Antecedentes de la investigación .1 Antecedentes internacionales

D. Kagklis

Hellenic Telecommunications Organization S.A., Greece S. Androulidakis

Hellenic Telecommunications Organization S.A., Greece G. Patikis

Hellenic Telecommunications Organization S.A., Greece T. Doukoglou

Hellenic Telecommunications Organization S.A., Greece

IntroductIon

Recent reports on market trends indicate that half the households in the United States intend to subscribe to next-generation broadband access in the immediate future, marking a shift to data rates in the range between 20Mbps to 100Mbps, as Seals (2006) states. According to the same studies, broadband penetration will rise to 75% by 2010, while at the same time, 10 to 20% of U.S. households will have subscribed to the highest access speeds. This data reflects the general tendency toward an increasing demand for more bandwidth, keeping pace with increasing computing power and memory/storage capacity.

The growing demand for higher speed broadband access connections is definitely connected to the emer-gence of high-quality bandwidth-consuming services such as massive-scale IP video transmission or multi-point video-conference. Examining the trends in the European market, Seals (2006) claims that it is evident that an increasing number of network access providers have already launched or plan to launch TV services over their respective IP networks (IPTV service). It is estimated that 40% of households in the U.S and Europe will have access to IPTV by 2010, while as much as 20% will have access to High-Definition IP-TV (HDTV quality video). To meet the inflated bandwidth needs at the network access, standard residential connections should be able to offer between 20 to 30Mbps in the downstream direction, which translates into support-ing two HDTV compressed streams concurrently or a range of standard-definition channels, together with high-speed Internet access and voice over IP (VoIP).

At the present time, ADSL is the most widely de-ployed DSL flavor in the world. It is however incapable of supporting the necessary speeds for offering IPTV and HDTV services, as it can only offer up to 8Mbps broadband connectivity, subject to subscriber loop conditions. A newly proposed standard, the ADSL2+

technology, providing greater reach, higher bit rates, and added quality of service facilities, is now emerging as a possible solution, as new equipment implementing this ITU standard is being deployed in commercial networks.

Furthermore, the ADSL2+ technology also implements the reach extension model (ITU-T G.992.3 Annex L, 2002), as well as the increased upstream option (ITU-T G.992.3 Annex J, 2002), referred to as ADSL2+ Annex J/M (ITU-T G.992.5 Annex J/M, 2003). Thus, the main advantages of the new technology are its potential for greater reach and higher speed. The increased rates enable the so-called triple-play service concept, con-sisting of converged Internet-access, telephone, and TV/VoD services over a standard telephone line. In this context, access rate offerings above the standard ADSL range of 3-6 Mbps downstream are gradually becoming available, while in certain countries access rates above 20 Mbps are already becoming the norm.

The greater reach allows more customers to enjoy such services over their local loop. However, just as in the case of standard ADSL, the actually achievable rate is subject to the condition of the subscriber loop.

This chapter presents and evaluates the ADSL2+

technology with respect to its physical layer perform-ance, which provides the foundation for supporting high-quality residential triple-play services on a mas-sive scale.

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ADSL2+ Technology

BAcKround

Over the past few years, there has been a substantial increase in the use of the Internet and its numerous applications by an ever swelling and ever more de-manding residential user community. The widespread adoption of the Internet among non business/corporate users has been powered by the emergence of low-cost high-speed residential broadband access and the great popularity of a number of related high-end applications as Androulidakis, Kagklis, Doukoglou, and Skenter (2004a) stated.

To support the extra traffic generated by the numer-ous residential clients, backbone network infrastructures have evolved to a level capable of sustaining high volumes of traffic belonging to bandwidth-intense applications. On the other hand, however, the cur-rently mass-deployed residential broadband access technologies are considered inadequate for supporting the increased needs of high-end multimedia/audio-visual services.

