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- Some Calling Number Display technical issues relating to privacy,
- usage and security
-
- Robin Whittle rw@firstpr.com.au
-
- http://www.ozemail.com.au/~firstpr/cnd/
-
- Written quickly on 26 November 1997 - as we await the finally crunch
- in which Telstra should admit, or be forced to admit, that it has
- not met the PAC requirements for public awareness/understanding.
-
- Please let me know of your suggestions for improving this!
-
-
-
- Here are a variety of technical aspects of CND which are not crucial
- to the immediate debate in the next few days about its activation,
- but of general interest in terms of privacy.
-
- This is a minimal technical description - so that non-technical
- readers can read over it and see where the shortcomings, security
- (hackablitity) and other issues arise.
-
- Points mentioned below include:
-
- * CND is not displayable on analogue PABXs.
-
- * CND initially will not display in a Call Waiting scenario.
-
- * Call Waiting CND may not be reliable with more than one box on the
- line.
-
- * Calling Name Display - how Telstra will not be sending customer
- names (at least at first) but will use it to display "Payphone"
- and "Mobile" instead of a number.
-
- * Not all boxes handle the Calling Number messages.
-
- * CND boxes which can dial the stored number typically do so without
- a prefix, such as for CND display / don't display.
-
- * CND displayable numbers from calls originating from PABXs and
- hunt-groups (business phone installations) may bear no resemblance
- to the actual caller's number and may not even lead to a number
- which will be answered.
-
- * Taxonomy of the various networks and what they are doing with CND.
-
- * Taxonomy of the various forms of connection to the Telstra
- analogue network and what they are doing with sending and
- displaying CND. Good privacy news here - analogue PABXs can't
- display CND.
-
-
- Its a complete dog's breakfast, and in order to understand the human
- implications of this, some technical explanations are necessary.
-
- You may well ask how this was planned. I am sure it wasn't. I have
- had a lot to do with various aspects of Telstra over the years, so
- its not an idle comment. Telstra is not so much an organisation as
- a seething mass of humanity. Optus isn't far behind. Calling
- Number Display is a can of worms from every perspective, not just
- privacy and its impact on telephone usage. "Making life easier"
- indeed!
-
-
- Displaying CND on an analogue POTS line
- =======================================
-
- Within the telco industry, the main network of telephone exchanges
- which connect to customers via analogue lines, is sometimes referred
- to the Public Switched Telephone Network (PSTN). I won't use that
- term, but keep in mind if someone is drawing distinctions between
- CND in the PSTN and, for instance, GSM or ISDN.
-
- The analogue fixed line type of CND display and calling is the most
- significant, because of the majority of phone services are of this
- type, but it is only one part of the bigger picture. For instance
- there are quite a few ways PABXs interface with the network - and
- the current analogue CND service does not allow display on these
- analogue PABXs.
-
- Looking solely at ordinary POTS (Plain Old Telephone Service)
- services, here are the technical issues. I have today obtained from
- the Telstra Tech Standards people (who I am old mates with in some
- respects at least - having fought side to side with them regarding
- the 1997 Telco Bill which was very nearly disastrous in terms of
- technical standards) their documents on analogue Calling Number
- Display.
-
- The CND aspect of telecommunications is not covered by mandatory
- technical standards, post July 97. Any device which connects to a
- carrier's network has to conform to certain basic safety, network
- integrity and electromagnetic compatibility standards (TS 002, TS
- 004 and the EMC Framework). There are no mandatory standards for
- Customer Equipment which displays CND. Prior to July 1997, this was
- a mandatory area, and consequently, AUSTEL WG 2/1 worked hard to
- produce a standard - AUSTEL TS 030 - which is still available from
- Standards Australia. TS 030 has no legal force any more. I was
- part of WG 2/1 for this and other work.
-
- TS 030 was an Australian adaptation of the so-called US Bellcore
- standards, with a nod to the European ETSI standard. Adaptation was
- required since these documents were incomplete, technically
- inadequate and/or unsuitable for Australian ring timings and voltage
- and impedance standards.
-
- Now Telstra has produced a series of its own technical documents,
- which constitute guidance on making CND display units - or the CND
- display function within a telephone or other CE (Customer
- Equipment). Email Merv Sewell at <MSewell@vcrpcsd1.telstra.com.au>
- and he will be able to email you a set of Word documents - which I
- think are well written and informative.
-
-
- It seems that Telstra cannot stop the sale of CND boxes which fail
- to meet its technical requirements. Any CND or other CE which fails
- to meet TS 002, TS 004 or the EMC Framework, can be sold - but only
- with a "Non-Complying" sticker - which I think is the old AUSTEL
- triangle with a circle and a cross through it.
-
- It seems that CND boxes manufactured for the US market, and produced
- for $10 in Taiwan will be sold here en-masse and that many of them
- they will probably work OK. Buyer beware! Best to check with
- Telstra before purchasing any particular box.
-
- Also, some US modems are capable of receiving CND modem tones - so
- these are already around, and could be hooked up to a phone line
- (not to answer the call) to get the number, and feed it to a
- database or reverse directory program while the phone is still
- ringing.
-
-
- POTS Calling Number and Name Display, and Message Waiting
- =========================================================
-
- This discussion relates to the idea of a separate CND box, but it
- could also be a function within a modem or most likely a CND
- function within a phone or perhaps within a "key system" - a multi-
- line "Commander" like phone system (which is not a PABX).
-
- The boxes perform three functions:
-
- A - Displaying a number or a reason for the number not being
- displayed.
