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ADSL: expediting Broadband Internet Access for society
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Unit: Benelux section
Senior Officer Name: Claude Oestges
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Unit: Benelux section
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IEEE Section: Benelux section
IEEE Section Chair name: Claude Oestges
Proposer name: Wim van Etten
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Long term vision In early 1990’s, the focus of the telecom industry for fixed access evolution was on Fiber-to-the-Home, with promise of near infinite bandwidth capabilities and future safeness. Hence, early proposals for ADSL were initially perceived as being only a “stopgap solution” and sometimes depicted as “only a niche segment”. The Antwerp team, supported by the Alcatel management, had a strong believe in the potential of this technology that could bring broadband to the masses much earlier than fiber and in an affordable way. Alcatel Antwerp also had the research, the analog and digital chipset design capabilities and other technical know-how for successful integration and productization. History proved that “the stopgap solution” would be of benefit to hundreds of millions of families across the globe. Raising the development funds, getting the development organized, commercializing and ramping up production of such innovation turned out to be a hard challenge.  provides a good overview of some of the hurdles that had to be taken, including the organizational and business management ones. Technical innovation At the time of the invention the technology was cutting edge, and still much research and massive technical effort was needed to develop and integrate the technology such that it was applicable in a reliable and performant way on most of the available copper telephone wires in the world. Telephone lines had been installed since the end of the 19th century. Although the largest volume was installed in the 20th century, most of these telephone cables were several decades old and sometimes degraded or impaired. There was also – and there still is – a wide variety of cable types and topologies, with a mix of aerial and underground cables. Also, telephone cables had been designed and developed for the transport of voice with a bandwidth of only 3.4 kHz while ADSL used up to 1104 kHz (and later xDSL variants multiples of this). All these aspects created plenty of technical challenges. Below sections provide a sample of the many technical challenges that were overcome by the research and development team in Antwerp. Deep level of ASIC integration The used DMT technology was very computation intensive, hence a very high level of integration was needed to fit the ADSL functionality in the available silicon technology of that time (CMOS 0.7µm). The paper  describes many of the challenges that were overcome. By end 1994, Alcatel became the first company worldwide with an integrated DMT ADSL chipset, also integrating the then innovative ATM (Asynchronous Time division Multiplexing) technology in the chips . Figure 1 illustrates the first digital ADSL chip from Alcatel.
Figure 1. DASP, implementing the Physical Medium-Dependent layer: 325 mm2, 144 pins PPQFP, 4.8W, 130 kGates logic, 130 kbits memory, 1.5M transistors
By June 1995, the chipset got integrated in a first-generation ADSL DSLAM (Digital Subscriber Line Access Multiplexer) for deployment in the Central Office, and CPE (Customer Premises Equipment) (Release 1.0). This was an extraordinary achievement knowing that DMT only got selected as line code for ADSL in March 1993 and the very first ADSL standard (ANSI T1.413) dates from August 1995. As the chipset was developed parallel to the writing of the standard, this first chipset was not fully standards compliant. This got corrected in subsequent versions of the Alcatel ADSL chipset that quickly followed. In October 1995 Alcatel was able to demonstrate an end-to-end operational system with Video-On-Demand over ADSL at the Telecom Geneva fair, managed via the Alcatel Network Management Expert platform (Figure 2). By mid-1997, Alcatel had its first product ready for volume deployment (Release 2.3).