Since it first became available, the ADSL technol-ogy (ITU-T G.992.1, 1999) has become the dominant technology for residential broadband access, deployed on a massive scale in order to address the Internet-access needs of the majority of residential end users. The data rates supported by ADSL and currently offered by most access service providers typically fall in the range of 512Kbps up to 8Mbps for the downstream direction, and 128Kbps up to 768Kbps for the upstream direc-tion, with the actual figures depending on a variety of technology- and business-specific considerations. Even though such speeds represent a significant improve-ment over typical PSTN/ISDN-based connectivity (i.e., 56/128Kbps), a typical ADSL line is unable to provide users with enough bandwidth for upstream-intense services (e.g., peer-to-peer networking) and ultra-high-quality audio-visual content access (e.g., HDTV).

Indeed, it is a fact established by monitoring current network usage practices that the majority of residential users (i.e., “mainstream” users) require quasi-symmetric access, with high upstream and downstream bitrates, in order to enjoy such services as bi-directional video conferencing, peer-to-peer networking, or online en-tertainment. Furthermore, Wolfe (2003) claims that the emergence of HDTV-based applications is expected to give rise to a category of “power” residential users, who will require very high downstream access rates in order to enjoy premium services. In this context, the ADSL

technology as it stands today is not a suitable choice for addressing the needs of both “mainstream” and “power”

users and their respective “profile” services.

With this in mind, enhancements were made to the original ADSL technology that produced ADSL2 as an immediate successor (ITU-T G.992.3, 2002). ADSL2 offered higher bitrates in both the downstream and upstream directions by using a more efficient G.DMT-based modulation mechanism, as described in the Aware White Paper (2003). The new modulation scheme was applied to the same frequency band as in the original ADSL standard (i.e., up to 1,1MHz), thus ensuring backward-compatibility with existing equipment.

However, ADSL2 never achieved prominence and has come to be considered a transitional technology.

Consequently, further improvements to the original ADSL specification have produced the next successor standard referred to as ADSL2+ (ITU-T G.992.5, 2003).

This technology will be the focus of the present chapter, in which ADSL2+ will be described with reference to its predecessors. In the course of the description, the new features, improvements, and advantages of this new technology will be presented, while physical-layer performance measurements will be used to provide a better understanding of what ADSL2+ has to offer.

FEAturES, IMProVEMEntS, And AdVAntAGES oF AdSL2+

This section is organized in two sub-sections, the first one describing and the second one evaluating the ADSL2+ technology.

The technology description offers an overview of functions, features, and improvements incorporated in the ADSL2+ specification. The evaluation is based on a performance and quality comparison between ADSL2+ and its predecessor technologies, on the basis of experimental results.

the components of the AdSL2+ technology

The ADSL2+ technology was specifically designed to offer almost three times the downstream rate offered by ADSL, as well as a substantially increased upstream rate. At the same time, backward compatibility with existing customer premised equipments, providing re-duced-speed connectivity, was also a prime concern in

formulating the new standard. To achieve these goals,

A

ADSL2+ employs a dual approach; firstly, by adopting the improved modulation mechanism of the ADSL2 standard (ADSL-compatible), it improves the bit car-rying efficiency per sub-carrier frequency; secondly, by doubling the downstream spectrum bandwidth¹ of the ADSL/ADSL2 specifications, it effectively doubles the downstream bitrate it can offer. Both these choices are intended to endower ADSL2+ with greatly improved access rates, while maintaining compatibility with both the ADSL and ADSL2 standard.

Furthermore, ADSL2+ has included the reach exten-sion model described in the ADSL2 Annex L standard (studied by Ouyang, Duvaut, Moreno, & Pierrugues, 2003), as well as the increased upstream option intro-duced in the ADSL2 Annex J standard. The latter feature has been included as the Annex J/M of the ADSL2+

standard (ITU-T G.992.5, 2003). The importance of these two features is demonstrated by Bong, Taek, Ginis, and Cioffi (2002) and Androulidakis, Kagklis, Doukoglou, and Skenter (2004b) in their respective work. The former has shown that an ADSL2+ Annex L transceiver can deliver an improvement in reach for distances beyond 5.5km, allowing for broadband connectivity even in remote areas. The latter has dem-onstrated how an ADSL2+ Annex J/M transceiver can offer the necessary high upstream rates for customers needing low-cost symmetric broadband access similar to a G.SHDSL service (ITU-T, G.991.2, 2001).