-
- B - Displaying a name or a reason for the number not being
- displayed.
-
- For both the above, time and date information is usually attached.
- Therefore the times and date come from the telephone line, not an
- internal clock in the CND display box.
-
-
- C - Displaying a "Visual Message Wait Indication" LED or similar.
-
-
- (CND boxes may also have a facility for dialling the numbers stored
- from previous incoming calls. In most cases these apparently do not
- allow any prefixes to be added. Of course this relies on the number
- being sent to the box in a form suitable for dialling - with STD
- codes where needed etc. That should be OK. However due to issues
- discussed below, related to calls from PABXs and hunt-groups, the
- number displayed is not necessarily one that will be a valid one to
- ring, or to reach the original caller on.)
-
-
- Calling Number
- --------------
-
- In A above, there are various options for the number being sent by
- the network to the phone - with or without STD prefixes. These are
- carried in the "Calling Number" field of the message the CND box
- gets from the network.
-
- If no number is sent, then there are two options for why it is not.
- These are carried in a separate field called "Reason for Absence of
- Number":
-
- O = Out of area. Telstra recommends the box display: "Unavailable"
-
- This would be when the call comes from somewhere that was not
- capable of providing the number, or was not capable of
- providing a valid CND display/don't-display bit.
-
- What the box actually displays could be anything, such as
- "Out of Area" or who knows what else.
-
- P = Private. Telstra recommends the box display: "Private".
-
- This is when the local exchange knows the caller's number
- but the CND display flag is set to "don't display".
-
- What the box displays could be anything, but it is quite
- likely to be "Private". Some in the UK apparently show:
- "Blocked" - which I consider to be prejudicial to privacy.
-
- (CND in the UK and in European countries is technically
- somewhat different from the US model we are following, so
- its unlikely we will get these boxes here.)
-
- However, see below on numbers from payphones and mobiles which do
- not have displayable numbers.
-
-
- Name Display
- ------------
-
- Calling Name display is not a function that Telstra is planning on
- implementing as far as I know. However, they are going to be
- sending "Name" messages to convey the text "Payphone" and "Mobile".
-
- The Name text is in its own field within the message and can be
- arbitrary ASCII text - upper and lower case, up to 15 perhaps (which
- isn't very many) but perhaps longer in the future. This 15 is a
- limitation stated by Telstra. How long a name the display box can
- store and display is another matter.
-
- As with numbers, there can be all sorts of messy things with boxes
- not remembering/displaying the full set of longer numbers or names.
-
- If a name is not sent, then a separate field carries the "Reason for
- Absence of Name, which can be "O", or "P" with the same meanings as
- listed above for Reason for Absence of Number.
-
- If there was a name service, how Telstra would assign the names, and
- compress them into 15 characters, is anyone's guess. I understand
- that in entrepreneurial Hong Kong, hundreds of thousands of dollars
- have been raised by *auctioning* the popular names!
-
- Both name and number can be sent in the one CND message - how the
- box displays this is another matter.
-
- The two uses of the Name function are to indicate that there is no
- number because the call is from a payphone or from an (analogue)
- mobile phone. In the latter case, since the analogue mobiles can't
- handle prefixes, they are never going to make calls with displayable
- numbers. In the former case, Telstra does not want to reveal the
- phone numbers of its payphones. However payphones which are not
- Telstra's (eg. those in hotels etc.) are just ordinary phones as far
- as Telstra is concerned, and so will be making calls with
- displayable numbers (unless their owners opt-out per line, or the
- caller dials the 1831 prefix).
-
- "Payphone" and "Mobile" are the two "Calling Names" which Telstra's
- exchanges will send to the CND box, in lieu of numbers and (I think)
- in lieu of using "O" or "P" for "Reason for Absence of Calling
- Number".
-
- The boxes should display and store this text verbatim, so as long as
- the box handles names, then this will show up. I think there are
- quite a few boxes from the US which don't handle names - so they may
- display nothing when they receive a CND message with no number, no
- reason for absence of number and only a "name".
-
-
- The "Visual Message Wait Indication" - VMWI - has nothing whatsoever
- to do with Calling Number Display, or with any phone call.
-
- It is a means by which the box can have an LED or a section of its
- LCD (Liquid Crystal Display) to indicate that the customer has one
- or more messages waiting in their voice mail system. The customer's
- voice mail is handled by some special equipment attached to the
- exchange. The customer does not want to call it every half hour to
- see if there is a voice message for them (for instance one could
- have arrived when they were out and did not answer the phone, or
- while they were on the phone). One approach is to provide a
- stuttering dial-tone to tell the customer there is a message waiting
- for them. Another is to provide that stutter dial-tone and have a
- box automatically pick up the line periodically, analyse the sound
- of the dial-tone and turn a LED on or off accordingly - messy!
-
- This VMWI works by the exchange sending a specific message,
- unlrelated to CND, to the box while the phone line is idle - ie. "On
- Hook". This is done without any ringing of the bell. The message
- simply tells the box to turn its VMWI indicator on or off. VMWI has
- no privacy implications I can think of.
-
-
- Modes 1, 2, 3 and 4
- ===================
-
- Now . . . . there are four potential modes in which all the above
- can happen, only two of which are of interest:
-
-
- Mode 1 After the first ringing signal and before the second.
-
- Mode 2 Just before the first ringing signal.
-
- Mode 3 With the phone on-hook, and without ringing the bell and
- therefore without any association with any call.
