TELECOM ACCESS STANDARDS NEWSLETTER NO. 117 NOVEMBER/DECEMBER 1999 CONTENTS 1. PUBLIC TELEPHONE NETWORK CHANGES IMPACTING ON NEW AND EXISTING CPE a. Lack of support for "new" Bothway Analogue 2-wire DDI services b. Reduced line feed voltage and current c. Ringing voltage d. Ringing cadence distortion e. Ringing detection via the earth reference f. Polarity reversals 2. CANCELLATION OF 3-WIRE JACKPOINT TELEPERMITS & INSTALLATION REQUIREMENTS 3. ADSL CPE: TELEPERMITTING DEVELOPMENTS RETURN TO MAIN INDEX 1. PUBLIC TELEPHONE NETWORK CHANGES IMPACTING ON NEW AND EXISTING CPE For many years now, Telecom has been installing large quantities of digital transmission systems into its network. At first, these only provided economic solutions in the trunk or inter-city network as they were too expensive for short-haul services. Over the past ten years or so, they have been almost universally used for inter-exchange junctions within cities and their use has also grown in local reticulation networks as Telecom progressively replaces its copper feeder network (the cabling between the local telephone exchange and the distribution cabinet) with fibre optic systems, PCM, etc. The distribution cabinet usually provides the local copper distribution cabling to the customers, but even this last few hundred metres may be over derived circuit systems in some cases. This is especially the case in new areas, such as industrial parks and major housing sub-divisions developed over the past 5 years or so. All in all, the probability of the customer being connected via a copper cable all the way to the local exchange is steadily decreasing. a. Lack of support for "new" Bothway Analogue 2-wire DDI services As mentioned in Newsletter 104, some derived systems are unable to support the bothway analogue 2-wire DDI arrangements popular with many earlier PABX systems. In view of the need to minimise the impact of future changes on PABX customers, Telecom has now placed a "cap" on bothway analogue 2-wire DDI and will no longer provide these services to new customers, or to customers moving premises and taking these older PABX systems with them. PABX suppliers and maintainers should be aware of this limitation and not promote the use of this service to prospective customers. This is especially important where the customer is being offered a second-hand PABX system which needs to make use of this service due to limited trunk interface types or trunk capacity. Telecom can still provide bothway DDI service, but only via ISDN or other digital interfaces to the PABX. Alternatively, unidirectional services are available to cover those cases where the PABX has capacity to support sufficient trunks to meet the peak traffic demand. Existing customers using this service will be catered for until such time as network changes impact on our ability to continue to supply copper pairs to support it. This will be progressive on an "as needed basis" although Telecom will endeavour to give reasonable notice to any customers likely to be affected. Telecom is not always aware of exactly how a privately-owned PABX has been programmed and most customers are unlikely to know such details either. We are thus not in a position to positively identify all customers using the 2-wire version of the service. In view of this, it is recommended that PABX suppliers and maintainers act as advisors if they know that a customer is using the service and contemplating a move to new premises. The aim is "early warning" of the need to set up a PABX system in a way that can be supported by Telecom in the new location and so avoid any last minute hitches with a move. b. Reduced line feed voltage and current In Newsletter No. 103 (February 1998), I advised that our increasing use of derived circuit systems would be likely to impact on some existing CPE and on our PTC specifications for new types of CPE. These derived circuit systems do not provide the "traditional" 50 V d.c. feed voltage of our telephone exchanges. Systems installed over the past 5 years or so provide an open circuit feed voltage of approximately 36 V. Future systems may be further reduced in line voltage, in accordance with the industry standard, which is a minimum of 21 V. Since our network equipment is purchased on the world market, we also have to recognise these industry standards. Because these systems are located between the local telephone exchange and the customer, their analogue line lengths (those portions of the circuit carried over copper pairs) are reduced, along with the voltage drop in the line. This largely compensates for the reduced feed voltage and the equipment, along with the final distribution to the customer, will be designed to provide at least 20 mA into a 450 Ohm resistive termination, as with "all copper" services. As such, this will not impact on the majority of single line CPE, which has been required to operate correctly with current as low as 20 mA for some years. Notwithstanding the minimal impact on most customers' equipment, the reduced d.c. voltage is known to affect some PABX and Key Telephone systems, which "look" for 50 V to determine whether an exchange trunk is available for service. This issue was covered in Newsletters No. 105 (May 1998) and No. 109 (December 1998). Business system maintainers should be aware of the likelihood of some customers having problems if they are still using such systems. Similarly, any party dealing in re-conditioned early business systems should make their customers aware of these potential service problems before they are asked to sign up for such products. While conversion to derived circuit access will not impact on minimum available total line current, it can be expected to reduce the current previously available on many existing copper residential lines. This may make handover of a call from one telephone to another difficult, as two telephones may not have enough current available to keep them both operational during the handover period. As a result, it may no longer be possible for the first party to hear the second enter a