The ADSL2+ standard has incorporated other im-provements, including:

• Better diagnostic methods such as SELT described in the Texas Instruments White Paper (2003), or DELT, (ITU-T G.992.5, 2003)

• Improved power management capabilities (the L0/L2/L3 power management scheme) as well

• asLine bonding functionality based on an ATM IMA layer, similar to the method described by the Aware White Paper (2004) and evaluated by Androulidakis, Kagklis, Doukoglou, and Sykas (2005)

All these features indicate that the new standard holds much promise, especially in view of the move to bandwidth-intense multimedia-based services targeted at the residential market. Given the rising demand for advanced “triple play” services, which the current ADSL

technology cannot fully support, and the fact that many content providers seek entrance into new markets (e.g., DVD- or HDTV-quality video broadcasts) through IP-based networks, the move to ADSL2+ technology seems perfectly justified. It is expected that deploying ADSL2+ on a massive scale for residential broadband access will address the inflated bandwidth requirements in the access network at a low cost per customer.

Evaluation of the AdSL2+ technology The following evaluation of the ADSL2+ technology is based on actual performance measurements regarding the synchronization of ADSL2+ ATU-R/ATU-C trans-ceiver pairs in a noise-free environment. The perfor-mance of ADSL2+ is assessed according to increasing subscriber loop length, with respect to bitrates offered in both the downstream and upstream directions.

All measurements were performed in-lab, using a Globespan-based ADSL2+-compliant DSLAM (Er-icsson, 2003) and two modems; a Broadcom chipset ADSL2+-compliant modem (Ericsson 410, support-ing Annexes A, L, J/M through configuration) and a Globespan Virata chipset standard ADSL modem (Lancom 821). Therefore two combinations were tested: an “ADSL2+ ATU-R/ATU-C” (ADSL2+) link and an “ADSL ATU-R/ADSL2+ ATU-C” (ADSL) link. A Telebyte Inc. 458-LM-HDE 0.4mm copper line emulator was used to observe the behaviour of the test links according to increasing subscriber loop lengths, ranging from 300m up to 6600m in steps of 300m. For each individual loop length, the corresponding bitrate values, for both the downstream and upstream directions of both test links, were recorded. These measurements were then crosschecked for validity, by performing a second set of measurements on real 0.4mm copper line lengths. The ATU-R/ATU-C training process had a fixed target SNR of 6dB (DSLAM equipment default) applicable to all measurement cases. This threshold represents a minimum for maintaining a low bit error rate and acceptable communication quality on the DSL access link.

The measurement results are presented in the fol-lowing figures, documenting the values acquired from the line emulator measurements, corroborated by the values acquired from measurements on the real 0.4mm copper loops (max divergence ~5%). Figures 1 and 2 demonstrate the decrease of downstream and upstream bitrates according to increasing loop length.



ADSL2+ Technology

The results in Figure 1 relate to the downstream bitrates for the ADSL2+ and ADSL test links. It clearly shows that the ADSL2+ technology consistently deliv-ers higher rates than ADSL, regardless of subscriber loop length. The ADSL2+ technology (in Annex A, L and J/M modes) almost triples the bitrate offered by the older ADSL at short loops. Moreover, at loop-lengths

over 3500m, the ADSL2+ technology consistently offers between 50% and 100% more bandwidth than ADSL. At the same time, ADSL2+ manages to sustain an active link over a longer subscriber loop, exceed-ing the ADSL loop length limit by a further 300m for ADSL2+ Annex A and 750m for ADSL2+ Annex L, respectively.