-
- Mode 4 While the phone is in use - "Off-Hook". (Call waiting!)
-
-
- Mode 1 is the main CND mode. This is used for both Name and Number
- display.
-
- Mode 2 is not going to be generated by Telstra's exchanges.
-
- Mode 3 is only going to be used for VMWI - and that is something for
- the future.
-
- Mode 4 is for providing Name and/or Number - but only in a call
- waiting scenario.
-
- Now looking at Modes 1 and 4 in more detail.
-
-
- Mode 1
- ------
-
- In Mode 1, the line is controlled by the exchange - this is before
- the phone is picked up and before the caller can send sound to the
- receiving line. Therefore this information is unhackable by the
- caller (except in that they may do things to their ISDN PABX
- configuration to cause some screwy number to be sent as the calling
- number).
-
- The normal ring cadence in Australia is:
-
- 0.4 sec ON
- 0.2 sec OFF
- 0.4 sec ON
- 2.0 sec OFF
-
-
- With CND in Mode 1 the phone rings once, for between 0.2 and 0.65
- seconds, then there is a period of a few seconds (5.5 absolute max,
- but more likely 2 or 3 seconds) when various modem tones are sent by
- the exchange to the CND box(es), then the normal ring-ring ringing
- cadence continues. This can be combined with the distinctive ring
- cadences with different timings - which are part of some Telstra
- services regarding calls from particular numbers, or having an
- automatic switch between a phone and a fax machine on the one
- physical line, but each with a different phone number.
-
- In that 2 or 3 second gap, there is a silent period, some
- synchronisation tones, then the 1200 baud (Bell 202) tones which
- carry the CND message. (These modem tones are pretty simple, just
- 1200 Hz for a One and 2200 Hz for a Zero.)
-
- The time depends somewhat on the length of the message. Then there
- is a silent period before the 90 volt ringing signals start again.
-
- If someone picks up the phone while the CND modem tones are being
- received, this should cause the CND box to discard this message.
- Therefore customers wanting to use CND have to let the phone ring
- once, wait, and not pick it up until it starts to ring a second
- time.
-
- The format of the CND message is in the technical doco from Telstra
- - and is the same as the Bellcore US spec as far as I can tell.
- There is a single 8 bit checksum at the end, as a means of detecting
- most, but not all, erroneous reception - so in most cases of
- glitches, the box should see the bad checksum and discard the
- message. (A checksum is the last byte sent - and is equal to the
- sum of all previous bytes. The receiver adds all the bytes it
- receives and compares it to what it interprets as the checksum. If
- there is a difference, then there has definitely been an error -
- however some errors could not be detected in this way, because the
- byte seen as the checksum just might happen to be the value which
- agrees with the total of the erroneous preceding bytes.)
-
- The CND box(es) do *not* send signals back to the exchange in Mode
- 1. If there is a glitch which disrupts the reception of the message,
- then there is no means for resending the message.
-
-
- Mode 4 - Call Waiting
- ---------------------
-
- The above is a full description of the main CND service - but not of
- CND in a Call Waiting scenario. The system should be quite
- reliable, assuming the boxes are sensitive enough. There are a few
- technical issues with having a dozen or so boxes on the line - for
- instance the current they may draw or other potentially nasty
- problems, but in general these do not present operational problems
- unless the box is truly sub-standard (which is quite possible, and
- entirely the customer's responsibility - although they probably have
- no way of knowing this).
-
- Now, the matter of CND in a Call Waiting situation. Telstra does
- not plan to introduce this immediately - I was told mid 1998
- perhaps. However it raises some real operational and "security"
- issues. The fact that the box can receive Mode 4 (and not all will
- be able to do so, unless they really do meet Telstra's spec) means
- that a malicious caller can put whatever messages they like into the
- box, under certain conditions. Those messages would be
- indistinguishable to the customer, unless perhaps they instructed
- him or her to call the Prime Minister, God or the name contained a
- threat or an insult.
-
- In ordinary operation, with a single CND box, there is no problem. A
- call in progress (either a received call or an outgoing call) is
- interrupted by an incoming call. The customer puts the first call
- on hold, a tone is sent by the exchange to tell the customer that
- there is about to be a CND message. Then another tone is sent to
- tell the CND box that a CND message is about to arrive. At this
- point, the "box" should cut off the audio path to and from the
- customer's hand-set - to stop them hearing the warblng modem tones,
- and most importantly to stop noise from their microphone being mixed
- with the modem tones and therefore disrupting the CND box's
- reception of them.
-
- When the CND box is part of the phone, or key-system, that's easy to
- achieve. Its not easy if it is a standalone box. This could be
- achieved by plugging the phone into a switched outlet of the CND
- box. Without this switching, customers should remain quiet during
- the modem tones, and probably would hold the earpiece away from
- their ear, since they could be reasonably loud.
-
- (Normally corrupted reception would be detected by the CND box
- recognising the received checksum is different from the checksum it
- calculated from the rest of the message it received. However, since
- it is only an 8 bit checksum, with 256 possible values, in a
- fraction of a percent of cases corrupt reception would not be
- detected - so a garbled number and/or name might be displayed.)
-
- After the CND box has cut off the audio path to and from the
- customer's handset, it should send a tone back to the exchange to
- confirm that there is a CND box ready to receive the message. This
- is the "Acknowledge" signal - a brief (0.06 second) burst of DTMF
- tone dial tone - either for the "A" or the "D" digits (which are a
- part of DTMF, but not normally buttons on phones). Which one to
- send depends on whether the CND box is an "ADSI" compatible box or
- not. Now what's ADSI???? I think I knew, once - its something to
- do with a whole raft of other functions for displaying arbitrary
- messages and doing other complex user interface things - none of
- which have anything to do with Telstra's plans for CND.
