 TELECOM ACCESS STANDARDS NEWSLETTER NO. 99 SEPTEMBER 1997
CONTENTS
1. NEW TELECOM ISDN SPECIFICATIONS
1. NEW TELECOM ISDN SPECIFICATIONS
Telecom issued its first ISDN PTC and TNA specifications in February 1990. These were PTC 131 and PTC 132, which were published as drafts for public comment, and TNA 133 and TNA 134, which were published as final versions. Little or no comment was received on the two PTC specifications, so we saved time and effort by not replacing them. They have since been regarded as "final" versions, even though they were marked "draft". TNA 135 was published in 1991 to describe ISDN Supplementary Services, and TNA 139 was published in 1992 to describe the differences between the Telecom New Zealand "ITU Blue Book" and the then Telecom Australia (now Telstra) "ITU Red Book" ISDN implementations.
Needless to say, the Telecom network has moved on since the early 1990's and we are about to publish a new set of specifications based on the latest ITU documentation and our current NEAX 61 implementations.
The original four specifications were essentially full reprints of the relevant ITU Recommendations, with the Telecom options and implementations marked by suitable highlighting. In contrast, the new and much briefer versions of PTC 131 and PTC 132 simply outline the very few differences between the Telecom implementations at Layer 1 and the relevant ITU Recommendations, I.430 and I.431. A similar approach has been used for TNA 133, which covers the differences between the Telecom implementation at Layer 2 and ITU Recommendation I.432. The actual ITU text has not been reproduced in these three documents. Layer 3 is where the number of options is much more extensive, so a good deal of ITU Recommendation I.432 and parts of some other relevant Recommendations, have been reproduced in TNA 134 for clarity. TNA 134 has now been split into four parts, the last of which covers Supplementary Services (previously published as TNA 135) in one integrated specification. Because only differences have been defined, we have omitted those parts of the ITU Recommendations covering particular services which Telecom has not implemented. Also, there are no SDL (ITU Specification Description Language) diagrams included in our publications. As a result, users will need to have access to the current ITU documentation in order to fully define Telecom's ISDN CPE requirements.
Our ISDN Layer 3 testing at Auckland and Wellington has been aligned with these latest PTC and TNA documents.
With Telstra and other networks introducing "ETSI" (the European Telecommunications Standards Institute) or "Euro" ISDN, TNA 139 is now relevant only to the earlier Australian ISDN. However, a comparison chart showing the high degree of commonality and relatively few differences between the Telecom implementation and "ETSI" ISDN (as defined in ETSI Specifications) is provided in the new issue of TNA 134. It is understood that Telstra's ISDN provides a sub-set of the full ETSI feature list, but CPE suppliers wanting full information on Telstra's "On Ramp" ETSI-compliant ISDN should contact Telstra Australia for full details.
As many suppliers will know, we have been granting Telepermits for ISDN CPE in the PTC 231 (basic rate) and PTC 232 (primary rate) classes, even though these two specifications have not been published, as such. This position is expected to be remedied over the next few months when we have finalised the requirements for ISDN terminal adapters, digital telephone levels, testing criteria, etc. In the meantime, the new set of documents will include brief summaries of the prerequisites and testing arrangements that have applied to ISDN CPE in general.
The new package of ISDN Specifications is published as a set in a white A4 loose-leaf folder. This set is available from Access Standards for $200, inclusive of GST and postage. Orders should be placed by fax to (04) 473 5927.
2. TELECOM'S ISDN AND "ETSI OR "EURO" ISDN
There are already about 300 ITU Recommendations on ISDN and these are continuing to be enhanced as more services and features are standardised. Many of these Recommendations provide numerous options from which network operators may determine their particular implementations.
ETSI aims to establish common standards and implementations for the countries within the European Union (EU). There are also around 300 ETSI Standards on ISDN, most are aligned with the ITU Recommendations. These standards provide for a wide range of services, with somewhat fewer options within these services than are permitted by the ITU.
ETSI also specifies a minimum set of services that should be implemented within the EU, but these are only a small proportion of the total number of services that are covered in their 300 standards. European network operators are also able to implement additional services and define how they will be controlled. As a result, describing a network as "ETSI ISDN" is not a formal definition of what services are actually provided.
