Some of these changes reflect into Appendices 2 & 3, covering testing arrangements. They also involve editorial changes to subsequent pages. These consequential amendments are all included in the Amendment No. 1 package, which will be sent free of charge to registered purchasers of PTC 200 over the next two weeks.
"Stutter dial tone" is currently used by Telecom to indicate to users of our "Call Minder" and "Message Manager" services that a message is waiting. A drawback of this arrangement is that customers need to pick up the handset from time to time to listen for this tone. Each "off-hook" generates a call attempt in the local telephone exchange. The customer can gain some improvement by the use of Stutter Dial Tone (SDT) detector, which automatically checks for stutter dial tone. If it detects stutter dial tone, its light will start flashing. This is a useful reminder for those who have been away from their phone for a while. These devices are in use overseas and clause 8.1.2 (2) of PTC 200 covers the conditions under which they may initiate calls to check for messages on the Telecom network. However, these SDT detectors do not necessarily reduce the number of call attempts, as explained below.
After a call answered by the network messaging system terminates, there is typically a delay of about 15 seconds before stutter dial tone is applied. This delay can be up to 60 seconds in some circumstances. There is a delay before the SDT "listens" for the stutter dial tone and, if the timings do not match, it is possible for the SDT to miss a message indication. Also, many "Call Minder" or "Message Manager" users are likely to check for SDT themselves rather than wait for the light to come on after they have completed a call. As far as the local exchange is concerned, this would lead to two perceived call attempts, rather than one.
Similar timing issues occur when the user checks voice mail messages after using a phone on the same line. About 15 seconds (possibly up to 60 seconds) elapses before the stutter dial tone is removed, so the SDT detector has to wait before checking for removal of this tone. In the meantime, the light is still flashing. SDT detectors also fall down where the messages have been checked remotely (or remotely created directly into the mail box), as the light will not change status until a phone used on the customer's own line triggers the unit to check for SDT. As a result of these problems, SDT detectors do not always provide a totally accurate indication of whether or not a message is waiting.
Our current PTC requirements are simpler, but similar in principle to those set by the FCC which, by means of a specific waiver to their Regulations, allowed SDT detectors to be connected to US public networks in 1995. Previously, FCC Regulations did not permit any CPE to go off-hook for other than call initiation, so such devices were only connected behind PABXs.
Like Telecom, the FCC were concerned that such devices should not load up an exchange by going off-hook an excessive number of times. Because each such operation is seen by the exchange as a call attempt, widespread use of these devices would load up the exchange processors.
Readers may be interested to note the conditions imposed under the FCC waiver, as these give some useful clues as to what might be termed "good design practice". They also illustrate some of the complexities, as there are 8 conditions in addition to general CPE connection requirements, whereby the SDT detector:-
- is not to perform periodic testing for stutter dial tone;
- makes an off-hook stutter dial tone check no more than once after a subscriber completes a call completing the check no earlier than 4 seconds and no later than 30 seconds after the subscriber hangs up (currently 20 - 30 seconds for Telecom, but this might have to be extended);
- makes no more than one off-hook stutter dial tone check after an unanswered incoming call, and does so within 4 minutes after the call;
- performs no off-hook stutter dial tone checks after an unanswered incoming call if the visual message waiting indicator is already lit;
- takes the line off-hook for no more than 2.1 seconds per stutter dial tone check (up to 5 seconds under PTC 200);
- synchronises off-hook checks when multiple stutter dial tone detection and visual signalling devices are attached to the same line so that only one check is made per calling event for a single line (not currently a Telecom requirement, but under review);
- does not block dial tone to a subscriber attempting to initiate a call as an off-hook stutter dial tone check is occurring (not currently a Telecom requirement, but a good design practice);
- does not use more than 8 ľA of direct current from the subscriber loop except that the device may draw loop dc sufficient to make authorised off-hook checks.
At this stage, we are not thinking of applying all of the above conditions, as we feel it would be preferable to encourage the use of a more positive system of call waiting indication, as explained below. The proposed service should avoid the potential problems of SDT detectors, both as regards meeting the above requirements and the potential exchange loading problems, even if the related CPE goes into very widespread use.
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VISUAL MESSAGE WAITING INDICATION
Visual Message Waiting Indication (VMWI) is a proposed new network feature which will enhance our current "Call Minder" and "Message Manager" services by using an on-hook signal to show customers when a message is waiting. This avoids the need for the user to pick up the handset from time to time to listen for "stutter dial tone". It also avoids the need for a SDT detector to go off-hook, as explained above.
VMWI is based on specific signals sent by the network to control a message waiting indicator associated with the customer's telephone(s). This can be an adjunct unit, like some caller display units, or be integral with the telephone. VMWI devices detect "non-ringing" signals sent by the network, which uses Frequency Shift Keying (FSK) signalling to the same Bellcore standard (NWT-TR-000030) as our Caller Display System. This new service is likely to be made available by the end of this year.
