Report on Global xDSL Standards from the ITU1

By Stephen Palm (

In the September issue of The TeleChoice Report, the global ITU xDSL standards were introduced in the context of ANSI T1.413 issue 2. This month we will take a detailed look at the ITU Study Group 15 G series Recommendations (G.lite, G.dmt, etc.) that provide a set of worldwide interoperable ADSL standards. The most noteworthy specification of the group is the G.lite, "splitterless" ADSL, - that will allow a significantly easier installation.

The ITU (International Telecommunication Union) is the standards body composed of experts from international manufacturers and telecomm service providers dedicated to developing standards usable throughout the world. Many people are familiar with the ubiquitous V-series voiceband modems such as V.90 (56kbit/sec) and V.34 (33.6kbit/sec) with an installed base of over 100 million units developed by ITU Study Group 16. Study Group 15 (SG15) is the group tasked with developing transport networks, systems, and equipment and is the lead study group for Access Network Transport. The subgroup referred to as "Question 4" in SG15 is responsible for developing the equipment standards for subscriber access, in other words, xDSL standards.

During 1998, approximately 100 experts met six times to develop a set of specifications that define the line code, activation, and management for ADSL, splitterless ADSL, and HDSL modems2. In October 1998, SG15 accomplished the first level approval of the specification known in ITU-speak as "determination". This indicates the material has reached technically stability. After editorial review in January 1999, the specifications are scheduled to gain the final approval in early July 1999 (known in ITU-speak as "decision"). Although the ITU terms its specifications as "Recommendations", they are essentially international standards.

The Rapporteur for Question 4 is Dick Stuart from 3com. The myriad of details for specifying the various components of ADSL has been divided into 6 Recommendations shown in the table below. The G.992 series specifies the basic components of the ADSL modems that people usually think about when they think of a xDSL specification. The G.994, G.996, and G.997 series are the support Recommendation for the G.992 series. Finally, the G.995 series provides an overview of all of the Recommendations in the G.99x series.

Let's take a look at each of the Recommendations in a little more detail.

ITU ADSL Recommendations
Rec. No. Recommendation Title Editor
Asymmetrical Digital Subscriber Line (ADSL) Transceivers S Palm
Splitterless Asymmetrical Digital Subscriber Line (ADSL) Transceivers C Hansen
Handshake procedures for Digital Subscriber Line (DSL) Transceivers L Brown
Overview of Digital Subscriber Line (DSL) Recommendations S Abbas
Test procedures for Digital Subscriber Line (DSL) Transceivers M Tzannes
Physical layer management for Digital Subscriber Line (DSL) Transceivers A Johansson


G.992.1 (formerly known as G.dmt) is probably the easiest to understand since it is the closest to ANSI T1.413 issue 2. In fact, much of the text of G.992.1 comes directly from T1.413. It describes Asymmetrical Digital Subscriber Line (ADSL) modems on a metallic twisted pair that allows high speed data transmission between the network operator end (ATU-C) and the customer end (ATU-R). However, G.992.1 has several important distinctions from T1.413 that help make it useable in more parts of the world and increase its robustness.

The first distinction is the separate sections that contain localization specifications in addition to the main body specification. The localization specifications are contained in 3 annexes and address these three scenarios:

  1. The traditional ADSL transmission simultaneously on the same pair with voice (band) service.
  2. ADSL transmission simultaneously on the same pair with ISDN services typically found in several European countries.
  3. ADSL transmission on the same pair with voice-band services but with TCM-ISDN services in an adjacent pair. The TCM-ISDN services found in Japan cause a particularly difficult type of crosstalk interference into ADSL transmission, so special techniques are required to combat it.
These localization variations allow manufacturers and purchasers to have a single specification with a toolbox of options to tailor a DSL modem to the local conditions. These are in addition to other options in G.992.1 (and T1.413) that allow trade-offs in performance, electrical power requirements, and price points. In many cases, the benefits of G.dmt can be realized with a simple software upgrade to existing ADSL modems since it was a design goal of G.dmt to retain as much T1.413 technology as possible.

The second distinction of G.992.1 is its use of an enhanced activation or startup sequence, G.994.1. Instead of a single tone being used to indicate optional features supported by a DSL modem, several tones digitally transmit the same information. This makes G.994.1 very robust and allows a DSL modem to activate under a wide range local and environmental conditions. The direct benefit to G.992.1 is that modems have a much better chance of activating, so G.992.1 service is more likely to be available. G.994.1 has some other service features I will discuss later in this article.

G.lite: Splitterless ADSL

The G.lite or splitterless ADSL modem work was the high profile topic during 1998 resulting in draft Recommendation G.992.2. G.lite (G.992.2) is in the same series as G.dmt (G.992.1) because they are very similar. However G.lite brings two very important distinctions that are critical for consumer or mass market deployment: splitterless operation and power management.

One of the often lamented issues when installing a traditional DSL modem is the necessity of a truck roll to install a separate filter box, known as a "splitter", at the customer premises. The splitter would separate the phone wire into a (high frequency) portion for the DSL modem and the other portion would continue to be used for the voice telephone service. A technician would be required to install the splitter in a location away from the modem, typically where the phone wires entered the home.