-
- If the exchange gets this Acknowledge tone, then it sends the silent
- period, synchronisation tones, modem tones and silent period as for
- Mode 1 - with the same message format for Number and/or Name, with
- reasons for absence of either or both, and with the time and date.
-
- If the exchange does not receive an Acknowledge signal, then it
- doesn't send the CND message and I think it would carry on with the
- normal call-waiting procedures for switching in the new call. (The
- CND box's audio muting would time out.)
-
-
- There are two serious operational / security issues here.
-
-
- Problems with multiple CND boxes in Mode 4
- ------------------------------------------
-
- Firstly, it cannot be guaranteed that if the customer has two or
- more CND boxes on the one line, that the exchange will recognise the
- Acknowledge signals they both generate. It is conceivable that the
- pairs of sine-waves which make up each DTMF tone pair, are actually
- out of phase on one box, with respect to the other. This means the
- completely or partially cancel out and therefore would not be
- recognised by the exchange, causing it to abort from the CND message
- sending procedure. This could be work OK most of the time, and fail
- a few percent or the time.
-
- As the internal timings of the boxes change, they may work together
- or null each other out. Having 3 or more boxes probably reduces the
- chances of complete cancellation - but it would still be unreliable.
- Also, what if some boxes are "ADSI" and some aren't? Some would be
- sending the "A" tone pair (697 and 1633 Hz) and some would be
- sending the "D" tone pair (941 Hz and 1633 Hz). If this caused the
- exchange to not recognise one or the other, then the CND message
- would not be sent. Customers who bought such boxes and plugged them
- into the line would be responsible for Telstra's costs in
- investigating the problem - and if I am only vaguely aware of what
- ADSI is, how is the average consumer going to know to buy CND boxes
- of the same ADSI type???
-
-
- Secondly, while Mode 1 cannot be hacked (it only happens after
- ringing voltages - 90 volts AC which only the exchange can generate)
- this Mode 4 CND message system works purely with audio tones which
- ostensibly come from the exchange. In fact, there is no way the box
- or the customer can be sure that the tones are coming from the
- exchange (and therefore contain a real CND message) - they could be
- generated by the other party to the call (for instance a hacker).
- Therefore, a hacker (perhaps with a modem or some PC software I
- could cook up in a few hours) could generate modem tones which the
- CND box recognises as one or more valid messages.
-
- These messages could contain any number, any name (including rather
- long messages), any time and date and potentially any other things
- the box was capable of receiving.
-
- Multiple messages could be sent - one every few seconds. This could
- overflow the CND box's memory - removing records of all recent
- genuine calls.
-
- This is probably not a problem in a real call-waiting situation
- because the customer would realise that one or more spurious
- messages arrived at that time. The danger is when people have a
- Mode 4 capable CND box on their line, with an answering machine.
- The hacker can call the number, wait for the answering machine to
- answer, send the tones and leave arbitrary messages. These could
- take place after the machine has stopped recording. I have heard of
- such hacking of CND boxes in the US a few years ago. I can't
- remember the details, but people got misleading and offensive
- messages in the names and false numbers, times and dates when they
- came home and checked their CND boxes.
-
-
- Taxonomy of networks
- ====================
-
- This is a rough guide to the various networks which make up
- Australia's phone system. Its not complete, nor authoritative, but
- its a guide to the ensuing CND shemozzle.
-
- Ultimately, all these networks (they are all made with modern
- digital exchanges and they all communicate with CCS7) will be
- connected properly, and each carrier will trust both the CND number
- (name later?) time and date and most importantly the validity of the
- "Display the Number" flag in every CCS7 call setup message the get
- from other carriers' networks. These messages, with the CND number
- and "Display the Number" flag have been part of CCS7 since its
- inception at least 8 year ago.
-
-
-
- Analogue - POTS, key-systems and PABXs
- --------------------------------------
-
- Telstra's analogue, fixed line, phone network - for POTS phones
- services and for a variety of PABX connections, plus a PABX
- connection called Siteline.
-
- CND Display is generally only available on POTS phones and
- key-systems (Commander etc.) - as described below in the taxonomy
- of analogue phone systems.
-
- Phone services of all kinds, POTS, key-systems, PABXs etc. will
- be generating calls with displayable numbers, unless the line is
- "silent" (and only residential lines are "silent" - no business
- line, including unlisted business numbers and extra residential
- lines for fax etc, are regarded as "silent") or unless the
- customer has opted out per-line or per-call.
-
- In the case of PABXs, and various other multi-line commercial
- services, there could be differences between the displayed number
- and the number of the caller. More on this below.
-
-
- Telstra's Analogue Mobile network
- ---------------------------------
-
- This carries both Telstra and Optus analogue mobile calls.
-
- Analogue mobile phones are incapable of displaying CND and since
- they are apparently (I don't know why) incapable of handling
- display / don't display prefixes, it has long been decided that
- calls from analogue mobiles will not be recognised has having
- a valid "display / don't display" flag in the messages sent
- between exchanges, and that therefore their number will never
- be displayed on a CND box. When an analogue mobile caller calls
- a Telstra POTS CND service, Telstra will probably send the text
- "Mobile" in the Name field of the CND message - and no number.