Caller Display CPE manufactured to the Bellcore specification commonly incorporates a VMWI facility, so local suppliers should not have much difficulty in purchasing product to support the new service. However there is a potential problem in that some US networks provide a ìwake-upî signal in the form of a short line break just before the FSK signals are sent. This ìwake-upî signal, which is used to activate the speech path on some transmission systems, may also allow the CPE designer to reduce power consumption for battery operation. Unfortunately, this signal will not be available from the Telecom network. On the other hand, many designers of FSK units have eliminated the need for the wake-up signal by using mains power packs and having the VMWI circuitry monitor the line continuously.
FSK VMWI units will react to signals as soon as they come from the network. The typical minimum 15 second delay of an SDT may be reduced to about 5 seconds and the indication will be correct even if remote clearance of voice mail was used. From Telecom's viewpoint, the unnecessary exchange processor loading caused by SDT detectors is eliminated by the use of FSK VMWI detectors.
Formal amendments to TNA 102 and PTC 200 will be published when full details of the new service are confirmed. However, any intending suppliers of SDT units should be aware that there is likely to be competition from VMWI units within the year.
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AUTOMATIC LINE TESTING
Telecom has been using an automatic line testing system for some years now , but there are still occasional complaints that "someone is interfering with our line" should a slight squawk be heard late at night from a sensitive telephone ringer.
Another feature of the tester is that it will disconnect the normal exchange battery supply from the line for the duration of the test and replace it with a sequence of test signals. The sudden changes in voltage may be detected by customer equipment, such as some PABX systems and early cordless telephones, leading to an apparent "phantom" call. While this is not a serious problem in terms of the number of customers affected, CPE suppliers should be aware of the need for equipment ringers or other alerting devices (such as PABX operator console calling indicators) to respond to ringing on incoming calls, not to changes in voltage level or polarity. Compliance with PTC 200, clause 7.5.2 (4) should go a long way towards satisfying this requirement.
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DIGITAL PAIR GAIN AND OTHER "DERIVED LINE" SYSTEM INTERFACES
Digital pair gain systems and various types of radio system are being widely used in the Telecom network in preference to laying more copper in the ground. These systems provide a high level of flexibility in that they can be added as and where needed in fairly short time. This allows us to address the needs of individual customers where spare cable pairs are not available, as they can be rapidly brought into use for second lines, for homes split into flats, etc.
Unlike the standard NEAX public exchanges, these systems generally do not use 50 V d.c. feed bridges. Instead, they are more likely to use voltages around 35 - 40 V d.c., but with much reduced line lengths.
As such, the current available at the customer's end of the line is consistent with what is available over the typical NEAX copper line range. In general terms, the line would be providing at least 20 mA into a 450 Ω termination at the customer's premises, with an open circuit voltage within the range 21V - 100V. The great majority of line interfaces will fall in the range 35V - 54V. In addition, there are still some 12 V line systems in service. Although any line interface may have a different Voltage/Current characteristic to that shown in TNA 102, we have so far retained the standard 50 V-based test conditions in PTC 200.
Nevertheless, with the many different types of system now in use, minor differences in the network interface are arising. As a result, we are endeavouring to establish a common set of interface characteristics which can be relied on by CPE suppliers for the great majority of customers. More is expected to be published on this issue in future Newsletters and, if necessary, any changes resulting from the use of these systems will be covered in an amendment to TNA 102.
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USE OF CAT 3 AND CAT 5 CABLES - NETWORK CONNECTION PROBLEMS
For almost 15 years, Telecom has been using 3-pair cable for jackpoint wiring. The three pairs in this cable used the colour coding Red/White, Orange/Black and Green/Blue. For 3-wiring, red and white are used for the speech pair and blue is used for the ringing wire. This led to the obvious choice of red and white for our more recent 2-wire system.
1997 saw the introduction of our new higher performance Category 3 and Category 5 cables which are based on internationally recognised standards. We have also adopted the "blue, orange, green, brown, grey" colour coding specified in these standards. This allows local suppliers to source existing product without the need to arrange special insulation colours. As a result, the pair colours for 2-pair cable are blue/white and orange/white.
Telecom's standard 2-wiring instructions mention that for the first-in line, either red/white (for older cables) or blue/white (for the new cables) are to be used. Unfortunately, some jackpoint instructions specify only the colour combinations. This has led to some problems where pre-wired homes and small businesses are being connected to the Telecom network, in that Telecom staff automatically connect the blue/white pair to the network lead-in cable. If the installer has used a different pair and the Telecom staff are unable to gain access to the premises to confirm network access or check the jack wiring, the customer can end up with no service. Sorting out this problem is inconvenient for the customer and for the other parties involved.
Non-Telecom wiring installers using the new cable types should note the need to either use blue/white for the first-in line (Telecom's preferred arrangement) or, where this is not possible for some reason, to clearly mark on the cable tail what other pair colour(s) have been used for the internal wiring.
The same applies where earlier 0.4 mm cable is used. Either use the red/white pair or show clearly that another pair is to be connected to the Telecom lead-in.
