292 lines
14 KiB
Plaintext
292 lines
14 KiB
Plaintext
Subject: Frequently Asked Questions About Caller-ID
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Organization: TELECOM Digest
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Sender: telecom@eecs.nwu.edu
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Approved: telecom@eecs.nwu.edu
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X-Submissions-To: telecom@eecs.nwu.edu
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X-Administrivia-To: telecom-request@eecs.nwu.edu
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X-Telecom-Digest: Volume 14, Issue 446, Message 9 of 9
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Lines: 281
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[TELECOM Digest Editor's Note: It is time to re-run this article by
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Padgett which appeared earlier this year in the Digest. We will let
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this be the authoritative answer to the commentaries running in this
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issue of the Digest. PAT]
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Frequently Asked Questions About Caller-ID
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v1.1 Mar. 1994
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1) What is Caller-ID ?
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First ask "What is ANI"
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2) OK, What is ANI ?
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ANI or Automatic Number Identification is a mechanism by which
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the different telephone companies determine what account is to be
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charged for a call, This information is passed between Telcos and
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was originally for billing purposes and predated both SS7
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(Signaling System 7) and (C)LASS (Local Area Signaling Services
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was the original AT&T designations, the "C" was added by Bellcore
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after divesture) services which make CNID or Calling Number
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IDentification as Caller-ID is more properly known, possible.
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Since the Telcos had ANI, the decision was made to make it
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available to authorized parties such as 911 service and law
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enforcement agencies. ANI is also used to let a Telco operator
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know who is calling.
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More recently, ANI is used to report to 800 and 900 subscribers,
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who made the calls they have received, in the first case so that
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the 800 subscriber knows who the charge is for, and so that 900
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number subscribers know who to charge.
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Thus while ANI is similar to CALLER-ID and may provide the same
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information, they are actually two different services and ANI
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information is not necessarily the same as what will appear on a
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CALLER-ID display.
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3) Now (maybe) what is Caller-ID ?
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Caller-ID is a Telco offering that is a byproduct of (C)LASS
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services. In this case, only those numbers reported by
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participating exchanges are returned, exactly which are and which
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are not is currently (March 1994) at the Telco's discretion.
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The Federal Government has stated that it is their intent that
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nationwide CNID be available by mid-1995. The full text of this
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decision may be found FCC Report No. DC-2571 issued on March 8,
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1994.
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The biggest effect of the ruling is to mandate transport of CPN
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(customer provided number) information between interconnecting
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networks eliminating the effective inter-LATA-only limitation
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that exists today in most areas.
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Currently there are two types of Caller-ID. The first (often
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referred to as "basic" service) just returns the calling number
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or an error message and the date/time of the call.
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The second ("enhanced" Caller-ID) also may return the directory
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information about the calling number. At a minimum, the name of
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the subscriber is returned (the subscriber is not the same as the
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caller, the phone company has no way to determine who is actually
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on the line).
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4) How is the Caller-ID information provided ?
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As a 1200 baud, 7 data bits, 1 stop bit data stream usually
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transmitted following the first and before the second ring signal
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on the line. Note that this is not a standard Bell 212 or CCITT
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v22 data format so a standard modem will probably not be able to
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receive it. Further, the serial information exists as such only
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from the recipient's switch to the callee's location. Between
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carriers the signal exists as data packets.
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The signal is provided before the circuit is complete: picking up
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the receiver before the data stream is finished will stop/corrupt
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the transmission.
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Currently there are two types of information returned: a "short
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form" which contains the date/time (telco and not local) of the
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call and the calling number or error message. The "long form"
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will also contain the name and possibly the address (directory
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information) of the calling phone.
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The "short form" stream consists of a set of null values,
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followed by a two byte prefix, followed by the DATE (Month/Day),
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TIME (24 hour format), and number including area code in ASCII,
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followed by a 2s compliment checksum. Most modems/caller id
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devices will format the data but the raw stream looks like this :
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0412303232383134333434303735353537373737xx
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or (prefix)02281334407555777(checksum)
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A formatted output would look like this:
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Date - Feb 28
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Time - 1:34 pm
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Number - (407)555-7777
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5) Can a Caller-ID signal be forged/altered ?
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Since the signal is provided by the local Telco switch and the
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calling party's line is not connected until after the phone is
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answered, generally the signal cannot be altered from the distant
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end. Manipulation would have to take place either at the switch
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or on the called party's line.
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However, the foregoing applies only to a properly designed CNID
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unit. For instance the Motorola M145447 chip has a "power down"
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option that wakes the Chip up when the phone rings for just long
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enough to receive, process, and deliver the CNID signal after
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which it shuts down until the next call.
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Should this option be disabled, the chip will be in a "listen
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always" state and it is theoretically possible to "flood" a line
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making a vulnerable box record successive erroneous numbers.
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I have received a report of a device called "Presto Chango" that
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can transmit an extra ADSI modem tone after the call has been
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picked up that will cause a susceptible box to display the later
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information. It was also reported to me that CNID boxes marketed
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by US-West as their brand and made by CIDCO have been used to
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demonstrate the "Presto Chango" box.
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6) What is "ID Blocking" ?
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Most Telco's providing Caller-ID have been required to also
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provide the ability for a calling party to suppress the Caller-ID
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signal. Generally this is done by pressing star-six-seven before
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making the call. In most cases this will block the next call only
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however some Telcos have decided to implement this in a
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bewildering array of methods. The best answer is to contact the
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service provider and get an answer in writing.
