5
SIGNALLING REQUIREMENTS
5.1
Signalling types
(1) The standard method of signalling between customer premises equipment and Telecom telephone exchanges is dual tone multi-frequency (DTMF) signalling. This method is also widely used for signalling from customer to customer after a call has been established.
(2) The use of decadic only signalling on new products is not accepted for Telepermitting purposes.
* Existing exchanges may support NZ-type decadic signalling, but this will not always be so in future.
* Decadic signalling designed for use in countries other than New Zealand is totally unacceptable for the reason given in clause 5.4.2.
(3) The Telecom network is, at present, set up to accept either "reverse decadic" or DTMF customer signalling on PSTN lines. Once a decadic digit has been received by the exchange, the DTMF capability is switched off and the exchange expects the remaining digits to also be decadic.
(4) It shall not be possible for any device to send either DTMF or decadic signalling pulses to line in the "on hook" condition.
* When initiated, 'Handsfree' operation of telephony devices is regarded as an off-hook condition.
(5) The test set-up for signalling tests is shown in Fig. 5-1. Return to Contents
5.2
Transmission of DTMF signalling
* Reference CCITT Blue Book, Recommendation Q. 23. Return to Contents
5.2.1
(2) Each transmitted frequency shall be within ± 1.5% of the nominal frequency.
(3) DTMF signalling devices shall support at least the 10 numeric digits plus '*' and '#' symbols.
DTMF frequencies
(1) The allocation of DTMF signalling frequencies shall be as follows:-
Low Group High Group (Hz) (Hz) 1209 1336 1477 1633 697 1 2 3 A 770 4 5 6 B 852 7 8 9 C 941 * 0 # D
* The 'A', 'B', 'C' and 'D' are rarely used at the date of issue of this Specification. Return to Contents
5.2.2
(2) All devices shall satisfy the following requirements during the transmission of DTMF signalling:-
(a) The impedance of the equipment shall be such that the return loss complies
with sub-clause 4.5.1(8).
(b) Each individual signalling tone of a burst, when measured against 600ohm on a zero length line, shall be at a power level between -4 dBm and -13 dBm.
DTMF signalling requirements
(1) For optimum performance, all transmitted DTMF frequencies shall comply with the requirements of this Specification at all line currents in the range 20 mA to full current.
(c) For successful operation of DTMF signalling between customer premises, it is recommended that DTMF send levels be in the range -4 dBm to -10 dBm.
(d) There shall be a pre-emphasis in the range 1 to 3 dB for the high frequency tone group relative to the low frequency group.
(e) During DTMF signalling, any individual distortion or intermodulation products shall comply with clause 4.3.2(2).
5.2.3
(a) The minimum valid tone duration shall be 60 ms, and the minimum inter- digital pause shall also be 60 ms, exclusive of any rise and fall times.
(b) The valid tone duration shall be timed when the signal is within the
acceptable tolerances stated in clause 5.2.1 and clause 5.2.2.
(c) The inter-digital pause shall be measured following rapid successive
operation of any of the DTMF keys.
Signal timing
The duration of transmitted DTMF signals shall comply with the following requirements:-
* Even if buffering of manually operated DTMF signals is not provided, the minimum tone duration is still likely to be around 60 ms. Return to Contents
5.2.4
(2) For terminating series connected devices, any DTMF signalling facility shall comply with the requirements of clause 5.2.1 to 5.2.3.
Series connected equipment
(1) Any harmonic distortion of DTMF signals introduced by series connected devices shall not exceed 5 %.
5.2.5
Speech circuit suppression
(1) For all devices capable of sending DTMF signals, the normal transmission path from any voice or similar source shall be attenuated in both directions of transmission by at least 30 dB during DTMF sending. This applies to terminating devices and also to other series connected devices capable of being used on the same line as a terminating device.
(2) On telephony devices, a confidence tone may be applied to the receiver so that the user is aware that signals are being sent to line.
* The application of confidence tone and the level selected are regarded as marketing features.
FIG. 5-1 TEST SET-UP FOR SIGNALLING TESTS
5.3
(a) Any receive level between -5 dBm and -30 dBm.
