Caller ID FAQ

Caller ID FAQ

Frequently Asked Questions About Caller-ID


Table of contents

  1. What is Caller-ID ?
  2. OK, What is ANI ?
  3. Now (maybe) what is Caller-ID ?
  4. Background
  5. How is the Caller-ID information provided ?
  6. What is Caller ID On Call Waiting ?
  7. What is "ID Blocking" ?
  8. What happens if a call is forwarded ?
  9. What happens if I have two phone lines and a black box to do the forwarding ?
  10. I called somebody from a company phone (555-1234) but their Caller-ID device reported 555-1000.
  11. What are Private and Out of Area Calls ?
  12. Why are so many people against Caller-ID ?
  13. Glossary

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  1. What is Caller-ID ?

    First ask "What is ANI"

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  2. OK, What is ANI ?

    ANI or Automatic Number Identification is a mechanism by which the different telephone companies determine what account is to be charged for a call. This information is passed between Telcos and was originally for billing purposes and predated both SS7 (Signaling System 7) and (C)LASS (Local Area Signaling Services was the original AT&T designations, the "C" was added by Bellcore after divesture) services which makes CNID or Calling Number IDentification as Caller-ID is more properly known, possible.

    Since the Telcos had ANI, the decision was made to make it available to authorized parties such as 911 service and law enforcement agencies. ANI is also used to let a Telco operator know who is calling.

    More recently, ANI is used to report to 800 and 900 subscribers, who made the calls they have received, in the first case so that the 800 subscriber knows who the charge is for, and so that 900 number subscribers know who to charge.

    Thus while ANI is similar to CALLER-ID and may provide the same information, they are actually two different services and ANI information is not necessarily the same as what will appear on a CALLER-ID display.

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  3. Now (maybe) what is Caller-ID ?

    Caller-ID is a Telco offering that is a byproduct of (C)LASS services. In this case, only those numbers reported by participating exchanges are returned, exactly which are and which are not is currently (March 1994) at the Telco's discretion.

    The Federal Government has stated that it is their intent that nationwide CNID be available by mid-1995. The full text of this decision may be found FCC Report No. DC-2571 issued on March 8, 1994.

    The biggest effect of the ruling is to mandate transport of CPN (customer provided number) information between interconnecting networks eliminating the effective inter-LATA-only limitation that exists today in most areas.

    Currently there are two types of Caller-ID. The first (often referred to as "basic" service) returns just the calling number or an error message and the date/time of the call.

    The second ("enhanced" Caller-ID) also may return the directory information about the calling number. At a minimum, the name of the subscriber is returned (the subscriber is not the same as the caller, the phone company has no way to determine who is actually on the line).

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  4. Background

    The method for sending and displaying the CID information during a silent interval between rings was invented by Carolyn A. Doughty. This invention was filed for a United States Patent on July 12, 1983. It was assigned patent number 4,582,956 on April 15, 1986 with AT&T Bell Laboratories (now Lucent Technologies Incorporated) listed as the assignee.

    Caller ID was first offered in New Jersey in 1987 by New Jersey Bell. The telephone company was interested in trying to earn additional revenue from its investments in new high-speed network signaling systems. These new systems use a separate call data circuit based on the SS7 standard to handle the setup, termination, supervisory signals, and other data concerning a call. This separate call data circuit can handle the processing of multiple calls very quickly and eliminates certain types of toll fraud.

    Prior to SS7 telephone companies used ANI for call billing purposes. With only a few exceptions, the billing information about the caller was not sent beyond the central office that provided that caller's service. The exceptions to this were authorized parties such as 911 services, law enforcement agencies, and more recently 800 and 900 number subscribers. Even today, ANI is still used by these parties. Since ANI is completely independent of CID it will deliver the originating number even when the originator has blocked their number for CID purposes.

    With the installation of the new SS7 systems it became practical to forward the calling party's identification through the telephone network to the central office serving the called party. This aspect of SS7 is known as Calling Party Number Message (CPNM). The CPNM includes the calling party's telephone number, and whether or not the calling party wants their number blocked from being seen by the called party. Note that the CPNM is sent out on all telephone calls that are made, regardless of whether or not the calling party wants their number blocked.

    A major issue that came up with the offering of CID was that of privacy. In several states the implementation of CID was slowed by privacy advocate groups. These groups claimed that CID was an invasion of privacy. In April of 1994, the Federal Communications Commission (FCC) issued a ruling that established a national standard for CID and the delivery of the caller's number into the switched network. Local and long distance telephone companies that were equipped to do so had to exchange CPNMs. Along with this, the telephone companies had to offer blocking on a per call basis so that the caller could block their number from being delivered to the called party. The CPNM is still delivered to the final central office serving the end subscriber, but the number is then blocked and the CID message marked as private.

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  5. How is the Caller-ID information provided ?