-
- What happens when an analogue mobile calls a GSM (digital) mobile
- or an ISDN service . . . who knows? Probably no displayable
- number and an "O" flag in the reason for no number field, which
- may result in a displayed message such as "Out of Area". Or maybe
- the name "Mobile" will be sent.
-
-
- Telstra's GSM Mobile network
- ----------------------------
-
- As far as I know, this will display and send numbers within
- itself, and according to Telstra's plan, to and from the
- analogue fixed network and the ISDN networks.
-
- Over time, when Optus and Vodafone improve the way in which
- their networks interface between Telstra's there will be
- interworking between these and all the other networks.
-
- What information and opt-out procedure has Telstra given
- its GSM customers?
-
- I presume the GSM standards (apparently they occupy at least
- a metre of shelf space) cope with Calling Name display as well.
-
- I haven't head of proposals to actually start displaying callers
- names, but who knows. I understand that some GSM phones can be
- configured by the user to associate particular numbers with the
- names of people - names they key in. So particular caller
- numbers might cause these names to be displayed instead.
-
-
-
- Telstra's *Overlay* ISDN network
- --------------------------------
-
- ISDN is a digital way of connecting to the exchanges. The
- exchanges carry phone calls in 64 kbit / second pipes - within
- the exchange and between the exchanges. The exchanges also
- talk to each other, to set-up and tear-down calls, using a
- protocol and packet switched network (a little like the Internet,
- but just for phone exchanges) called the CCS7 network.
-
- An analogue phone service carries audio signals, ringing and
- dialling signals to and from the customers site and the exchange
- via analogue voltage variations on a single pair of wires.
-
- ISDN, by one means or another, provides two or more 64 kbit
- channels between the customer and the exchange, plus another
- digital channel to the exchange to carry commands, related to
- CCS7, for setting up and managing calls.
-
- For instance one way of connecting to ISDN is "Basic Rate ISDN"
- (BR-ISDN). This works over a standard pair of phone wires - if
- they are in good shape and less than a few km long. It provides
- two 64 k channels, plus a 16 kbps signalling channel for sending
- and receiving information to set up calls, and to be notified of
- incoming calls. Its very flexible, and each 64 kbit/sec channel
- can handle a voice call or a "data" call.
-
- The main advantage is that data calls between ISDN services
- are the full 64 kbit sec - with special computer adaptors, rather
- than modems, which are limited to 33.6 kbps.
-
- ISDN services are generally more expensive and even local calls
- are timed. They messages in the control channel (16 kbps in BR-
- ISDN) can send commands to allow or disallow the display of the
- caller's number. They also receive the CND messages.
-
- You can't plug analogue phones into an ISDN service - but special
- ISDN phones, or more likely computer interface cards or PABXs do
- plug straight in, and these typically have CND software or
- hardware built in.
-
- A second form of ISDN connection is "Primary Rate ISDN" - 10, 20
- or typically 30 channels of 64 kbit/sec plus a control channel
- for managing all those calls. The same CND messages travel on
- this control channel as for BR-ISDN. PR-ISDN is typically used
- to connect to larger PABXs in banks, government departments or
- for instance, the TIO. PR-ISDN is typically delivered via
- two optical fibres.
-
- The PABX connects to handset on the desks of users. They
- typically do "CND" within the organisation, and some handsets
- may be configured to display numbers from outside.
-
- Telstra's "Overlay" network has been operational since about
- 1990. It consists of a number of Ericsson AXE exchanges, and
- various data links and "B-Muxes" to connect BR-ISDN customers
- in areas far removed from the relatively small number of
- "Overlay" exchanges.
-
- These Overlay ISDN exchanges, have always (or almost always)
- supported CND on an opt out basis. Its called "CLIP" (Calling
- Line Identification Presentation) and the per-call control of
- whether the caller's number is displayed or not is called CLIR
- (Calling Line Identification Restriction). CLIR is best done
- by the handset or PABX sending special messages to Telstra's
- ISDN exchange, but for reasons related to the special technical
- "standards" used in the overlay network, this is not usually
- practical. I can't remember what the situation is with prefixes
- - I have the guff somewhere - but I think it is messy too.
-
- I think there are not separate "display" and "don't display"
- prefixes - but rather the one prefix which reverses whatever
- the current status of the line is. This is damn stupid
- programming on the part of Ericsson, but the price of a software
- rewrite to change even something as seemingly trivial as this is
- apparently prohibitive.
-
- One way or another, most Overlay ISDN customers are generally
- sending their number, and if equipped, displaying it - but
- only within the Overlay network. I am not sure if they
- pay for it or not - its in the price list, but I am not sure
- if it is in fact charged for. "Line-block" is technically
- possible, but I don't know how often it is used.
-
- This has been going on for years - and callers are often
- surprised when the ISDN customer they call knows their number.
- This is because there are not a lot of display handsets - except
- perhaps in the higher offices of the companies.
-
- This happened well before the AUSTEL Privacy Inquiry started -
- and AUSTEL turned a blind eye to it because it was already there
- and because it was "only" business customers.
-
- I don't accept that the privacy of businesses or of individuals
- in their workplace should be given less consideration than that
- of "residential" users - who are the same people, just at work.
-
- There have been "glitches" in the past where a call from a
- normal POTS service caused a number to be displayed on a
- Overlay ISDN connected PABX. I know one definite case of this
- occurring ca. 1992. More recently, I heard a similar thing
- happened with a call to the TIO.