(b) DTMF frequencies within ±3 % of the nominal values (ref. clause 5.2.1(1)).
DTMF receivers
(1) Any device incorporating a DTMF receiver shall be capable of responding to DTMF signals in the following ranges:-
* It is strongly recommended that devices respond down to -40 dBm (ref Document TNA 102, clause 5.2.2).
(2) The receiver shall recognise any valid DTMF signal that is present for a minimum of 40 ms, as long as it is preceded by a continuous pause of 40 ms. Return to Contents
5.4
Decadic Signalling
DECADIC SIGNALLING IS NO LONGER SUPPORTED BY TELECOM NEW ZEALAND
5.5
Breaks in line current during signalling
(1) During the switching of any call through the PSTN there may be short periods when the line current feeding CPE is interrupted. Where such interruptions occur they are usually within the inter-digital pauses, or at the end of dialling, during the call set-up period.
(2) The dialler circuitry on any device shall be capable of tolerating line breaks of up to 50 ms at any time during call set-up without affecting the signalling performance.
* Although this requirement is mandatory, products will only be subjected to formal compliance testing if other tests indicate that performance is being adversely affected. Return to Contents
5.6
Automatic dialling devices
5.6.1
General requirements
(1) Devices which have automatic dialling features, but which do not forward a pre-recorded voice or data message when the call is answered, shall only be activated manually. This includes devices with features such as last number re-dial, memory dialling, automatic dial back, etc.
* Requirements for automatic calling equipment with calling messages, such as memory fax machines, telemarketing equipment, alarm security equipment, etc., are covered separately in Section 8.
(2) For the purpose of this clause, there shall be no more than one call attempt to the same number for each manual operation.
* Any device which makes more than one call attempt to the same number is classified as an "automatic calling device" and is covered separately in Section 8..
(3) The user instructions associated with any automatic dialling equipment shall clearly state the following:-
"This equipment shall not be set up to make automatic calls to the Telecom "111" Emergency Service"
(4) Equipment designed for automatic dialling shall be capable of having the called numbers re-programmed or edited. This may take the form of installer programming locally or from an external source, internal software programming or associated DTMF keypad for the purpose of manually editing individual programmed numbers.
* It is sometimes necessary to insert prefix digits additional to the number programmed (see clause 5.6.2 below).
(5) Where an adjunct unit has an integral keypad, it shall be regarded, for dialling purposes, as equivalent to CPE with an integral display unit.
(6) When dialling a local number, only the 7 digit number shown in the directory needs to be dialled. Nevertheless, a customer may dial the "0" and the area code for a local call wholly within the Telecom network. This does NOT incur a toll charge.
(7) Should a Telecom customer also be the customer of a toll by-pass carrier and subscribe to Telecom's non-code access service, local calls preceded by the "0" and area code will be passed to the other carrier (see clause 5.6.3).
* Such customers will need to choose whether or not the use of an automatic dial back feature is justified. In some cases, such units will be unacceptable. Return to Contents
5.6.2
Caller display equipment
(1) Caller display devices which offer a dial-back facility represent a potential difficulty for users. The number forwarded from the network comprises only the area code and the local number, with no leading "0". Wrong numbers will result in MOST cases when such devices dial only the number received.
* Without the "0" prefix, the area code is seen by the exchange as the first digit of a local number. The last digit of the true local number is then ignored. The only cases for which the correct number is shown and dialled is where the device is used within a Centrex group.
* See also clause 11.4.5 for further information on number formats.
(2) There are three possible ways to avoid the problem described in (1) above, and one of them should be employed on adjunct caller display units. The possibilities are as follows:-
(a) Dispense with the dial back feature totally.
(b) Provide for digit insertion in accordance with sub-clause (5) below.
(c) Incorporate a keypad for manual insertion of digits.
(3) Telecom pre-sets each line in the local exchange to either "normally display" or "normally withhold" the calling number, as determined by the customer concerned. It should be noted that callers (who may or may not be the actual customer) may wish to control the display of their numbers on a call by call basis as follows:-
(a) To withhold a normally displayed calling number, dial the prefix "0197".