    As a 1200 baud, 7 data bits, 1 stop bit data stream usually transmitted following the first and before the second ring signal on the line. Note that this is not a standard Bell 212 or CCITT v22 data format so a standard modem will probably not be able to receive it. Further, the serial information exists as such only from the recipient's switch to the callee's location. Between carriers the signal exists as data packets.

    The signal is provided before the circuit is complete: picking up the receiver before the data stream is finished will stop/corrupt the transmission.

    Currently there are two types of information returned: a "short form" which contains the date/time (telco and not local) of the call and the calling number or error message. The "long form" will also contain the name and possibly the address (directory information) of the calling phone.

    The "short form" stream consists of a set of null values, followed by a two byte prefix, followed by the DATE (Month/Day), TIME (24 hour format), and number including area code in ASCII, followed by a 2s compliment checksum. Most modems/caller id devices will format the data but the raw stream looks like this : 0412303232383134333434303735353537373737xx or (prefix)02281334407555777(checksum)

    A formatted output would look like this:

    Date - Feb 28
    Time - 1:34 pm
    Number - (407)555-7777

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  6. What is Caller ID On Call Waiting ?

    In carrying CID a step further and combining it with Call Waiting, a service commonly called Caller ID on Call Waiting has been developed. This feature is also sometimes called Calling Identity Delivery on Call Waiting. With CIDCW the identification of the party calling can now be seen without putting the current call on hold. CIDCW is done the same way as CID with only a few exceptions. One of those exceptions is that CIDCW only uses MDMF so the message type will always be a binary 10000000 (decimal 128).

    Since CIDCW will only take place if a call is already in progress there will never be a ring signal preceding the message. The channel seizure signal is not used with CIDCW either. The central office gets the CPE's attention by sending it a CPE Alerting Signal (CAS), a two-tone signal described later. If the customer's equipment is ready to receive the CID information, it responds to the CAS by sending the central office an acknowledgment (ACK) signal. Note that the CPE will only respond to the CAS signal if it is the only extension that is off-hook. If someone else is involved in the call on another extension, the CPE will not send an ACK signal to the central office because it cannot mute the second device. Most manufacturers detect whether an extension is off-hook by monitoring the line voltage. If the line voltage drops below a certain point then it is assumed that an extension is off-hook. This can lead to problems in situations where the CPE is placed on a very long loop. The long loop reduces the voltage at the CPE and causes its detector to continuously register an extension off-hook. At least one manufacturer is getting around this by designing the equipment to learn the line voltage that it should see during normal operation. This is done by taking the device off-hook two or three times so that it can register the normal off-hook voltage. If the voltage ever drops below this setting, it assumes that there is another extension off-hook. Once the central office sees the ACK signal it sends the FSK data.

    When the CPE sees the CAS from the central office it mutes the telephone's handset before sending the ACK and receiving the FSK data. This act of muting serves two purposes.

    First, it keeps the telephone user from hearing the FSK signal. But, more importantly it avoids interference from speech and other noises that would be picked up by the telephone's microphone. The muting only has to take place on the end receiving the FSK data since before sending the CAS, the central office momentarily removes audio to and from the far end of the current call. The audio is reconnected after the FSK data has been transmitted.

    The sequence of events that displays the information about the caller begins when the central office temporarily removes the far end party of the current call and sends a Subscriber Alerting Signal (SAS) to the near end party. The SAS is a single frequency of 440 Hz that is applied for approximately 300 ms. This is the tone that is heard when a call is in progress and call waiting beeps to indicate a second call. The SAS tone is mainly for the user and is not required for the CPE to receive the CID information. The SAS tone is followed by a CAS to alert the CPE that it has CID information to send. The CAS is a dual tone signal combination of 2130 Hz and 2750 Hz that is 80 ms in length. Once the CPE hears the CAS it mutes the handset of the telephone and returns an ACK signal to the central office. This ACK signal has a nominal tone duration of 60 ms and is either a Dual-Tone Multifrequency (DTMF) "A" or a DTMF "D". The DTMF "D" is the most common ACK signal and it consists of the frequencies 941 Hz and 1633 Hz. A DTMF "A" consists of the frequencies 697 Hz and 1633 Hz. Once the central office receives the ACK signal it in turn sends the CID information. The CPE un-mutes the handset as soon as it finishes receiving the FSK signal.

    A problem that is sometimes experienced with CIDCW is Talkoff. Talkoff occurs when the CPE falsely detects a CAS. Speech can sometimes be interpreted by the CPE's detector as a CAS from the central office. When talkoff occurs both parties will hear a brief interruption in the conversation and the party on the far end will hear the ACK signal that is sent to the central office by the CPE. When the handset is muted by a talkoff, the CPE will timeout and un-mute the handset after waiting a set period of time for an FSK signal. Bellcore SR-3004 specifies that this timeout period should be no more than 501 ms from the end of the ACK signal. A CPE's detector will behave differently with different people because of the variations in each persons voice and speech patterns. Speech can also cause a valid CAS signal to not be recognized by the CPE. When this occurs, it is called a Talkdown.

