INTRODUCTION                                                                    
                                                                                
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 four second silent interval in the ringing
cycle.  The customer must contact a Bellcore Client Company to
initiate CND service.

According to Pacific Bell representatives, the following states and
district currently support CND service: Delaware, District of
Columbia, Florida, Georgia, Idaho, Kentucky, Louisiana, Maine,
Maryland, Nebraska, Nevada, New Jersey, Oklahoma, Tennessee, Vermont,
Virginia, and West Virginia.

The following states are scheduled to support CND service by April,
1992: Alaska, Arizona, California, Colorado, Illinois, Indiana, Iowa,
Massachusetts, Mississippi, New Hampshire, New York, North Carolina,
North Dakota, Ohio, Oregon, Rhode Island, and South Carolina.

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
(I have copied this data as presented.  I believe the transmission
level is meant to be -13.5 dBm.)

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 SEIZURE 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 CND 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.

(I will mail the Rockwell data sheet, which includes the suggested
schematic diagram.)

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.

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
Modem manufacturers will soon be implementing new modem features
based on CND information as this service becomes widely
available.

Once CND information is received the user may process the
information in a number of ways.

1. The date, time, and calling party's directory number can be
   displayed.

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. 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.