Telecom's ISDN complies with the latest ITU Recommendations and the various services it provides have been selected to best meet our customers' requirements. Telecom's ISDN also complies with and exceeds the ETSI minimum set of services. In addition, all services in the ETSI minimum list are implemented in the same way by Telecom. In view of this, we could also fairly describe the Telecom ISDN as "ETSI ISDN"!
Terminal Compatibility (for basic calls)
On the other hand, not all ISDN CPE is necessarily compatible with our network or with any network, other than the one for which it has been designed. The many options within the ETSI and ITU standards means that differences can arise in terms of which services are supported and in the way they are implemented. Also, any ISDN terminal that supports speech or 3.1 kHz audio services cannot be moved between (-law networks (North America and Japan) and A-law networks (used by Telecom, ETSI and most of the rest of the world). Speech calls can be made between these networks only because the speech encoding is converted in the (-law network. Should a CPE supplier have the choice of whether to use American, Japanese or European versions of software for product intended for New Zealand use, the ETSI version is the most suitable.
As it might have an impact on customer's terminals, I should mention one difference between our implementation of Primary rate and the ETSI version. In Telecom's network, a "primary group" (e.g., a link between the switch and a PABX) can have up to 4 Primary rate lines (each providing up to 30 B channels) and all signalling is done over just one D-channel. In comparison, ETSI uses each D-channel of each line to carry the signalling for that line. Many terminals are flexible enough to support both methods.
Terminal Compatibility (for supplementary services)
a. Keypad
b. Feature key
c. Functional
Further to the announcement in Newsletter No. 98, PTC 200:1997, which covers "Requirements for the Connection of Customer Equipment to Analogue Lines" should be available within the next two weeks. This new Issue 2 version has been extensively revised and now includes a formal test schedule.
Copies can be ordered from Access Standards (preferably by fax to (04) 473 5927). The price to those who we have recorded as having purchased Issue 1 is a nominal $50. This does not include a folder, as one was supplied with the original order. For those who are purchasing this specification for the first time, the price is $120. However, we recommend that all new purchasers take advantage of our offer of PTC 200 and TNA 102, which are supplied together in a white loose-leaf folder at the overall price of $170. If purchased separately, TNA 102 costs $60 without the folder. The two documents need to be read as a "pair" to gain a full understanding of our requirements. All these prices quoted are inclusive of postage and GST.
With the re-issue of Specification PTC 200, it is timely to stress what is becoming a major issue to us. We have in the past, somewhat reluctantly, accepted test reports against a few "close equivalent" overseas specifications for devices designed for connection to analogue lines. This was done as part of our efforts to be as reasonable as possible. Because assessment against such specifications is always time-consuming, we introduced an additional charge for this activity.
Developments in recent years have resulted in Issue 2 of PTC 200, which now incorporates several new services and testing procedures. These have widened the gap between our requirements and those of other countries. Examples are CLI and private number ringing (Distinctive Alerts), both of which are quite specific to the Telecom network. Our method for assessing ringing detectors is also different from that used by other networks (see item 5 below).
Because of these differences, we will require full assessment against PTC 200 for virtually all analogue line devices. Test reports against overseas specifications, such as those from the Australian Communications Authority (ACA) will be subject to increased assessment charges and, almost always, some parameters will have to be referred back to the applicant for additional testing. Testing to PTC 200 will avoid these costs and delays. Where the nature of a product is such that the overseas test report does appear to be sufficient, it is recommended that the potential Telepermit applicant discusses the matter with Access Standards and does not assume that overseas test reports will be adequate.
One of the areas covered in some detail in PTC 200: 1997 is a replacement for the Ringer Approximate Load (RAL) concept we introduced in 1988. More and more items, each of which may have different ringing characteristics, are being connected to the same line. While each is individually compatible with the network, a lot more attention now needs to be placed on whether or not these devices are compatible with one another. The new Ringing Number (RN) system takes some additional parameters into account, as explained below.