(b) To display a normally withheld calling number, dial the prefix "0196".
(4) Where dialling such prefixes involves the use of an associated CPE device, potential problems may arise, as explained in clause 5.6.4.
(5) Where a dial back feature is provided on an adjunct caller display unit and there is no associated keypad on the unit:-
(a) the digit "0" shall be inserted as the first digit where the number received from the network comprises 8 or 9 digits.
* Local and national calls are indicated by a number length of 8 or 9 digits.
(b) no digits shall be inserted where the number received from the network comprises 7 digits or less.
* Such short numbers indicate either a Centrex call or an incoming international call for which the number is not available.
(6) For international length numbers, an adjunct caller display unit should be capable of storing and/or displaying up to 15 digits. If a dial back feature is provided on such units, the digits "00" should be inserted as the first digits for all cases where the number received from the network comprises 10 or more digits.
* The majority of numbers of this length will, in due course, represent international incoming calls.
* Note that there may be some incoming international calls which result in number lengths of less than 10 digits, but these are expected to be in the minority. Typical examples are those from the Pacific Islands.
(7) Display units providing auto dial back facilities may also provide the following facilities:-
(a) For dialling local numbers, a device may be programmed to automatically delete the "0" prefix and area code digit and dial only the 7 digits of the local number.
* It should be noted that every area code used in the Telecom PSTN serves more than one local calling area. Any dial back logic will thus need to examine the one digit area code and one or more digits of the following 7-digit number in order to accurately determine whether the number received from the network relates to a local call. Since the digits indicating local calls vary from area to area, this arrangement is likely to be practicable only for Computer-Telephony Integration (CTI) systems.
(b) Where an exchange line access prefix is necessary, the device may be programmed to automatically insert it. In such cases, the access prefix must only be inserted for local and national calls, and not for calls internal to a Centrex group or PABX network.
* To avoid any customer misunderstanding, it is recommended that suppliers clearly state in the product brochures whether or not the product is "Centrex compatible". Return to Contents
5.6.3
Non-code access to other networks
(1) Many Telecom customers use another carrier's toll services for their national calls and have their line set up so that their chosen carrier's access code is automatically inserted by the Telecom exchange. This service is activated when the caller dials a "0" or "00" prefix before the wanted number. Should such a customer dial the "0" prefix and area code for local calls, these calls will also be passed to the other carrier as if they were national calls. As a result, a toll charge may be incurred.
(2) Telecom area codes serve more than one local calling area. As a result, on caller display devices, complex user programming would be necessary to automatically separate local calls from national calls, as covered in clause 5.6.2 above. Return to Contents
5.6.4
Manual insertion of additional digits
(1) In some cases it is necessary to dial digits manually in addition to those dialled by an automatic dialling device. Usually this will be carried out from an associated telephone. Use of such arrangements may give rise to the following difficulties:-
(a) If both the adjunct automatic dialling unit and the associated telephone draw current from the line, there may be insufficient current available to power both devices.
(b) If both devices terminate the line, the combined impedance of the two devices is unlikely to meet the requirements of clause 4.5.
* This is acceptable for a limited period before actual speech transmission takes place, provided that the automatic dialling device goes on-hook as soon as dialling is completed.
* From a user viewpoint, the need to manually dial part of the number via the telephone is not very convenient. Also, if the two devices are connected in parallel rather than series, then the sound level from the receiver may be objectionable when the display unit is dialling the remaining digits (series connected devices are required to comply with the speech path suppression requirements of clause 5.2.5).
(c) Adjunct dialling units use DTMF signalling and can therefore only be used in conjunction with DTMF telephones.
* See also clause 5.1(3).
* These potential problems are likely to result in complaints from users and, possibly, product returns.
(d) An adjunct dialling unit is unlikely to be able to suppress the speech performance of an associated telephone during dialling (clause 5.2.5). Use in high ambient noise areas or during loud conversation could lead to digits being ignored by the local exchange and calls timing out.
* Note that this affects ONLY the calling party, as missed digits usually do not result in a wrong number.