Figure 1. Downstream bitrate performance

Figure 2. Downstream bitrate performance

Similarly, the results of the upstream performance

A

measurements on the test links show that the upstream bitrates offered by ADSL2+ (Annexes A and L) exceed those offered by ADSL, which is in accordance to their respective specifications (Figure 2). The same figure indicates that the Annex J/M link can reach up to 3Mbps in the upstream direction, thus offering about 225%

more upstream bandwidth than the ADSL2+ Annex A and Annex L links, or almost three times more up-stream bandwidth than the ADSL link. This means that ADSL2+ Annex J/M can be used to support customers needing a symmetric broadband access service, even being able to substitute for the G.SHDSL technology for corporate access.

The upstream measurements also reveal that when connecting a standard ADSL modem to an ADSL2+-compliant DSLAM port, even when the latter is operat-ing in standard ADSL mode, the upstream link capacity will clearly exceed the upper limit of ~800Kbps of a pure “ADSL ATU-R/ATU-C” link. In fact, the upstream bitrate was measured consistently above 900Kbps at loop lengths up to 3300m, possibly achieving an up to 32% increase over the pure ADSL link, as calculated by Androulidakis et al. (2004a).

FuturE trEndS

Today ADSL2+ can be considered a mature technol-ogy as more and more ADSL2+ chipsets are developed and marketed by the leading manufacturers, including Globespan, Broadcom, and others. Almost all major access equipment manufacturers, including Alcatel, Ericsson, Paradyne, UTstarcom, Huawei, Siemens, and others, have made available ADSL2+-compliant DSLAMs and modems, continuously expanding the range of available products and incorporating more so-phisticated features (e.g. Annex B for access over ISDN lines or and Annex J/M for symmetric services).

As “triple play” services become commonplace among residential customers, directly competing against traditional media networks (e.g., high-quality video broadcast over terrestrial or satellite), ageing ADSL-based broadband access networks will be upgraded to ADSL2+. However, the adoption of ADSL2+ as the access technology of choice does not immediately enable high-end multimedia-rich services for all. The measurement-based evaluation of the technology has shown that the performance of ADSL2+ is highly

dependant on the loop length, just as all the various flavours of the DSL family. Even the extended-reach feature, specifically developed to counter the adverse effects of increased loop lengths, only offers about 500m of extra coverage. Furthermore, even though HDTV-based services can (marginally) be offered over ADSL2+ broadband access, even more advanced future Super HDTV- and Ultra HDTV-based services can certainly not. The bandwidth needed by the latter two may exceed by a large margin the ADSL2+ limits and indeed the limitations of copper-based broadband access.

In the short term and in view of the limitations of ADSL2+, a new DSL access technology has been de-veloped as an alternative to a full Fibre-to-the-Home access solution, namely the VDSL2 technology. Its short distance reach however requires a supporting Fibre-to-the-Curb network, which, in most countries, is only now beginning to materialise.

concLuSIon

This chapter studies the new ADSL2+ technology, de-scribing the new functionality it offers and comparing its performance against the older ADSL technology.

The comparison is based on a set of physical layer measurements, conducted in a controlled laboratory environment. The physical layer speeds achieved by each technology were measured at increasing loop lengths without the presence of noise. At the same time the corresponding SNR values were also measured in each case. The charts plotting the measured bitrates and SNR values according to length, for both the ADSL technology and three different flavours of ADSL2+, offer a visual perspective of ADSL2+ performance and how it relates to ADSL. Comparing the results in these charts, it can be concluded that ADSL2+ offers higher access rates as loop length increases.

All in all though, the greatest advantage of ADSL2+

is the high downstream rate it can offer over a single copper loop. With 24Mbps of downstream bitrate, ADSL2+ is ideal for offering bandwidth-intense multimedia services on a massive scale. Furthermore, ADSL2+ in the Annex J/M flavour can support sym-metric connections at speeds up to 3Mbps, which is better than the G.SHDSL technology. However, HDTV or better services may not be effectively offered over ADSL2+, mainly due to coverage issues. Even though



ADSL2+ Technology

it is expected that ADSL2+ will be the next dominant broadband access technology for the residential market, ultra-high-quality audio-visual services will have to be offered through VDSL2 (either cell-mode or frame-mode) access connections, studied by Walko (2005), which offers backward compatibility with all previous DSL flavours, or an all optical access network (passive or active optical networks).