    The last difference between CID and CIDCW is that instead of being preceded by the 300 bits of Channel Seizure Signal and 180 bits of Mark Signal, as with standard CID, the Caller ID message for CIDCW is only preceded by 80 bits of Mark Signal. CIDCW still uses the checksum just like CID.

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  7. What is "ID Blocking" ?

    Most Telco's providing Caller-ID have been required to also provide the ability for a calling party to suppress the Caller-ID signal. Generally this is done by pressing star-six-seven before making the call. In most cases this will block the next call only however some Telcos have decided to implement this in a bewildering array of methods. The best answer is to contact the service provider and get an answer in writing.

    Currently this is supplied as either by-call or by-line blocking. By-Call is preferred since the caller must consciously block the transmission on each call. By-Line blocking as currently implemented has the disadvantage that the caller, without having a second caller-id equipped line to use for checking, has no way of knowing if the last star-six-seven toggled blocking on or off.

    Note that blocking is provided by a "privacy" bit that is transmitted along with the CNID information and so is still available to the Telco switch, just not to the subscriber as a CNID signal. Consequently related services such as call trace, call return, & call block may still work.

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  8. What happens if a call is forwarded ?

    Generally, the number reported is that of the last phone to forward the call. Again there are some Telco differences so use the same precaution as in (6). If the forwarding is done by customer owned equipment there is no way of telling but will probably be the last calling number.

    Note that as specified, CNID is *supposed* to return the number of the originating caller but this is at the mercy of all forwarding devices, some of which may not be compliant.

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  9. What happens if I have two phone lines and a black box to do the forwarding ?

    If you have two phone lines or use a PBX with outdialing features, the reported number will be that of the last line to dial. Currently there is no way to tell a black box from a human holding two handsets together.

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  10. I called somebody from a company phone (555-1234) but their Caller-ID device reported 555-1000.

    Often a company with multiple trunks from the Telco and their own switch will report a generic number for all of the trunks.

    There is a defined protocol for PBXs to pass true CNID information on outgoing lines but it will be a long time before all existing COT (Customer Owned Telephone) equipment is upgraded to meet this standard unless they have a reason to do so.

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  11. What are Private and Out of Area Calls ?

    Each telephone company has an area in which they support Caller ID. The extent of this area varies with each local telephone company. In some cases this may be multiple states, and in other cases only your local area. Private and Out of Area calls are special calls supported by telephone companies.

    If you receive a call from someone from out of the calling area supported by your telephone company, the Caller ID software will indicate Out of Area. Further identification of the caller is not possible. You can add this number to the Contacts as 'O', which is the actual telephone number supplied by the telephone company for Out of Area calls.

    The telephone company must also provide an option to have the Caller ID information blocked. This is a special service that can apply to all calls made by a Private caller, or on selected calls.

    If you receive a call from someone what has their Caller ID information blocked, then you will receive only an indication that the caller is Private. Further identification of the caller is not possible. You can add this number to the Contacts as 'P', which is the actual telephone number supplied by the telephone company for Private calls.

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  12. Why are so many people against Caller-ID ?

    FUD - Fear, Uncertainty, & Doubt or 10,000,000 lemmings can't be wrong. There were some justifiable concerns that some people (battered wives, undercover policemen) might be endangered or subject to harassment (doctors, lawyers, celebrities) by Caller-ID. As mentioned above there are several legitimate ways to either block Caller-ID or to have it return a different number. It is up to the caller. The advantage is that with Caller-ID, for the first time, the called party has the same "right of refusal".

    Expect yet another Telco service (at a slight additional charge) to be offered to return an office number for calls made from home. Crisis centers could return the number of the local police station.

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  14. Glossary
    ACK
    Acknowledgment
    ANI
    Automatic Number Identification
    ASCII
    American Standard Code for Information Interchange
    BFSK
    Binary Frequency Shift Keying
    CAS
    CPE Alerting Signal
    CID
    Caller Identification or Caller ID
    CIDCW
    Calling Identity Delivery on Call Waiting or Caller ID on Call Waiting
    CNAM
    Calling Name Delivery
    CND
    Calling Number Delivery
    CPE
    Customer Premise Equipment
    CPNM
    Calling Party Number Message
    DTMF
    Dual-Tone Multifrequency
    FCC
    Federal Communications Commission
    FSK
    Frequency Shift Keying
    ID
    Identification
    LATA
    Local Access and Transport Area
    LSB
    Least Significant Bit
    LSSGR
    LATA Switching Systems Generic Requirements
    MDMF
    Multiple Data Message Format
    OSI
    Open Switch Interval
    PC
    Personal Computer
    SAS
    Subscriber Alerting Signal
    SDMF
    Single Data Message Format
    SPCS
    Stored Program Control Switching System
    SS7
    Signaling System 7



Parts of this FAQ are (c) Padgett Peterson. Some information used from Michael W. Slawson Caller ID Basics.