27. What is Caller-ID?

This FAQ answer is stolen from Rockewell:
Calling Number Delivery (CND), better known as Caller ID, is a telephone
service intended for residential and small business customers. It allows
the called Customer Premises Equipment (CPE) to receive a calling party's
directory number and the date and time of the call during the first 4
second silent interval in the ringing cycle.
Parameters
~~~~~~~~~~
The data signalling interface has the following characteristics:

           Link Type:                              2-wire, simplex
        Transmission Scheme:          Analog, phase-coherent FSK
        Logical 1 (mark)              1200 +/- 12 Hz
        Logical 0 (space)             2200 +/- 22 Hz
        Transmission Rate:            1200 bps
        Transmission Level:           13.5 +/- dBm into 900 ohm load

Protocol
~~~~~~~~
The protocol uses 8-bit data words (bytes), each bounded by a start bit and
a stop bit. The CND message uses the Single Data Message format shown
below.

| Channel  |  Carrier  |  Message  |  Message  |  Data       | Checksum |
| Seizure  |  Signal   |  Type     |  Length   |  Word(s)    | Word     |
| Signal   |           |  Word     |  Word     |             |          |

Channel Siezure Signal
~~~~~~~~~~~~~~~~~~~~~~
The channel seizure is 30 continuous bytes of 55h (01010101) providing a
detectable alternating function to the CPE (i.e. the modem data pump).
Carrier Signal
~~~~~~~~~~~~~~
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:

   * o The first two words represent the month
   * o The next two words represent the day of the month
   * o The next two words represent the hour in local military time
   * o The next two words represent the minute after the hour
   * o 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 CNS Single Data Message
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
An example of a received CND message, beginning with the message type word,
follows:

04 12 30 39 33 30 31 32 32 34 36 30 39 35 35 35 31 32 31 32 51

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

Data Access Arrangement (DAA) Requirements
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To receive CND information, the modem monitors the phone line between the
first and second ring bursts without causing the DAA to go off hook in the
conventional sense, which would inhibit the transmission of CND by the
local central office. A simple modification to an existing DAA circuit
easily accomplishes the task.
Modem Requirements
~~~~~~~~~~~~~~~~~~
Although the data signalling interface parameters match those of a Bell 202
modem, the receiving CPE need not be a Bell 202 modem. A V.23 1200 bps
modem receiver may be used to demodulate the Bell 202 signal. The ring
indicate bit (RI) may be used on a modem to indicate when to monitor the
phone line for CND information. After the RI bit sets, indicating the first
ring burst, the host waits for the RI bit to reset. The host then
configures the modem to monitor the phone line for CND information.
Signalling
~~~~~~~~~~
According to Bellcore specifications, CND signalling starts as early as 300
mS after the first ring burst and ends at least 475 mS before the second
ring burst
Applications
~~~~~~~~~~~~
Once CND information is received the user may process the information in a
number of ways.

  1. 1. The date, time, and calling party's directory number can be
     displayed.
  2. 2. Using a look-up table, the calling party's directory number can be
     correlated with his or her name and the name displayed.
  3. 3. CND information can also be used in additional ways such as for:
          a. Bulletin board applications
          b. Black-listing applications
          c. Keeping logs of system user calls, or
          d. Implementing a telemarketing data base

References
~~~~~~~~~~
For more information on Calling Number Delivery (CND), refer to Bellcore
publications TR-TSY-000030 and TR-TSY-000031.
To obtain Bellcore documents contact:

        Bellcore Customer Service
        60 New England Avenue, Room 1B252
        Piscataway, NJ   08834-4196
        (908) 699-5800




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