conversation before going on-hook. This can be overcome by using the "Hold" facility provided on many telephones. Similarly, some customers like to have two telephones "off-hook" during a call for a three-way conversation. This is discouraged by Telecom, as speech quality is seriously impaired when two telephones are off-hook together, even when this is possible. A "handsfree" telephone is recommended when more than more people want to participate in a call. These are expected to be the most likely changes noted by residential customers. The limitations of feed current described above may cause mis-operation of CPE connected behind any series-connected equipment, such as ringing decoders, fax switches, etc, which introduce an additional voltage drop. However, this has not been a problem to date because few lines are operated at limit conditions and the majority of CPE works correctly with feed current marginally below the 20 mA design value. c. Ringing voltage The other significant change resulting from the use of derived access systems was explained in Newsletter No. 112. This is the reduced open circuit ringing voltage available on such lines. Instead of the 75/90 V rms ringing output from our public exchanges, these derived access systems will generate voltages of around 40 V rms. Also, this will not necessarily be a pure sinewave. Despite the lower open circuit voltage, the network will still deliver 107 V peak-to-peak at the customer's premises into a ringing loading of no greater than 5 microFarads in series with 1600 Ohms - corresponding to a total Ringing Number of 5. The 25 Hz ringing waveform will have a varying crest value, typically but not necessarily in the range of 1.2 and 1.6, dependent on the nature of the ringing load presented by the customer's equipment. These same criteria apply to both derived and copper lines and already form the basis of the Ringing Number tests specified in PTC 200, so there should be little or no impact on more recent CPE. Satisfactory ringing performance is very dependent on the amount and types of CPE connected by the customer. With multiple CPE items connected, the use of 2-wire jackpoints, wired in accordance with Code of Practice PTC 103, is strongly recommended. d. Ringing cadence distortion These new derived circuit access systems may introduce a delays in ringing of up to 50 ms, such that the ringing "on" period is extended to 450 ms and the silent periods are shortened by the same amount. This could impact on the accurate detection of our four Distinctive Alert ringing cadences. In some rural situations these systems may even be connected in tandem with other transmission equipment, potentially leading to even more distortion. Again, in reality, this is little different to the situation in our old electro-mechanical exchanges, where the ringing frequency and cadence varied with the loading on machine-generated ringing. A related CPE design factor is the low frequency of ringing. At 25 Hz, each half cycle takes 20 milliseconds, so decoders also have the problem that they too cannot detect timings to any great accuracy. This means we have to set what are effectively compromise conditions and accept that problems can sometimes arise. Despite the possibility of higher distortion on some lines, we have accepted correct decoding performance at ± 10% of the "on" cadence durations as adequate for Telepermit purposes, since this covers the great majority of customers. These limits have already been stated in TNA 102 and PTC 200. e. Ringing detection via the earth reference The ringing voltage covered in "c" above is applied across the line wires. Ringing voltages generated by telephone exchanges and derived systems are referenced to earth, but do not necessarily provide the same ringing voltage to earth or dc voltage to earth, and do not necessarily detect the same voltage when the line is looped. Some business systems detect an incoming call by looking for an earth, the ringing voltage to earth, or a d.c. voltage, rather than by detecting the loop ringing voltage. Such systems may be unable to operate correctly should a derived system be installed. Suppliers are asked to check that ALL CPE currently offered for sale uses loop ringing detection. Where any equipment is found to use the other forms of incoming call detection described above, it may have to be modified to ensure correct operation under all line conditions. f. Polarity reversals I mentioned in Newsletter No. 113 that we may not always be able to provide answer supervision by means of polarity reversals on derived access systems. So far, this has not eventuated and newer derived access systems have this capability. Nevertheless, suppliers are advised to be wary with respect to any systems or features that depend on polarity reversals, especially for any new equipment designs. Specification changes TNA 102 and PTC 200 are to be amended to cover the above changes, as well as those announced in previous newsletters. Timing of network changes Unfortunately, we can not provide much advance information on which lines may be affected by these new network interfaces or when changes are likely to take place. The network is subject to ongoing change as we extend it to meet customer demand and gradually incorporate these new technologies. In view of the progressive and continuing nature of these changes, suppliers should assume that all new equipment will be required to operate under the above conditions. Supplier obligations Now that we have made the likely future operating conditions known to suppliers, it is the supplier's responsibility to ensure that the CPE currently being sold complies with these conditions. If this is not the case, prospective customers need to be warned of any potential problems in accordance with New Zealand consumer protection legislation. Product review It has been suggested that all Telepermits issued more than, say, 5 years back should be automatically cancelled in order to ensure that all 5+ year old products still on sale as new items would automatically be