TELECOM ACCESS STANDARDS NEWSLETTER NO. 162

April 2007

CONTENTS
1. ADSL Evolution
2. ADSL2+ PTC Specification and process
3. Interoperability/Performance
4. Timing
5. Splitters and House Wiring
RETURN TO MAIN INDEX



1. ADSL Evolution

Dial up modems in the 1990s, improved from 2.4kps to 56 kbps, with each generation of modem profiting from the experience gained from the previous generation. Similarly with DSL technology, experience gained from ADSL1 (ITU-Recommendation G.992.1) lead to ADSL2 (ITU-T Rec G992.3) and ADSL2+ (G.992.5).

The specification for ADSL2 raised the maximum speed with some coding improvements and the handshake procedures between the modem and DSLAM were tightened and mandated leading to the ability of ADSL2 connections to adapt to line conditions dynamically and reliably. ADSL2+ effectively doubled the downstream bandwidth, thus doubling the downstream speed up to 24Mbps and if some options are invoked, up to 30Mbps. These speeds however are only possible on short lines, and by about 4 km the max speed has reduced to pretty much the same as an ADSL1 modem, i.e. between 1.5 and 2Mpbs.

Another development introduced with ADSL2 is the Reach Extended option (G.992.3 Annex L). This option reduces the frequencies used by turning off the higher tones.  This has the affect of concentrating the power of the output amplifier into a smaller band allowing the PSD to increase by up to 5.1 dbm/hz. Because line attenuation increases with frequency, the lower the frequency used, the further the signal will travel. While READSL2 has considerable lower maximum speeds on short lines, the speed diminishes more slowly and by about 3.5 km it is similar to ADSL2 (up to 2.5 Mbps), and actually faster than ADSL2/2+ for line lengths above 3.5 km.

Future developments include VDSL2 with maximum speeds up to 100Mbps, but rapidly diminishing over distance, so that the useful range is measured in hundreds of metres rather than kilometres.


2. ADSL2+ PTC Specification and process

Telecom New Zealand is currently trialling ADSL2+, and it is now appropriate that ADSL2+ modems be tested and Telepermitted. The new Specification is PTC273:2007, and this replaces PTC270:2000 which will be withdrawn. The new Specification is very similar to the PTC270 with the following differences:

Each modem now stores Vendor Information which is interrogated by the DSLAM when the modem is first connected. This also existed in ADSL1 but is now a mandatory requirement. As part of the testing, this information will be extracted from the DSLAM and recorded. Telecom sees this as an important tool for fault diagnostics, and should help differentiate between modem specific issues from network specific issues. It should also be able to identify modems which are running old firmware, from which corrective action can easily be taken.(see PTC273 clause 6.1)

There are now 4 distinct modes of operation:

1. ADSL2+ G.992.5 Annex A

2. ADSL2 G.992.3 Annex A

3. ADSL2 Reach Extended G.992.3 Annex L Mask 1

4. ADSL1 G.992.1 Annex A

The upstream PSDs of all of these modes are now tested. The test procedure is similar to that used in PTC270. (see PTC273 clause 6.2)

There is now a requirement for external power supplies to be labelled so that they can be positively identified as belonging to a specific modem. It has been found that substituting the original power supply with generic ones of the same rating can substantially alter the performance because of different noise characteristics of the power supply. (see PTC273 clause 6.4)

One additional test has been added to confirm that the modem behaves satisfactorily in the presence of incoming ringing on the POTS line, and on-hook/off hook transitions. (see PTC273 clause 6.6(4))

If any new ADSL1 modems are submitted for testing, they will be tested against PTC273, and will be granted a PTC272 Telepermit on the basis of compliance with the ADSL1 specific requirements.


3. Interoperability/Performance

Overseas experience has shown that modem performance can vary considerably, resulting in comprehensive interoperability and performance specifications being written, which require modems to be tested against a wide range of network conditions. Telecom will shortly be publishing a second ADSL PTC specification, PTC275, which will cover requirements for interoperability and performance in addition to the requirements of PTC273. Full compliance with PTC275 will indicate that the modem not only performs well over a range of network conditions, but is able to follow the correct negotiation procedures with the DSLAM to get the optimum performance for a given set of conditions. Compliance with either PTC273 or PTC275 will enable modems to be connected to the Telecom network, although Telecom will encourage modem suppliers to use PTC275 compliance as an indication of good performance as part of their marketing communications.


4. Timing

Exact timings will be announced in the next newsletter. The timings include:


5. Splitters and House Wiring

As modem speeds increase, the quality of the network becomes increasingly important. The cable attenuation higher at high frequencies, so the higher frequencies used by ADSL2+ to obtain the additional speed are attenuated to the point that beyond about 4km, there is insufficient signal at the higher frequencies to show any performance advantage over ADSL1. This is for ideal conditions! Any impairment that degrades the signal further will further reduce the speed at any given distance from the DSLAM.

Experience both here and overseas has shown that the quality of premises wiring is variable, and can have a big effect on the final speed. The best way of nullifying the effect of house wiring is to isolate it by the use of a splitter at the demarcation point between the network and house wiring. The ADSL modem is connected directly to the incoming line, and the house wiring is connected behind the splitter, which is essentially a high performance low pass filter, and which is also designed to negate the effect of poor balance of the house wiring. This is of particular importance in New Zealand, where the inherently unbalanced 3-wire house wiring was standard practice from 1983 to 1996.

When ADSL was introduced, all installations used a splitter, but the expense of having a splitter installed was seen as slowing the uptake of ADSL. Although originally intended for ADSL lite, line filters were introduced in 2001. These fit between each jackpoint and item of CPE (Phone, Fax, Modem etc), and have the huge advantage that they can be simply installed by the customer. Filters are tested against PTC280.

The advantage and disadvantage of using filters is that the ADSL modem can be plugged into any jackpoint, but the ADSL signals are carried over all of the house wiring. Wiring practices which were adequate for voice frequencies, are often inadequate for frequencies up to 1 MHz for ADSL1 and even more of a problem for frequencies up to 2.2MHz used by ADSL2+.

Two initiatives are underway to improve this situation. Firstly, we are in the process of developing a PTC specification for splitters, and secondly we are looking house wiring practices, and how these can be matched to the requirements of the next generation services. PTC106:2006 covers the recommended wiring practice for new houses, but there is a need to address the range of practices whihc are used in existing houses which may have been wired any time over the past 50 years.

In conclusion, the introduction of PTC275, CPE Splitters, and new house wiring practices should all contribute towards significant improvements in broadband performance.



Richard Brent
Access Standards