caller ID to work properly, IOS uses an internal setting based on the CPtone that is configured against the voice port, for example:
! voice-port 0/1/0 cptone BE !
* Telcordia Caller-ID support in North America Australia, China, Hong Kong, and Singapore, Telcordia Technologies (formerly Bellcore) - based on the GR-30-CORE specification.
* European Telecommunication Standards Institute (ETSI) Caller-ID support in France, Germany, Norway, Italy, Spain, South Africa, Turkey, and UK (CCA), - based on the European Telecommunication Standard ETS 300 648, ETS 300 659-1
* ETSI Caller-ID support in UK (British Telecom) - based on the British Telecom SIN227, SIN242
* NT FSK Caller-ID support in Japan - based on the Nippon Telegraph & Telephone Corporation standard
* DTMF-based Caller-ID support in Brazil, Belgium, Sweden, Denmark, Holland, Iceland, and Saudi Arabia - based on the ETS 300 648, ETS 300 659-1
To receive CND information, the voice port monitors the phone line between the first and second ring bursts without causing the voice port to go off hook in the conventional sense, which would inhibit the transmission of CND by the local central office. The CLID is sent as a burst of FSK modem tones using 1200 baud Bell 202 tone modulation. The data may be sent in Single Data Message Format (SDMF) which includes the date, time and number or in Multiple Data Message Format (MDMF) which adds a NAME field. The CLID data is based on the following format -
CHANNEL SEIZURE SIGNAL
The channel seizure is 30 continuous bytes of 55h (01010101) providing a detectable alternating function to the CPE
The carrier signal consists of 130 +/- 25 mS of mark (1200 Hz) to condition the receiver for data.
MESSAGE TYPE WORD The message type word indicates the service and capability associated with the data message. The message type word for CND is 04h (00000100).
MESSAGE LENGTH WORD The message length word specifies the total number of data words to follow.
DATA WORDS The data words are encoded in ASCII and represent the following information:
The first two words represent the month The next two words represent the day of the month The next two words represent the hour in local military time The next two words represent the minute after the hour The calling party's directory number is represented by the remaining words in the data word field
If the calling party's directory number is not available to the terminating central office, the data word field contains an ASCII "O". If the calling party invokes the privacy capability, the data word field contains an ASCII "P".
CHECKSUM WORD The Checksum Word contains the twos complement of the modulo 256 sum of the other words in the data message (i.e., message type, message length, and data words). The receiving equipment may calculate the modulo 256 sum of the received words and add this sum to the reveived checksum word. A result of zero generally indicates that the message was correctly received. Message retransmission is not supported.
EXAMPLE CND SINGLE DATA MESSAGE
An example of a received CND message, beginning with the message type word, follows:
04h= Calling number delivery information code (message type word) 12h= 18 decimal; Number of data words (date,time, and directory number words) ASCII 30,39= 09; September ASCII 33,30= 30; 30th day ASCII 31,32= 12; 12:00 PM ASCII 32,34= 24; 24 minutes (i.e., 12:24 PM) ASCII 36,30,39,35,35,35,31,32,31,32= (609) 555-1212; calling Party's directory number 51h= Checksum Word