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Currently this is supplied as either by-call or by-line blocking.
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By-Call is preferred since the caller must consciously block the
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transmission on each call. By-Line blocking as currently
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implemented has the disadvantage that the caller, without having
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a second caller-id equipped line to use for checking, has no way
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of knowing if the last star-six-seven toggled blocking on or off.
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Note that blocking is provided by a "privacy" bit that is
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transmitted along with the CNID information and so is still
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available to the Telco switch, just not to the subscriber as a
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CNID signal. Consequently related services such as call trace,
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call return, & call block may still work.
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7) What happens if a call is forwarded ?
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Generally, the number reported is that of the last phone to
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forward the call. Again there are some Telco differences so use
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the same precaution as in (6). If the forwarding is done by
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customer owned equipment there is no way of telling but will
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probably be the last calling number.
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Note that as specified, CNID is *supposed* to return the number
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of the originating caller but this is at the mercy of all
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forwarding devices, some of which may not be compliant.
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8) What happens if I have two phone lines and a black box to do
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the forwarding ?
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If you have two phone lines or use a PBX with outdialing
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features, the reported number will be that of the last line to
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dial. Currently there is no way to tell a black box from a human
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holding two handsets together.
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9) I called somebody from a company phone (555-1234) but their
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Caller-ID device reported 555-1000.
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Often a company with multiple trunks from the Telco and their own
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switch will report a generic number for all of the trunks.
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There is a defined protocol for PBXs to pass true CNID
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information on outgoing lines but it will be a long time before
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all existing COT (Customer Owned Telephone) equipment is upgraded
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to meet this standard unless they have a reason to do so.
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10) I run a BBS. How can I use Caller-ID to authenticate/log
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callers ?
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There are two ways. The first utilizes a separate Caller-ID box
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with a serial cable or an internal card. This sends the
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information back to a PC which can then decide whether to answer
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the phone and what device should respond. Some of these are
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available which can handle multiple phone lines per card and
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multiple cards per PC.
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The second (and most common) is for the capability to be built in
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a modem or FAX/modem. While limited to a single line per modem,
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the information can be transmitted through the normal COM port to
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a program that again can decide whether or not to answer the
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phone and how. There is a FreeWare Caller-ID ASP script for
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Procomm Plus v2.x available for FTP from the Telecom archive.
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Most such software packages will also log each call as it is
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received and the action taken.
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Of course for true wizards, there are chips available (one of the
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first was the Motorola MC145447) that can recognize the CNID
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signal and transform it into a proper RS-232 (serial) signal.
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11) How is security enhanced by using Caller-ID over a Call-Back
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service or one-time-passwords for dial-up access ?
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Caller-ID has one great advantage over any other mechanism for
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telephone lines. It allows the customer to decide *before*
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picking up the receiver, whether to answer the call.
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Consider hackers, crackers, and phreaks. Their goal in life is to
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forcibly penetrate electronic systems without permission (sounds
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like rape doesn't it ?). They employ demon dialers and "finger
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hacking" to discover responsive numbers, often checking every
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number in a 10,000 number exchange.
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If they get a response such as a modem tone, they have a target
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and will often spend days or weeks trying every possible
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combination of codes to get in. With Caller-ID answer selection,
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the miscreant will never get to the modem tone in the first
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place, yet for an authorized number, the tone will appear on the
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second ring. Previously the best solution for dial-ups was to set
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the modem to answer on the sixth ring (ats0=6). Few hackers will
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wait that long but it can also irritate customers.
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12) What error messages will Caller-ID return ?
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a) "Out of Area" - (Telco) the call came from outside the Telco's
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service area and the Telco either has no available information or
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has chosen not to return what information it has.
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b) "Blocked" or "Private" - (Telco) the caller either has
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permanent call blocking enabled or has dialed star-six-seven for
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this call. You do not have to answer either.
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c) "Buffer Full" - (device manufacturer) there are many Caller-ID
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devices on the market and exactly how they have chosen to
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implement storage is up to the manufacturer. This probably mans
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that the divide has a limited buffer space and the device is
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either losing the earliest call records or has stopped recording
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new calls.
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d) "Data Error" or "Data Error #x" - (device manufacturer)
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signal was received that was substandard in some way or for which
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the checksum did not match the contents.
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e) "No Data Sent" - (device manufacturer) Signal was received
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consisting entirely of nulls or with missing information but a
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proper checksum.
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13) Why are so many people against Caller-ID ?
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FUD - Fear, Uncertainty, & Doubt or 10,000,000 lemmings can't be
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wrong. There were some justifiable concerns that some people
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(battered wives, undercover policemen) might be endangered or
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subject to harassment (doctors, lawyers, celebrities) by
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Caller-ID. As mentioned above there are several legitimate ways
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to either block Caller-ID or to have it return a different
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number. It is up to the caller. The advantage is that with
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Caller-ID, for the first time, the called party has the same
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"right of refusal".
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Expect yet another Telco service (at a slight additional charge)
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to be offered to return an office number for calls made from
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home. Crisis centers could return the number of the local police
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station.
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Compiled by Padgett Peterson. Constructive comments to:
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padgett@tccslr.dnet.mmc.com Brickbats >nul.
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Thanks for additional material to:
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David J. Kovan
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Robert Krten
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John Levine
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David G. Lewis
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Karl Voss
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but the mistakes are all mine - Padgett (Ignorance is curable)
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___ Blue Wave/QWK v2.12
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