Figure 2. video-on-demand over ATM-based ADSL at Telecom Geneva, October 1995 The high level of integration allowed to achieve cost points that enabled the large-scale deployment of ADSL across the globe. At the end of the nineties, end-to-end equipment volume pricing below $200 per subscriber turned out to be possible, successfully enabling the market. Development and tuning of the DMT ADSL technology The Alcatel Antwerp team did much research on and tuning of the DMT technology such that it could be applied to all real-life copper lines, with often very different characteristics , with strong frequency dependent attenuation, with interference between adjacent pairs in the same cable , susceptible for pickup of (semi-)stationary alien noise and impulse noise , with single pair faults , or with imperfections in the topology . The intrinsic copper cable impairments (e.g. attenuation, interference, and susceptibility for noise pickup) can get worse as the result of ageing or water ingress . These advances of technology were situated in many different domains, including efficient multiplexing and solutions for DMT clipping, equalization, synchronization, error correction, optimal power allocation, and energy efficiency. Duplexing The ADSL standard allowed for two duplexing variants: Echo Cancelling (EC) and Frequency Division Duplexing (FDD). Selecting EC with the wide downstream band overlapping the narrow upstream band was very tempting, as it promised better downstream performance. The Antwerp team analyzed both options in detail in terms of performance and integration complexity and concluded that FDD with frequency separated upstream and downstream bands was the better pick . It turned out that the more modest FDD choice was the more viable one. It finally got adopted by the whole industry. Clipping DMT systems, that consist of the sum of many independently QAM modulated signals (sub-channels or carriers), inherently have a high crest factor that is defined as the ratio of the peak amplitude to the average (root mean square) amplitude value. Amplitude clipping reduces the dynamic range of the analog front-end by hard clipping the signal above a well-chosen amplitude. The effect of amplitude clipping on the DMT system performance was in depth analyzed by Alcatel researchers and the results were reported in . An expression was derived for the SNR degradation due to clipping. The publication describes how the noise introduced by clipping can be compensated for by a reduction of the quantization noise of ADC and DAC due to the resulting smaller dynamic range. Therefore, the same overall ADSL system performance can be obtained with less severe requirements for the analog front-end, with easier integration in silicon (ASIC), lower costs and lower power dissipation as major benefits. The outcome of this theoretical study was applied in the design of the Alcatel ADSL chipsets. Equalization The highly dispersive nature of telephone pairs would require very long guard bands between DMT symbols to minimize interference between consecutive symbols and between sub-channels within a symbol. This would result in intolerable reduction of the throughput efficiency. This could be avoided by developing advanced equalization schemes. As an example,  presented a novel optimization criterium for the Time Domain Equalizer (TEQ) and Frequency Domain Equalizer (FEQ), called Constrained Mean Square Error (C-MSE). It resulted in a 35% bitrate increase on an ADSL reference loop. Synchronization Synchronization and clock recovery are crucial elements in every modem design. In ADSL, the receiver locks its clock on the remote transmitter. In the Alcatel ADSL chipset, a unique approach has been adopted for clock recovery. To compensate for the frequency difference between the remote transmitter clock and the local receive clock, a ROTOR was implemented in the frequency domain and a sample SKIP/STUFF function in the time domain. The mechanism got patented by Alcatel  and implemented in the Alcatel ADSL digital chips . Many incremental improvements were studied (in collaboration with Professor Marc Moeneclaey at the Ghent University), patented and contributed to standardization. Alcatel also invented a method to transparently transport an incoming clock signal over a network segment . Alcatel brought this concept to the standardization committee T1E1.4 in November 1996 (T1E1.4/96-364). It got adopted by the ITU-T for the first international ADSL standard in 1999: G.992.1 (06/99). Continued innovation beyond ADSL ADSL was not the end of a road. Alcatel pursued its research to further boost transmission speeds over telephone pairs and became also a front-runner for the next generations of copper access technologies. The Alcatel research team developed a first DMT VDSL chip that was demonstrated at the Telecom Geneva 1999 exhibition, including video services, four years after the introduction of ADSL at the event.  provides a high-level description of this first VDSL chipset and its main characteristics.
Figure 3. First VDSL line card in 1999 based on the first-generation VDSL TDD DMT digital chip integrating all digital PMD, PMS-TC and ATM TPS-TC functions
The Alcatel Antwerp team kept innovating and rejuvenating the technology after the first step, continuously driving the performance limits to the technical boundaries, including less obvious domains such as optimal power allocation and energy efficiency. Over time, this led to ADSL2, ADSL2plus, VDSL, VDSL2, VDSL2 vectoring, and currently G.fast products with MGfast appearing at the horizon. Scalable product architecture based on ATM multiplexing technology The Alcatel ADSL solution provided a very low entry cost level for operators, with a DSLAM that could scale cost effectively from 4 lines up to 576 lines. In the mid-nineties, several equipment vendors used a system architecture with modem banks connected to a central router. Such architecture was not sufficiently scalable and was too expensive at the time. The Alcatel Antwerp team selected and developed a highly scalable and cost-effective ATM based DSLAM architecture named ASAM (ATM Subscriber Access Multiplexer). The hart of the architecture is the ATM based IQ-bus, a parallel bus providing 155 Mbit/s up- and downlink capacity. This turned out to be a good balance between cost and throughput requirements of that time. For operators, the upfront fixed cost investment turned out to be in the range of an extremely low $1 per household passed, with a very linear incremental cost as end-users were connected. No other fixed access technology could ever match this. Next to the ADSL Central Office equipment, Alcatel also supplied ADSL CPEs. To enable mass adoption these CPEs needed to be user friendly, compact, low power, 100% self-installable, and compatible with the then popular PC interfaces. Two successful products that fully matched these requirements were developed. An Ethernet based CPE (Figure 4) that reused the same chipset described above reached the right levels of cost and compactness. The product was easy to install for the end-user: basically, plugging a POTS splitter in a telephone socket (to isolate the telephone from the ADSL modem), connecting the POTS splitter with the modem by means of the supplied cable and connecting an Ethernet patch cord to the PC. In combination with a BRAS (Broadband Remote Access Switch) in the core network, the end-user start-up process was very similar to that of the then popular dial-up modems. But 100x faster. Having the start-up process the same as the legacy technology helped with the mass adoption of the product.
