CallerID FAQ

Note: Collection of various FAQs, information, and Internet snippets that hope to answer your questions.


[Document Version: 1.03] [Last Updated: 10/7/97]

Chapter 1) CallerID Introduction

  1.1) Section Info

From: Jeff Sicherman
Date: 1 Dec 91
Posted to: comp.dcom.telecom

Note: This is a copy of the data sheet picked up at the Rockwell booth at the COMDEX show.

  1.2) 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.

[(From the Editor)

The following are dated and HAVE changed... anyone know to what?]

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.

  1.3) Parameters/Characteristics

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.)

  1.4) 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.

   1         2         3            4                  5
Channel | Carrier | Message | Message Data        | Checksum
Seizure | Signal  | Type    | Length Word(s) Word | 
Signal  |         |         | Word  Word          |

    1.4.1) 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).

    1.4.2) Carrier Signal

The carrier signal consists of 130 +/- 25 mS of mark (1200 Hz) to condition the receiver for data.

    1.4.3) 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).

    1.4.4) Message Length Word

The message length word specifies the total number of data words to follow.

    1.4.5) Data Words

The data words are encoded in ASCII and represent the following information:

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".

    1.4.6) 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.

    1.4.7) 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

    1.4.8) 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.

Look at the DTMF FAQ for some more info. (The Rockwell data sheet includes the suggested schematic diagram)

    1.4.9) 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.

(I'm skipping some Rockwell-specific information here.)

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: 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
(201) 699-5800

This article is also be filed in the Telecom Archives ( as a separate article. Check there for further information.

Chapter 2) Other CallerID information

(From John Schuch)

Phone gets first ring, duration 2 seconds
Delay 0.5 seconds after first ring
Send 30 bytes of '01010101', duration 250ms (channel seizure signal)
Send 150ms of marks (carrier signal)
Send an 8 bit "parameter word type"
        00000100 = Caller ID
        00001010 = message waiting indicator
        10000001 = Test for caller ID
Send an 8 bit number representing the number of WORDS in the message

Now, the actual data:

The first two digits are the month with a leading zero for JAN to SEPt.
The next two digits are the day of the month w/leading zero for 1 to 9.
The next two digits are the hour in local military time.
The next two digits are the minutes in local time.
The remaining digits (message length -8) are the telephone number of the
              calling party. The last field may include the '' or 'O' for
              blocked or out-of-area calls.
The last 8 bits of the message are the checksum. The check sum is the two's complement of the modulo 256 sum of the other words in the data message. This sum does not include the channel seizure signal or the carrier signal.

Total transmission duration = 718ms

The above description is as stated in the Motorola MC145447 Caller ID Receiver data sheet, and in the Cermetek Application Note AN-107.

Chapter 3) Simple Q&A

(From the Editor)

You can submit any other Questions AND Answers with this [Feedback Form]

  3.1) Can I use a 2400baud modem?

(From Henry Spencer)

[A 2400 baud] modem would need substantial changes. Normal 1200bps modem signalling is NOT FSK, and your modem almost certainly is not set up to receive 1200-baud FSK. Moreover, it would have to do it WITHOUT "picking up the phone", which again it probably cannot do.

(From the Editor)

Your 2400baud modem can not do it. There have been some more recent high-end modems that DO include CallerID detection. But that's not the original question.

  3.2) Does every phone have CallerID?

(From Henry Spencer)

The CallerID information isn't ON the telephone lines unless you pay for it. It's an extra-cost option, not part of basic phone service.

(From the Editor)

There have been various legal struggles by people who want CallerID and those who feel it invades privacy. Last I heard (or paid attention) there was talk of making CallerID an OPTION but making CallerID BLOCKING standard with the phone - so those who needed to use it would need to specifically enable it. I have no idea what the scumsuc.. er.. lawyers have done now. Also, please realize that ANY law on this level varies from state to state and definitely from country to conuntry. Also, don't expect states to talk to other states - the country to country communication may take years (or decades...). Here's what Ketil Albertsen wrote in some article:

Even here in Norway, which I believe is among the countries with the highest percentage of all-digital switches in the world, SS7 wasn't introduced until something like 3 or 4 years ago.

Countries with a large share of electromechanical switches won't have SS7 all over for many years to come, and won't be able to tell you the Caller ID. Although the ID in principle might be carried by modem-like signals between the first and second ring (isn't that the way you do it in the US?), don't expect other countries to invest in equipment for generating this sort of singals. It is cheap if you already have an all-digital exchange, but not if it is electromechanical.

But also note that SS7 (as well as DSS1, the subscriber signalling interface used in ISDN) allows caller IDs to be 'non-disclosure' marked, so the receiver end is not permitted to show it to the called party. I wouldn't be surprised if a teleadministration (or politicians, through laws) decide to mark all calls going out of the country as 'non-disclosure', as a form of protection of their subscribers/citizens. This is a political issue, not a technical one.

It's best to call your local phone company and ask them for specifics.

Chapter 4) CallerID chips

(From the Editor)

If you know of additional chips, please submit them with this [Feedback Form]

Chapter 5) References/Magazine Articles

(From the Editor)

If you know of additional references, please submit them with this [Feedback Form]

Contributors: and John F. Woods.


Magazine Articles

Chapter 6) Commercial CallerID components

(From the Editor)

If you know of additional Commercial offerings, please submit them with this [Feedback Form]

(From Don Davis)

Not a card, but a box that connects to the serial port -- available from:

Zeus Phonestuff
Atlanta, GA
Tel: 800-240-4637
Tel: 404-263-7111

Their product costs $99 (they may give you a discount if you order several). Has 2 RJ-11 phone jacks, and several neat features:

  1. The unit does NOT pass the first ring through to any telephone plugged into the phone jack -- first, it delivers the Caller*ID info out the serial port, then you have the option (under software control) to let the phone number determine whether you want subsequent rings passed through or not.

  2. The unit can provide the Caller*ID data in several formats. A checksum feature indicates whether the data received is valid.

  3. The unit can store up to about 30 sets of call info in case your system goes offline for a while.

  4. Software for PC-compatibles is included -- caller database and key macro TSR that'll lurk in the background and hit whatever keys you please depending upon the Caller*ID info.

  5. Straightforward hardware interface with good documentation -- I wrote my own little application in a couple of hours (I wanted to do some things that went beyond their software).

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