A G.lite DSL modem would typically not need a splitter and could be installed by a consumer in much the same way as V.34 or V.90 voiceband modems are installed today. To help improve the sharing of the single phone wire by the DSL modem and telephones, the customer in some cases may want to improve the DSL modem speed or reduce noise by installing a micro filter near individual phones. Use of micro filters would be decided by individual preferences and would be simple to install: plug in a small adapter between the phone and where it connects to the wall.

In order to combat voice and DSL services interfering with each other when splitters or filters are not installed, fast retrain techniques were developed for DSL modems. Fast retrain allows the DSL modems to change their operating characteristics very quickly in order to adapt to such events as the telephone ringing or someone placing a telephone call. Those seemingly harmless events would have had a devastating effect on previous generation DSL modems.

While splitterless improves the installation situation, the other feature of G.lite, power management, improves the long-term day to day operation for the service provider. Service providers are typically concerned about the amount of electrical power consumed and the amount of transmitted power from all of the DSL modems in their service area. When the DSL modems are transferring lots of data, they can consume and transmit relatively large amount of power. However, the typical consumer user is not transmitting data all of the time, so power management is an effective tool for reducing power. This is very similar to the power management tools found in PCs and laptops today: screen savers, suspend to RAM, and suspend to disk. The DSL modem can be "woken up" and resume transmitting large amounts of data in a very short amount of time.

Most of the technical improvements incorporated in G.lite are a result of the great work from the UAWG.

Handshaking for DSL modems (G.hs)

G.994.1 is the new common handshaking or activation procedure for DSL modems. It is fully interoperable with the activation procedure for T1.413 DSL modems but offers several benefits for service providers: robustness, exchange of capabilities information, notification of the application and service requests, backwards and forwards compatibility for other DSL modem standards, etc.

As mentioned earlier, previous generation DSL modems relied on single tones to negotiate the modem operating options during startup. With G.994.1, simultaneous multiple redundant sets of tones are modulated with digital information. If one or more tones are blocked by interfering noise, bridge taps or otherwise unavailable, at least one will be available to complete the startup information exchange. The tones were selected and the procedures were designed to be compatible with the many other existing services including other DSLs in the network.

The digital information carried by G.994.1 can now go beyond the simple selection of options in previous generation DSL modem startup procedures. Higher layers on each end of the DSL modems can indicate service and application requirements during the handshake so that the appropriate DSL Recommendation from the family of DSL Recommendations will be activated following the G.994.1 handshake. This allows providers to flexibly tailor the service bandwidth and latency requirements of the customer's dynamic applications and environments including mobile DSL modems or modem pooling. The G.994.1 data architecture is flexible enough to allow exchange of new or non-standard information without upgrading the installed software base.

Testing procedures (G.test)

Recommendation G.996.1 describes the testing procedures for ITU DSL modem Recommendations. This is useful for manufacturers during the design and development phase. It does not describe the procedures for installing DSL modems, but a product designed using the procedures will help ensure consistent installation results. G.996.1 does not contain the performance criteria for DSL modems. Instead, individual DSL Recommendations call out various performance and loop reach criteria based on the information in G.996.1. Test loops and in-home wiring models are specified for different regions of the world for use during DSL performance testing.

Network management (G.ploam)

Recommendation G.997.1 continues the physical layer management protocol for ADSL transmission systems that was initiated with the T1.413 standard. It specifies control data exchange on a sub-channel of the DSL modems. It uses a MIB on the SNMP protocol to easily integrate with existing network management systems.

Technical Overview of the DSL modem Recommendations (G.ref)

Recommendation G.995.1 provides a reference and overview of the DSL family of Recommendations. Readers who are interested in a more technical overview of the ITU xDSL Recommendations are encouraged to consult G.995.1. It contains a description of each of the Recommendations and how they relate to one another. Further it provides generic reference and configuration diagrams that are expanded to show the details for each Recommendation and the interrelationship. The text of the Recommendation is approximately 15 pages and includes a glossary of xDSL terms and mnemonics.

Future ITU DSL Recommendations

Looking to the future, Question 4, embarked on the study of two new Recommendations in the October 1998 meeting. The first project, currently referred to as G.vdsl, will look to develop a standard in the 10's of Mbit/sec range for short copper loops. This is useful for environments where fiber optical cable passes near homes, but the last access into the home passes over copper wires. The second project, G.shdsl, will be a technology improvement of G.hdsl (G.991.1) that will work on a single pair. This work is envisioned to improve the T1 and E1 line deployment for business customers.

Stephen Palm is a senior staff engineer with Matsushita (Panasonic) Graphic Communication Systems in Tokyo. He is also the editor of ITU Recommendation G.992.1 (G.dmt) and can be contacted at

Note 1: Appeared in the December 1998 issue of The Telechoice Report on xDSL.

Note 2: I use the term "DSL modem" to describe the box or card performing an xDSL. The ITU chose to call them "DSL transceivers" because a different study group (SG16) has claim to the word "modem" (which to some people indicated voiceband modems).