(2) Where it is necessary to use an associated telephone with an adjunct dialling unit for dialling prefix(es), suppliers shall include the following warning notices in sales brochures and user instructions:-
"This unit will only work in conjunction with a tone signalling (DTMF) telephone, but some telephones may not be compatible. Since noise or
speech from the telephone can upset dialling from this unit, errors may result if the devices are used together in other than quiet conditions"
"Where it is necessary to dial prefix digits, such as a Caller Display
override code ("0196" or "0197"), this unit will have to be used in conjunction with an associated tone signalling (DTMF) telephone.
Note that some telephones are NOT compatible with this unit and
dialling errors may result if the two devices are used together in other
than quiet conditions. The Telecom Faults Service is NOT to be called should such problems arise. In such cases, it is recommended that the prefix and wanted number are dialled manually".
5.6.5
User instructions
(1) In view of the above complications resulting from use of automatic dialling devices, clear and comprehensive User Instructions shall be supplied.
* This is particularly important for devices intended to operate with Telecom's Caller Display service.
(2) In addition to the specified mandatory warnings, it is recommended that the User Instructions also include a brief explanation of the reasons behind these requirements.
(3) The user instructions shall clearly explain:-
(a) the appropriate dialling procedures for local and national calls for both direct line and Centrex customers.
* The most commonly used exchange line access prefix for New Zealand PABX systems, Telecom Centrex lines, etc, is the digit "1". However, other access digits may also be encountered.
(b) the impact of "non-code access" to a network other than the local network to which a display unit is connected.
(4) To minimise liability under New Zealand consumer protection legislation, suppliers shall make purchasers aware of this issue by inserting the following warning in the User Instructions.
"If a charge for local calls is unacceptable, the "Dial" button should
NOT be used for local calls. Only the 7-digits of the local number should
be dialled from your telephone. DO NOT dial the area code digit or the
"0" prefix"
* Under New Zealand consumer protection legislation, the supplier is responsible for ensuring that a product is fit for its intended purpose. A warning to the above effect is likely to minimise the number of product returns or claims by purchasers who make use of Telecom's non code access service. Return to Contents
5.7
Keypad layout
5.7.1
International standard layout
(1) The standard requirement for keypads is that they be 'alpha-numeric'. The layout agreed internationally at the World Telecommunications Standardisation Conference,
March 1993, has been adopted by Telecom in New Zealand. The layout is as shown below in sub-clause (2) and the nominally agreed date for implementation was 1 April 1996.
* This new standard is published in the following standard documents:-
ITU-T Recommendation E. 161
International Standard ISO/IEC 9995-8:1994
AUSTEL TS 002:1996
* ANSI and ETSI publications also include the same standard.
(2) The internationally agreed standard alpha-numeric keypad layout is as follows:-
| 1 | 2 ABC | 3 DEF |
| 4 GHI | 5 JKL | 6 MNO |
| 7 PQRS | 8 TUV | 9 WXYZ |
| * . | 0 | # |
(3) For equipment which does not comply with this standard layout, details of the actual layout shall be stated in the application or in an associated test report.
* It is strongly recommended that all products marketed comply with the layout in sub-clause (2) above. Failure to do so will inevitably cause confusion to callers. Alpha-numeric "wordnumbers" are very popular, particularly for customers with "0800" or "0900" numbers.
* At some future date, the alpha-numeric layout shown in sub-clause (2) above may become mandatory. If so, from that time no Telepermit applications will be accepted for products with layouts that are numerical only, or that are similar to the above but with 'Q' and 'Z' on digit 1, or with 'Q' and 'Z' omitted.
(4) Equipment that does not use a keypad for sending (e.g. modems and PC's) shall be exempt from the requirement to comply with the above layout.
(5) "Inverted" layouts produced on overseas-supplied decadic signalling telephones by simply dismantling and reassembling the telephone keypad with the buttons changed around are not acceptable.
Return to Contents
5.7.2
Tactile performance and service life
There is no formal requirement for keypad performance.
* The operating life of a keypad, its construction and function are all regarded as marketing features.