Background
Unfortunately, this simple assessment method has proved inadequate for some types of CPE. Some ringers have non-linear voltage/current characteristics and may not "switch on" until some minimum voltage is reached. As a result, even with a RAL of less than "5", there can still be ringing problems due to poor current sharing if two or more items of CPE with markedly different ringing characteristics are connected to the same line. When several items of CPE are connected to a line, the relative "sensitivity" of each ringing detector is an important factor. Some need a specific minimum voltage to trigger, others have widely disparate impedances or impedance varying with the current drawn. The result is that RAL based solely on current drawn in a static test has not proved a reliable indication of ringing performance when mixed items of CPE are connected in parallel.
For telephones, this is not usually critical, as a call can be manually answered whenever ringing is heard. It is immaterial whether the telephone actually used to answer the call is ringing properly. However, this is not the case with auto-answering devices, such as fax machines, modems and answering machines. It is essential that every one of these is capable of reliably detecting the incoming ring when connected in parallel with other CPE.
RAL was originally based on ringer impedance, which mainly determines the current drawn from the common "Master" capacitor. This "3-wire based" scheme was then adapted for 2-wire connected devices. These generally have a RAL of less than "1" - typically 0.7 - because they draw ringing current directly from the line via the integral series capacitor (usually about 1 µF), and do not share the limited current drawn via the "Master" capacitor.
To overcome the limitations of "RAL", the Ringing Number or "RN" concept has been developed. This is based on the highest of two separate test ratings, one based on the load current drawn and the other based on the "sensitivity" or ability to accurately follow ringing cadences.
Ringer/ring detector testing
For test purposes, 10 "half-unit" loads are used so that we can have steps of 0.5 in the overall ratings. Each "half-unit" load consists of a 16 k ohm resistor and a 0.5 µF capacitor. These can be switched in or out to simulate parallel-connected ringing loads and so determine the ringing performance of the device under test. Measurements on typical lines has indicated that a limit voltage of 38 V rms is typical where several items of CPE are connected to the same line. Because of frequency distortion due to non-linear loads and the fact that many ringers are "peak sensitive", the equivalent test voltage is set at 107 V peak to peak with the 10 test loads connected to simulate a total load of "5".
The rating of any device can be assessed by counting the maximum number of "half-unit" loads that can be connected in parallel with it such that it still rings (or detects ringing reliably). No "stuttering" or discernible shortening of the "on" period for each cadence is acceptable for audible ringers. For Auto Answer ringers,
Loading Test
Sensitivity Test
The Ringing Number is the larger of the above two ratings. Ringing Numbers of zero are rounded up to 0.5.
Application of Ringing Numbers
Because there will be a mixture of RAL and RN-labelled CPE in service for many years to come, we have retained the same limit of "5". For most types of existing CPE, the RAL number will correspond with the Ringing Number. Where there are exceptions known to be the cause of ringing problems, we will make these known to our staff and other interested parties. Typically, any device with a RAL of 1.3 or more is somewhat suspect as regards load sharing. Otherwise, most RAL and Ringing Numbers can generally be intermixed.
Where problems are experienced on 3-wired installations, even with loads of less than 5, it may be best to convert to 2-wiring.
Where the maximum loading is exceeded and trouble arises, it will be necessary to turn off ringers or disconnect some of the directly connected load. Use of Distinctive Alerts and one or more ringing decoders may be an acceptable solution in some cases, as this will allow at least some of the CPE to be indirectly connected. This will be discussed in more detail in the next Newsletter.
Telecom has been considering the impact of changing to 0.5 mm conductors for its premises cabling for some time. The heavier gauge offers a lot of advantages as regards robustness and lower risk of breakage, but there has also been the problem of connecting mixed gauges in our earlier 3-wire jackpoints. Should customers use two different wire gauges in the same slot, the reliability of the termination is seriously compromised.
With the introduction of 2-wire jackpoints and their capability to terminate up to three different wire gauges, the opportunity arises to overcome the termination problem. It also means that we can introduce cabling with the much higher transmission performance needed to support future network services.