I would like to express my thanks to MM Cables Ltd, who are assisting us in overcoming this problem by printing special labels on their drums and reels of telephone cable. These labels not only explain the need to use the right pair colours, but also warn that these types of telecommunications cable should be used only for wiring services operating at Telecommunications Network Voltage. Needless to say, we are all anxious to avoid "home handypersons" using this cable at hazardous voltages.
I also thank ModemPak Ltd, who have agreed to overprint their wiring instructions to cover the use of the Blue and White pair in place of the Red and White pair for first-in lines when the new cables are being used.
The prompt steps these companies are taking should help all concerned to ensure that incorrect network connections are kept to a minimum.
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CONNECTION OF "SOHO" CABLING IN RESIDENTIAL PREMISES
The publication of AS/NZS 3086:1996 has led to increased interest from cable and hardware suppliers as to Telecom's position with respect to the use of "SOHO" (Small Office/Home Office) cabling systems in residential premises. As explained in Newsletter No. 93, Telecom's present "2-wire" system is quite adequate for the great majority of residential customers. It uses the BT plug and socket connections introduced in New Zealand in 1983 and the vast majority of CPE is directly compatible with this wiring system.
The trial introduction of ADSL may have some impact on the overall wiring system design, but our new PTC 222 cable provides the improved transmission performance needed for the various high bit rate network
services now under development. As such, customers can pre-wire new premises with PTC 222 cable and have reasonable assurance that it will not be necessary to replace that wiring at great expense within the next few years.
Nevertheless, the level of prospective "future proofing" is something for the customer to decide in relation to expected future needs. The SOHO system specifies 4-pair category 5 "star-wired" from each "TO" ("Telecommunications Outlet - the "jackpoint", or "socket" as we usually refer to them in this country) back to a central cross-connect panel. This system provides more flexibility for allocating specific functions to each TO as needs grow or services change, but it is a lot more expensive than our simpler 2-wiring. In addition, it uses the RJ 45 8-way modular sockets throughout. As such , 2-wire connected CPE needs to either be fitted with an RJ 45 compatible plug or used with some sort of plug adapter to fit these TO's. 3-wire connected CPE, a vast quantity of which is in service in this country, needs a "master" adapter fitted with a ringing capacitor to provide the third "ringing wire".
Notwithstanding these disadvantages, SOHO cabling is desirable for those customers considering multiple lines (the standard system serves up to 4 exchange lines) or wanting a home PABX system. Typical SOHO-based cabling hardware overseas also provides for co-axial cabling or even fibre distribution to each room in addition to the Cat. 5/RJ 45 "network services" outlets. Again, the price is higher, but the customer makes the choice.
From Telecom's viewpoint, while such cabling systems are not necessary for the majority of residential or small business customers, we have no objection to their use and connection to our network. However, because Telecom does not install or maintain these systems, our residential faults staff do not carry spare components to service them. As a result, they will not be covered by our residential wiring maintenance service. Suppliers of such hardware are thus required to ensure that prospective purchasers are made aware of this point.
Should these SOHO systems become more "mainstream" over the next few years, maintenance by Telecom staff and contractors may prove viable. At this stage, suppliers are required to appoint and train their own installers and that these parties will maintain their own hardware and wiring.
It is expected that a draft PTC Specification covering "SOHO" cabling will be published in the near future.
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COMMENTS ON PTC 222 DRAFT
The "Draft for Public Comment" edition of PTC 222 was published in October 1997 to cover the introduction of Category 3 and Category 5 cables, as explained above. I intend to publish the "final" version of this document in March. In view of this, cable suppliers are asked to submit any comments or suggestions by the end of February.
One change worth mentioning relates to the above problem of pair identification. PTC 222 will require suppliers to provide an advisory label on their reels/drums to the effect that the blue/white pair should be used for the first (or only) line in new installations.
There are also two minor errors in each of the two tables published in the draft. These will be corrected in the published version.
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TELEPERMIT GRANTS DURING 1997
Further to the brief outline in Newsletter No 102, the final tally of Telepermits issued for 1997 was 628. In addition, there were 31 Limited Permits granted for the more complex classes of terminal product.
As usual, non-voice products - predominantly modems and fax machines - were the largest class and there were 287 permits issued in the PTC 211-series. The next main class was PTC 207, which covers customer switching systems and associated telephones and terminals. 76 permits were issued in this series. New models of telephones still continue to pour in, with a total of 43 permits in the PTC 202-series.
It was interesting to note a total of 56 new ISDN products, despite claims in some quarters that "ISDN is dead!"
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TELEPERMIT OVERVIEW
The Telepermit Overview has just been updated and is now at Issue 23. Most regular clients of Access Standards will be familiar with the current arrangements, but those who have not made any recent Telepermit applications need to be aware of the new Application Form. A copy of this is appended to the Overview.
Any potential Telepermit applicant requiring a copy of the Overview is invited to request one free of charge from Janine Jackson,either by phoning her on (04) 498 9713 or by faxing her on (04) 473 5927.
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DOUG BURRUS
Manager, Access Standards