Figure 4. Ethernet CPE, introduced around 1996
Figure 5 illustrates the first USB based ADSL modem that was released by Alcatel in 1999. The design was iconic and a strong attractor at trade shows. The level of integration was un-surmounted at the time.
Figure 5. USB "Mantra" CPE introduced in 1999
Alcatel has been a key contributor to the standardization of ADSL and of all xDSL variants that followed as it valued the importance of common standards and technology that could be deployed on all copper lines around the world. Moreover, Alcatel licensed its DMT technology to 4 technology vendors, making it available for the entire telecommunications community. These efforts enabled a true mass market for ADSL, avoiding market fragmentation, and secured interoperability between the DSLAM and the CPE. Without a universally accepted and sustained solution, the economies of scale that drive prices down and bring technology within reach of the average end-customer would never have been achieved. Alcatel started to become a very active contributor to xDSL standards in 1993 and continued to do so till today. That is for more than 25 years. Alcatel has been taking up several leading roles in different standards bodies, e.g. Rapporteur of ITU-T Study Group 15, Question 4 that develops Recommendations for Transceivers for customer access and in-premises phone line networking systems on metallic pairs. Alcatel also acted as (co-)editor of several ADSL standards, e.g. the ANSI ADSL Issue 2 standard (T1.413 Issue 2) that was the first ADSL standard suited for mass deployment, and the later international ADSL2 and ADSL2plus recommendations (ITU-T G.992.3 and G.992.5, respectively). Around 1998 Alcatel decided to license its ADSL technology to different chipset suppliers. This was an unconventional decision as it gave Alcatel’s competitors access to its market leading technology. Key benefits and objectives were to get the technology accepted on a global scale and offer operators a choice of vendors for their equipment at both ends of the line. It gave Alcatel’s competitors the opportunity to integrate Alcatel’s standard compliant technology in their own systems, contributing to interoperability in the early days of ADSL. Through lump sums and royalties, it also helped Alcatel to sponsor its R&D investments. As ADSL became a mainstream technology for end-users to connect to the Internet from home, a large variety of ADSL CPE’s appeared on the market, from several brands and based on different ADSL chipsets. By 2002, Alcatel estimated that more than 400 different CPE models were connected to the Alcatel DSLAM central office equipment. As a result, an increased number of interoperability issues were detected. Such issues stemmed from a different interpretation or partial implementation of the ADSL standard. Alcatel was strongly committed to address such problems. To that end, Alcatel agreed with its customers on an interoperability policy. Its goal was to ensure that new Alcatel software releases continued to be interoperable with the operators’ installed base of CPE modems. For that purpose, Alcatel built an “interop wall” in its Antwerp lab that included hundreds of CPE types of its customers and against which extensive regression tests were run. Alcatel also facilitated public CPE interop qualification via the DSL-Forum, later the Broadband Forum (BBF), and set up private interoperability tests with leading ADSL suppliers, including but not limited to its own licensees. ADSL was the first in a range of residential DSL technologies. It had the biggest impact on society, as it enabled the step from narrowband Internet access (max 56 kbit/s downlink for voiceband modems, 144 kbit/s for ISDN), to true broadband access of several Mbit/s downlink at least. With an improvement of up to two orders of magnitude in one technology step, the service impact was enormous.
What obstacles (technical, political, geographic) needed to be overcome?