In view of the increasing need for more bandwidth and higher data rates, 4-pair Category 5 ("Cat 5") cable is a key component of the international building cabling standard, ISO/IEC 11801, the North American Standard, EIA/TIA 568A , and the Australia/New Zealand version, AS/NZS 3080. It is also specified in the SOHO wiring standard, AS/NZS 3086. Cat 5 cable is expected to support all projected new services for at least 10 years, even the new 1000 Mbit/s LAN services now under development. However, as its additional cost is probably not justified for most Telecom customers, who only want network-based services, we have arranged for our cable supplier to make a lower cost 2-pair version.
This new Telecom cable is intended primarily for low frequency applications and has a somewhat reduced high frequency capability relative to the standard 4-pair Cat 5 cable. However, it is more than adequate for all current PSTN-based services. The 2-pair version uses the same 0.5 mm wire, with the same wire colours and varying close twist, but it has only the first two pairs (Blue with White as one pair, and Orange with White as the second pair). The two pairs are not laid with a twist and the sheath is smaller in diameter. From a cable supplier's viewpoint, this approach allows the use of the main Cat 5 production line for the actual wire pairs.
To cover the introduction of these modern cable types for residential wiring, we are planning to publish PTC 224. This will cover both the Telecom 2-pair version and the standard 4-pair Cat 5 cable. Pending publication of a formal PTC specification, we are trialling the first batches of this new cable to make sure there are no unforeseen operational problems.
The earlier cable to PTC 203 will still be in wide use and it is expected to continue to be on sale for some time to cover extensions and additions to 3-wire installations. Where it is practicable to convert to 2-wiring, and we recommend that this is done, the new high performance 2-pair cable can be used for additions to provide the customer with at least partial "future-proofing". As far as new installations and additional lines are concerned, the new 2-pair cable is strongly recommended in preference to the earlier 3-pair cable.
In those situations where the customer is more concerned about the need for future service capability than first cost, the 4-pair higher performance cable can be used. If installed to AS/NZS 3086, this would be "star-wired" back to a common point, rather than "loop-wired" from socket to socket, as in Telecom's standard practice. Star wiring and Cat 5 cable incurs a higher first cost, but should largely reduce the risk of the customer needing to replace the cabling if service requirements change in the future. Installers should note that SOHO systems are not covered by Telecom's residential wiring maintenance service.
A number of modems with analogue Caller Display facilities have been submitted recently for Telepermit. In some, the facility has been implemented by taking an a.c. feed directly from the line side of the on-hook/off-hook relay. The on-hook impedance of this feed is often quite low - in one extreme case, down to only 1.4 k ohms! This affects the transmission levels for any other devices on the same line.
Raising the impedance of this a.c. path to meet the minimum 10 k ohms on-hook impedance requirement often significantly reduces the sensitivity of the device to the caller display data. Neither the reduced sensitivity nor the low impedance is acceptable.
Telecom routinely uses automatic line testing equipment. This is a pro-active means of detecting faults before they develop to a stage at which they actually affect a customer's service. Since this equipment treats low on-hook impedance as a fault, its detection is likely to result in a service visit.
In view of these constraints, suppliers should ensure that such Caller Display features have been correctly implemented in accordance with PTC 200, Issue 2, in any products submitted for Telepermit.
Any correspondence relating to Telepermit should either be addressed to Access Standards or attention Janine Jackson. This ensures that Janine will record in and acknowledge applications. Should correspondence be addressed directly to another member of the team, such recording and acknowledgement may be delayed. Also, Janine will ensure that any separate follow-up correspondence is associated with our main application file. Our main aim in making this request is to ask you to help us help you by avoiding mislaid papers. With applications running at around 600 per year, and many of these submitted in several stages, we are anxious that we do not mix or mislay the many thousands of sheets of paper that we receive from our clients.
Telelabs Ltd in Auckland are offering a new service, whereby they can provide photographs of equipment for suppliers. Typically, such photographs are required for Telepermit applications, but they are often needed for quite unrelated purposes. Photographs can be provided, independent of whether Telelabs is asked to carry out testing. Enquiries should be addressed direct to Telelabs. Their telephone and fax number is (09) 625 8200.
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