checked for compliance with current network conditions. The number of such products still being offered for sale is not known but, with the present high speed of technology development, most products are superseded within 5 years, many within a year! However, the real problem is to determine ANY current products which fail under the new network interface conditions, no matter what age their design. It must be remembered that under New Zealand consumer protection legislation, suppliers are responsible for the correct operation of the equipment they provide to their customers. In view of this, suppliers are urged to check their product range to confirm that everything operates correctly under the currently specified conditions. NOTE: This is particularly important in relation to the supply of SECOND-HAND CPE, especially PABX and KEY TELEPHONE systems, which were originally supplied by the Post Office before deregulation in 1989. Such equipment was "grandfathered" under the Telepermit system and is unlikely to meet current PTC Specification requirements in many respects. 2. CANCELLATION OF 3-WIRE JACKPOINT TELEPERMITS & INSTALLATION REQUIREMENTS Manufacturers, suppliers and installers of residential customer premises wiring are reminded that the Telepermits for 3-wire "Master" jackpoints expired on 1 April 1999 and that those for 3-wire "secondary" or "Extension" jackpoints will expire on 1 January 2000. It is understood that manufacturers ceased production some time back, but there are still a few on sale. Since the expiry was announced in August 1998, suppliers have had plenty of opportunity to change over to the various 2-wire types now available from manufacturers. Also, of course, Telecom has since permitted the use of 2-wire jackpoints for extensions to 3-wire installations in those cases where a full conversion to 2-wire is not justified. This is not a recommended approach, but it could be an acceptable option for those customers who are unlikely to want to connect modems to their lines. For those who already have or expect to connect a modem, Telecom strongly recommends full conversion to 2-wire. Installers should also note the need to use only 2-wire jackpoints for residential-type installations from 1 January. In setting this requirement, Telecom is conscious of the very rapid growth in internet users and the expected move from 56 Kbit/s modems to ADSL over the next year or so. 2-wiring has been clearly demonstrated to provide speed improvement with 56 kbit/s and V.34 modems. It is essential for those customers connecting to our "JetStream" (ADSL) service. Any remaining stocks of 3-wire "Extension" jackpoints may continue to be used for commercial cabling installations, the components of which are not subject to Telepermit requirements. This is actually necessary in any proprietary wiring installations using BT-style sockets and requiring 3 or more wires. e.g., key telephone systems using an additional pair of wires for signalling. As announced in Newsletter No. 111, another aim is to rapidly phase out use of the earlier 3-pair cable to PTC 204. This move is likely to be well under way already because the later 2-pair cable is usually less expensive and offers much better performance. 3. PHASING OUT 3-WIRE CONNECTED CPE The move to 2-wiring in 1996 has brought significant improvements in reliability and service continuity in comparison with the earlier 3-wire systems, and we are now reduced to only one really significant mode of jackpoint corrosion causing service interruption for our customers. 2-wire connected CPE uses only pins 2 & 5 of the socket, leaving the two unused intervening pins to provide plenty of clearance to deal with the usual 50 V continuous on-hook line voltage. Unfortunately, this level of corrosion protection does not exist when 3-wire connected CPE is plugged into the socket and makes use of the integral capacitor. In this situation, the full line potential exists between pins 4 & 5. Not surprisingly, the most common form of breakdown now arises between these two pins. There are still many earlier 3-wire connected telephones in service, but they are gradually being replaced by newer and better featured 2-wire telephones, so this corrosion problem should gradually reduce. To avoid any increased risk, we now intend to decline new Telepermit applications for 3-wire connected CPE after 1 March 2000. This is expected to have almost nil impact on CPE suppliers, as we have not had any applications for 3-wire connected CPE for some time. Nevertheless, formal warning is being given in this Newsletter, just in case any suppliers are already considering the possible introduction of new 3-wire products. All new analogue CPE designs will thus have the line connections on pins 2 & 5 of the plug, with no connections on the other pins. Exceptions will still exist for CPE connected to proprietary systems, such as key telephone systems, which need to connect more than 2 wires for signalling or other special purposes. 4. ADSL CPE: TELEPERMITTING DEVELOPMENTS Although the ITU announced publication of the relevant Recommendation, this has still not been published. Once it is available, we will select appropriate options for our service and negotiate with our supplier for the provision of "ITU standard-compatible" DSLAM's. In the meantime, Nokia have advised that the ADI chipset used in the present DSLAMs, M10 modems and Efficient Networks NICs is based on the ANSI T1.413 issue 2 standard. It does not interoperate with equipment designed to the new ITU G.992.1 and G.992.2 standards. Nokia is expected to make ITU-compliant DSLAM's available to Telecom next year. Due to this change in network equipment, Telecom will have to take steps to either continue to support the initial rented modems and network cards or, perhaps, upgrade them as and when required. This change may also impact on any prospective CPE suppliers who have been following up on the earlier announcement that ADI chipsets were being used for the first batch of modems and network cards. DOUG BURRUS Manager, Access Standards