Caller ID
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.
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.
The channel seizure is 30 continuous bytes of 55h (01010101) providing
a
detectable alternating function to the CPE (i.e. the modem data
pump).
The
carrier signal consists of 130 +/- 25 mS of mark (1200 Hz) to
condition the receiver for
data.
The message type word indicates the service and capability associated
with
the data message. The message type word for CND is 04h
(00000100).
The message
length word specifies the total number of data words to
follow.
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".
The Checksum Word contains the twos complement of the modulo 256 sum
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.
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)
party’s directory number
51h= Checksum Word
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.
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
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
For more
information on Calling Number Delivery (CND), refer to
Bellcore publications TR-TSY-000030 and
TR-TSY-000031.
Always test as much as possible before relying on any method of
blocking
Caller-ID. Some of these methods work in some areas, but not in others.
Dial *67 before you dial the number. (141 in the United Kingdom)
Dial your local TelCo and
have them add Caller-ID block to your line.
Dial the 0 Operator and have him or her place the
call for you.
Dial the call using a pre-paid phone card.
Dial through Security
Consultants at (900)PREVENT for U.S. calls
($1.99/minute) or (900)STONEWALL for international
calls ($3.99/minute).
Dial from a pay phone. 