-
- No doubt, Telstra is preparing to display the number of POTS,
- GSM and On-Ramp ISDN callers on the Overlay ISDN network. It may
- already be happening. What proportion of people on these PABXs
- have CND capable handsets, I don't know - but it is bound to
- grow.
-
-
- Telstra's "On-Ramp" ISDN "network"
- ----------------------------------
-
- Much as Telstra would like to pretend otherwise, On-Ramp ISDN
- is not a separate network. It was activated early in 1997.
-
- On-Ramp ISDN is BR-ISDN and PR-ISDN services but not with the
- small set of "Overlay" exchanges. On-Ramp ISDN conforms to the
- modern European ETSI standards and is provided from the
- main Telstra network of Ericsson AXE and Alcatel System 12
- exchanges which now comprise its main network. These are the
- exchanges which provide all the analogue services - so On-Ramp
- ISDN simply means a digital link to the exchange from the
- customers site, rather than analogue POTS or PABX lines.
-
- Its a scandal, but earlier this year, the Privacy Commissioner's
- office bought Telstra's argument that On-Ramp was an extension
- of the existing ISDN network (I wish I was at those meetings!)
- and that because it was currently aimed at business customers
- it would be OK to do CND on an opt-out basis - *without* any
- reference to the PAC CND Report.
-
- This should never have been allowed.
-
- On-Ramp ISDN is intended to be a mass market product, although it
- is a bit pricey now. OnRamp can be used to give 64 kbps access
- to an Internet Service Provider while another channel can still
- take ordinary phone calls - all with the existing wiring to
- the home. A family with Internet-keen members would find this
- most attractive.
-
- Telstra has agreed (to the Privacy Commissioner's Office I think
- and to me in an article for Australian Communications in
- June 1997) that when On-Ramp is aimed at residential customers
- then whatever guidelines apply for analogue POTS customers will
- also apply to On-Ramp.
-
- My understanding is that On-Ramp CND has interworked with
- the Overlay ISDN network since its introduction.
-
- No doubt, Telstra is preparing to display the number of POTS,
- Overlay ISDN and GSM callers on On-Ramp ISDN services. It may
- already be happening.
-
-
-
- Telstra's DMS Centrex network
- -----------------------------
-
- In addition to the Overlay ISDN network, the main analogue / On-
- Ramp "Future Mode of Operation" exchanges (its main phone
- network) and the GSM and AMPS (analogue mobile) exchanges,
- Telstra has some Nortel DMS-100 ISDN exchanges. These have been
- around for a while, and I think they generally serve business
- customers, via ISDN links (?), analogue lines and potentially
- "Siteline" connections (discussed below).
-
- These DMS exchanges have been retained because the provide the
- "Centrex" services which can link offices all over the country
- as if it was all on one PABX - without the need for a PABX
- at all.
-
- I don't know what the CND behaviour of these DMS exchanges is -
- maybe they can't display because they have neither ordinary POTS
- analogue lines or ISDN connections (perhaps they do have ISDN
- connections). I imagine they will be by default generating
- calls with displayable numbers - presumably the software in the
- exchanges has been updated to handle the prefixes and to do
- "line-block" like the AXEs and System 12s of Telstra's main
- network.
-
- (If you think this is complex - it is actually beautifully
- elegant compared to the menagerie of 17+ different exchange
- types which Telstra was running just a few years ago -
- communicating via a mixture of analogue and digital
- signalling techniques. The Future Mode of Operation is now
- largely complete and has got rid of all the analogue exchanges
- and simplified the main network to just 200 AXE and System 12
- exchanges, the mobile exchanges, 20 System 12 inter-exchange
- exchanges, the DMS-100s and special exchanges for International
- and linking to other carriers.)
-
-
- Telstra's and Optus' International connections
- ----------------------------------------------
-
- These are separate exchanges, and I don't think that CND
- will be sent or received for international calls for
- the foreseeable future.
-
- In principle it can be with any country that uses CCS7 signalling
- - but there are technical compatibility issues, matters of
- reliability and trust about the accuracy of the information.
- Also, I expect there is some recognition of the privacy problems
- of disclosing callers numbers internationally.
-
-
-
- Optus' GSM network
- ------------------
-
- Optus, I think, has been doing CND *for free* on an opt-out
- basis for month or so - just within its GSM network.
-
- The call it "Caller ID" - a misnomer which should be resisted.
-
- They have a little form to fill at the back of their brochure:
- "I would like to opt-out of the Optus Caller ID service."
-
- This is a lot more user-friendly than Telstra's awkward
- arrangements.
-
- I think this means that your number is not sent, and nor do
- you get CND numbers displayed. (GSM phones are generally
- capable of displaying CND as a matter of course.)
-
- I haven't investigated this at all - any further info?
-
-
-
- Optus' HFC telephone network
- ----------------------------
-
- Optus has a growing number of residential (and maybe a few
- business) customers with phone lines connected to its Hybrid
- Fibre Coaxial cable network. This pioneering system is now
- working well and gaining customers. According to a report
- in the Australian 25 November 1997, they are adding 1200
- customers a week. Unfortunately, without number portability
- this means the customers cannot keep their old numbers.
-
- All customers are connected to large DMS-100 exchanges just
- for the HFC systems. These are technically capable of displaying
- CND according to the tech standards that Telstra has set
- for its service. As far as the user is concerned, its
- just an ordinary analogue phone service, although all sorts
- of digital technology stands between their socket and the
- remote exchange.