Around 1995, the Internet was still in its infancy, with around 20 Million active users. For most of those 20 Million users, the Internet experience was very different than today. It involved establishing a dial-up connection, which after a minute or so, provided a connection in the range of 14 to 34 kbit/s. Although such speeds allowed for basic Web services, none of today’s plethora of Internet and Cloud services were possible at all: YouTube, Maps, Netflix, Microsoft OneDrive, … A fundamental bottleneck was affordable and scalable access technology. Yes, fiber access technologies existed - either point-to-point or Passive Optical Networks, e.g. APON (ATM Passive Optical Network) - but FTTH (Fiber-to-the-Home) technology was not affordable on a large scale. The other available option was cable networks (coax based). It took till the late ‘90s until the DOCSIS 1.0 standard started to get traction (reference Wikipedia – Cable Modem). But cable only reached a limited part of the global population. If only a technology would exist that could provide Broadband Internet access over the legacy of 800 Million of copper telephone lines. Early 1990s, Digital Subscriber Line technology was in full development. Be it as part of the ISDN (Integrated Services Digital Network) standard providing 144 kbit/s over approximately 5km of copper (standardized in the mid to late 1980s), or as the HDSL (High bit rate Digital Subscriber Line) technology enabling symmetrical 2 Mbit/s or 1.5 Mbit/s over shorter loop lengths, with repeater capabilities (standardized by ANSI in 1994 and by ETSI and ITU in 1998). HDSL was mainly intended for high-speed connections to corporations and businesses. ADSL (Asymmetric Digital Subscriber Line) technology was in the prototyping phase, a.o. driven by Prof. John Cioffi, with a major milestone at the Bellcore ADSL Olympics early in 1993. At this competitive testing event the Discrete Multi-Tone (DMT) prototype of Amati (founded in 1991 by Prof. John Cioffi) showed its superiority over the single-carrier (CAP and QAM) technologies. This resulted in the selection of DMT as line code for ADSL by the ANSI T1E1.4 Committee in March 1993. This line code decision by ANSI was the start for the Antwerp based Alcatel team to accelerate its research and development of a scalable and affordable ADSL technology, that would revolutionize fixed access, and expedite Broadband Internet for society (). The historical significance of Alcatel’s ADSL achievements is also described in  that examines how Alcatel successfully explored and exploited the promises of Broadband Access Technology. In the period between 1993 and 1997, the Alcatel team invented and developed an end-to-end ADSL solution that had all the right capabilities to bring Broadband Internet to millions, and later multiple hundreds of millions of households. In parallel, Alcatel actively contributed to the standardization of ADSL in ANSI T1E1.4 and took up the editorship role of the Issue 2 of this standard (T1.413-1998 Issue 2, approved Nov. 1998). Moreover, from the beginning, Alcatel addressed all challenges inherent to large-scale deployment by creating an eco-system of partners, organizing interoperability events, licensing its technology and patents, facilitating end-user self-installation and enabling large scale manufacturing and multi-sourcing. These efforts established a viable ADSL business model for telecom operators, at an affordable price point and excellent service experience for the end-user. As a result of this, ADSL had a tremendous impact on the acceleration of global affordable Broadband Internet Access. Although the underlying technology itself is highly complex, Alcatel created a standard compliant product that was easy to deploy on the 800 Million legacy copper telephone lines present in the world. Alcatel’s ADSL took full benefit of the telecom operators’ already available infrastructure, i.e. the existing copper lines and the existing Central Offices. And the end-user could simply self-install the ADSL modem. No truck-roll or professional service was required to set up the connection at the end-user. No trenching, digging or other civil works were needed. The solution could be deployed in overlay on any existing type and brand of telephony central office switches, creating a truly open market, without restrictions and lock-in of legacy equipment and vendors. Moreover, the interface between the central office equipment and the end-user ADSL modem was fully standardized and open, creating a wide choice of end-user devices. Through a high level of integration, the Alcatel solution realized a breakthrough in cost, making the technology extremely affordable, also for lower income regions. With all these benefits, Telecom Operators around the world successfully deployed the ADSL technology, providing affordable Broadband Internet access to hundreds of millions of households around the world. The spread of the ADSL Broadband technology around the world was a key contributor to the further development of the Internet and World Wide Web. The ADSL technology provided the bandwidth capacity and low latency that was required to develop a wide range of applications and services. Including as example: high speed file download, IPTV and video streaming, videoconferencing, work at home, interactive gaming, cloud services, advanced advertising, surveillance services, … Thanks to the invention of affordable and scalable ADSL technology the availability of these application and services was accelerated.
What features set this work apart from similar achievements?
The former work of Prof. John Cioffi and Amati laid the technology foundation and set the direction for ADSL. The achievements of the Alcatel Antwerp team however go much beyond this. The team innovated and developed the technology such that it was compatible with the hundreds of millions of copper lines around the world. It also created and developed a performant, cost effective and scalable ADSL end-to-end solution, enabling global coverage. It also scaled production, global distribution and support to fuel the mass market. Hundreds of technical challenges were tackled and resolved, witnessed by the amount of (standard essential) patents generated and scientific publications made. The high level of ASIC integration and the design of ATM based and linear scalable Central Office products that could be produced in high volume at low cost, enabled the global mass adoption of the technology. The magnitude of investment in standardisation and interoperability and the technology licensing resulted in the creation of a single global ADSL market.
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