AcKnoWLEdGMEnt

The authors would like to thank the Research Depart-ment of the Hellenic Telecommunications Organization S.A. (OTE) for allowing the use of the Broadband Access Laboratory as well as the use of the equipment used for the measurements.

rEFErEncES

ADSL2 and ADSL2+: The New ADSL Standards, Revision 3. (2003). Aware White Paper.

Androulidakis, S., Kagklis, D., Doukoglou, T., &

Skenter S. (2004a). ADSL2: A sequel better than the original? IEE Communication Engineer Magazine, 2(3), 22-27.

Androulidakis S., Kagklis D., Doukoglou T., & Sken-ter S. (2004b). ADSL2 vs. SHDSL for symmetric broadband access networking. IEE Electronic Letters, 40(16), 1027-1029.

Androulidakis S., Kagklis D., Doukoglou T., & Sykas E. (2005). Bonding techniques for symmetric services over ADSL and ADSL2/2+. In Proceeding of the IEEE Eurocon Conference (Vol. 2, pp. 1770-1773).

Bonded ADSL2+ New Standards for Multi-Line Ser-vices. (2004). Aware White Paper.

Bong, S. K., Taek, C. S., Ginis, G., & Cioffi, J. M.

(2002). Dynamic spectrum management for next-gen-eration DSL systems. IEEE Communications Magazine, 40(10), 101-109.

Ericsson EDN-110 IP DSLAM. (2003). Retrieved 15^th March 2003 from http://www.ericsson.com/

ITU-T G.992.1. (1999). Asymmetrical Digital Subscrib-er Line (ADSL) TransceivSubscrib-ers. ITU-T standard SSubscrib-eries

G: Transmission Systems and Media, Digital Systems and Networks, Digital transmission systems—Digital sections and digital line system—Access networks.

ITU-T G.991.2 (2001). ‘Single-Pair High-speed Digital Subscriber Line (SHDSL) Transceivers’. ITU-T standard Series G: Transmission Systems and Media, Digital Systems and Networks, Digital transmission systems—Digital sections and digital line system—Ac-cess networks.

ITU-T G.992.3. (2002). Asymmetric Digital Sub-scriber Line (ADSL) transceivers 2 (ADSL2). ITU-T standard Series G: Transmission Systems and Media, Digital Systems and Networks, Digital transmission systems—Digital sections and digital line system—Ac-cess networks.

ITU-T G.992.5. (2003). Asymmetrical Digital Sub-scriber Line (ADSL) transceivers—Extended band-width ADSL2 (ADSL2+). ITU-T standard Series G:

Transmission Systems and Media, Digital Systems and Networks, Digital transmission systems—Digital sections and digital line system—Access networks.

Ouyang, F., Duvaut, P., Moreno, O., & Pierrugues, L. (2003). The first step of long-reach ADSL: Smart DSL technology, READSL. IEEE Communications Magazine, 41(9), 124-131.

Seals, T. (2006). Expanding the value of copper.

Xchange Magazine. Retrieved January 2, 2006, from http://www.xchangemag.com

Single-Ended Loop Testing (SELT) Expectations and Realities. (March 2003). Texas Instruments White Paper.

Walko, J. (2005). Click here for VDSL2. IEE Com-munications Engineering Magazine, 3(4), 9-12.

Wolfe, A. (2003). HDTV—Ready for the long drive?

IEEE Spectrum, 40(6), 13-15.

KEY tErMS

ADSL: Asymmetrical digital subscriber line.

ATU-C : ADSL transceiver unit--centre.

ATU-R: ADSL transceiver unit--remote.

DELT: Dual ended line test.

A

DELT: Dual ended line test.

A