-
- I haven't investigated, but I would expect Optus to provide
- CND display here at some stage - and probably charge for it.
-
- Whether these services send calls to Optus GSM with displayable
- numbers or not, I don't know. I presume these exchanges would
- be programmed to accept the 1831/1832 and/or *31* and *31#
- prefixes.
-
-
-
- Optus' DMS business ISDN network
- --------------------------------
-
- Optus has, since its inception, a network of DMS exchanges for
- business customers. I think these generally connect with ISDN
- to PABXs in the customer's premises. I don't know, but I would
- expect these to be brought into CND displaying and receiving
- over time as all the interoperability issues are sorted out
- within Optus and with other carriers.
-
-
-
- Vodafone's GSM network
- ----------------------
-
- I heard they are doing opt-out CND within their own network -
- for free I think.
-
- Over time, I would expect this to be linked to all the other
- carriers' networks, once the carriers trust the other carriers to
- give them CCS7 call setup messages with reliable, valid CND
- "display / don't display" flags.
-
-
-
- Other carriers and Service Providers
- ------------------------------------
-
- Some phone customers use the Telstra lines and exchanges, but are
- not Telstra customers - they are the customers of another
- company. I really don't know what the technical and
- administrative arrangements are here regarding CND.
-
-
- Last but not least, is Northgate Communications - the Californian
- pay TV operator who is installing HFC in Ballarat and I think
- Geelong. They had an HFC telephony trial, which may soon be a
- service. Ultimately that would do CND in the standard analogue
- way, since the HFC telephony interface units are designed to
- replicate most or all of the standard POTS line functions.
-
-
-
-
- Freecall numbers
- ----------------
-
- I have no idea what is happening here! There are quite a few
- variations - and in some cases they have been getting the
- caller's number for years, at least as part of the billing
- system. Even with a call made with CND display disabled,
- they would still get whatever they are getting at present.
-
- What might change is their ability to get the information
- at the time of the call, rather than later on a bill.
-
-
-
- Internal services within Telstra
- --------------------------------
-
- Theoretically only 000 always gets your number, but I have
- reliable reports of wake-up call operators knowing the
- caller's number. This is irrespective of CND. Within
- Telstra, they can probably do what they like.
-
-
-
- Taxonomy of Telstra analogue services
- =====================================
-
-
- The CND situation with ordinary, analogue POTS lines (as virtually
- all home phones are) has already been described. Now its time to
- look at the variations which are used by businesses, in respect of
- their ability to display CND, and to control the sending of their
- number, and the relationship between the number which is displayed
- and the actual originating line of the call.
-
- This is necessarily incomplete, but it gives a reasonable picture of
- what is going on.
-
-
- Key-systems - "Commander"
- -------------------------
-
- A key-system is not a PABX - its just a centralised box, with
- multiple handsets, and multiple ordinary POTS analogue phone lines
- to the Telstra exchange.
-
- As such, the standard POTS CND display signals could be sent to the
- lines if the customer paid for the service. I think that existing
- key-systems would not support CND display - there are various things
- going on, and the handsets are not ordinary phones. You could
- certainly put a CND box on each of the lines, and display numbers
- there - but the key-system would need to be specially designed to
- receive CND if it was to display this on the handsets. I understand
- that some suppliers are making the necessary additions to their
- designs. Whether these can be retrofited, I don't know.
-
-
- Since a key system, with say 5 lines, may have those lines all
- configured in the exchange to take incoming calls for a "Directory"
- number, on a rotary basis (is this aka a "hunt group"?), all
- incoming calls to the key-system are generally caused by people
- calling that Directory number.
-
- Each line does however have its own telephone number. Calling that
- number will cause a call on that specific line - which the key
- system will simply see as one outside line, rather than as a call to
- a specific handset.
-
- However, since each outside line has its own number, the calls made
- *from* these lines will each have their own displayable CND number.
- This reveals the real numbers of the key-system's exchange lines -
- which many customers do not want to reveal. All they want the
- public to know is the Directory number which distributes calls to
- all lines on a rotary basis. I don't know what Telstra is going to
- do about this, but unless something is done, then people will get
- calls from one particular business, showing a range of numbers (the
- set of numbers which are the real numbers of the lines) - none of
- which are the Directory number of the business.
-
- Perhaps a few key systems will prevent the use of 1831 and 1832
- prefixes - since they may have some tricky processing of dialled
- numbers built into their software.
-
-
- Analogue PABXs
- --------------
-
- Analogue PABXs have special analogue, but non-POTS, lines to the
- exchange. Instead of the exchange "driving" the line (with 50 volts
- when the phone is on-hook), and so driving the phone (this is the
- normal arrangement of a POTS line), and the phone being picked up to
- activate the line, in an Analogue PABX line, the *PABX* drives the
- line (with 50 volts), and Telstra's exchange behaves like a phone!
- Its weird and the reasons are lost on me.
-
- One thing the exchange has to do, is tell the PABX which indial
- extension a call is meant for. So a 100 extension PABX may have 10
- exchange lines. The Telstra exchange line tells the PABX there is
- an incoming call (by "picking up" the line - rather than sending
- ringing signals). Then the Telstra exchange has to tell the PABX
- which numbered extension the call should be sent to. There are no-
- less than three ways of doing this! One, DTMF tones, is hardly
- used. Another, Multi Frequency Compelled (MFC) signalling is used
- on a few more modern analogue PABXs. Most PABXs use clunky old
- decadic pulse dialling to be told the extension number of the
- incoming call.
-
- Pulse dialling goes "clack, clack, clack" for "3" etc. So a 0 is
- ten "clacks". You can hear this sometimes - for instance when
- calling AUSTEL/ACA. You hear a few seconds delay while the Telstra
- exchange sends three series of decadic pulses (each up to ten
- pulses) to the line - you can just hear the clicking. Then when the
- PABX knows the extension number, it makes that extension ring, and
- sends an audio ringing signal to the exchange so the caller can hear
- that the phone bell is being activated,
-
- Then the PABX waits for someone to pick up the handset.
-
- This means that ordinary analogue POTS CND messages simply cannot be
- sent to any analogue PABX in Mode 1. The exchange does not generate
- the ringing signals - the PABX does. It is totally different from a
- POTS line, although it is analogue, and uses the same twisted pair
- wires.
-
- Theoretically you could send the ten to 12 CND digits via DTMF, or
- MFC - but you wouldn't want to try sending all the other things,
- like Calling Name and date and time. It would take several minutes
- to send all that information with pulse dialling!
-
- There are no plans to send CND display data to analogue PABXs. To
- do so would require radical reprogramming of the exchange, and major
- hardware and software changes to the PABXs themselves - and they are
- being superseded by ISDN PABXs.
-
-
- However, these PABXs will generally be *making* calls with
- displayable numbers. Whether or not the PABX would prevent the 1831
- or 1832 prefix would depend on the system. I think it might be
- prevented on quite a few, since they have all sorts of pre-
- programmed rules to decide when the full number has been dialled.
- They have to have this because of the screwy way they interface to
- the exchange lines. Those PABXs are likely to need software
- upgrades or re configuration to allow the 1831 and 1832 prefixes to
- be sent.
-
- Apparently these PABXs often interface to the exchange via three
- types of analogue lines:
-
- 1 - Indial lines (purely for incoming calls to the PABX).
-
- 2 - Outdial lines (purely so the PABX can make outgoing calls.)
-
- 3 - Bi directional lines - which can do both.
-
-
- Now the PABX may have a range of directory numbers assigned to it,
- and the exchange is programmed to direct any call to a number in
- this range to any one (it doesn't matter which) of the Indial lines
- (or bi directional lines) and to tell the PABX (as described above)
- which extension, corresponding to the last two or three digits in
- the dialled number it is meant to go to.
-
- However, when someone at an extension, say ext. 17, picks up their
- handset and dials to the outside world, then that call could go out
- on any of the outdial or bi-directional lines. Each such line would
- have its own number, and none of those numbers would be related to
- the extension or to the official Directory number range the PABX is
- assigned to. So the number displayed on a CND box for those calls
- is not one that is instantly recognisable to someone as a number
- belonging to that business. If that displayed number was called -
- what would happen? If it was the number of an Outdial line -
- nothing - because the PABX doesn't answer those lines. A bi
- directional line? Who knows. At best the PABX would answer it, but
- there would be no way of it going to a particular extension.
-
-
- Siteline links to PABXs
- -----------------------
-
- I have only just learned about this variation. Functionally the
- PABX is like an analogue PABX. That is, it does not connect to the
- Telstra exchange via a PR-ISDN link, with all its sophisticated call
- management messages. (ISDN is a very rich and well developed set of
- protocols).
-
- Basically Siteline is a way of carrying 30 calls, and a signalling
- system call CAS (Channel Associated Signalling) to do the same kinds
- of things described above for analogue PABX indial, outdial and bi
- directional lines - but in a somewhat cleaner and more reliable
- digital form. So CND display probably cannot be carried over such a
- link to the PABX, and even if it could, the PABX would need to be
- radically redesigned to be able to receive it. The problems of
- mismatch of CND number for outgoing calls would presumably be the
- same.
-
-
- Mismatch of displayed CND numbers
- ---------------------------------
-
- These problems with inappropriate CND numbers for calls emanating
- from key-systems and analogue PABXs are fundamental. Neither has a
- way of telling the exchange which extension the call actually came
- from - so no information about the extension could ever affect the
- displayed CND number for that call. The best thing that could
- happen would be for Telstra to program their exchanges so that the
- CND number of all calls made from the lines of a key-system, or PABX
- group or lines, was the directory number of that business.
-
- I get the impression this is the sort of thing they will have to do-
- but that it could take a few years.
-
- ISDN PABXs are capable of much smarter operation - and its worth
- noting that the PABX owner themselves can do things to control the
- displayed number. What the limits to this are, I don't know - but I
- think there is a lot of scope for accidental, or perhaps deliberate,
- mismatches between the displayable number and the real source of the
- calls.
-
- This stuff is an obvious source of confusion to all users - its been
- happening on a big scale in the USA, and they have had CND for quite
- a few years.
-
-
- Can of worms
- ============
-
- This treatise has not considered the major privacy problems, nor has
- it looked into the vastly more complex, indefinite and confusing
- activity that CND can turn an ordinary telephone call into. The
- usage impact could be significant - more calls not answered, more
- calls made because the first one was not answered . . . it goes on.
-
- Telstra will ultimately regret CND - if they haven't got to that
- point already. It is a true can of worms. The only reason they are
- pushing ahead with it is because they are not enough of an
- organisation to have actually thought through all the costs,
- complications and negative changes to telephone usage. Marketing
- greed - and hunting and herding - rules supreme. I have been
- watching this for years - and have never seen any evidence that
- anyone sat down and did a comprehensive survey of what all the
- complications would be.
-
-
-
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