CAP is dead in Europe. This is a recent Alcatel White Paper.
USING DMT TECHNOLOGY FOR ADSL SYSTEMS
Martin De Prycker
Vice-President and General Manager Broadband Division
Alcatel Telecom
December 11, 1996
ú In 1992-1993, the ANSI (American National Standards Institute) T1E1.4 committee decided to create a standard for asymmetrical digital subscriber line (ADSL) service based on discrete multi-tone (DMT) technology. This decision was based on technical evaluations between DMT and an alternative method, known as carrierless amplitude phase modulation (CAP). These evaluations included simulations, models and measurements on a wide range of real line conditions with different noise environments. The committee found that, especially due to its higher flexibility and robustness against noise, DMT is very well suited for existing cable plants with unpredictable interference and, thus, superior to the CAP system. The next year the European Telecommunications Standards Institute (ETSI) endorsed also DMT as the standard for ADSL. To complete all the physical parameters of the standard, it took the ANSI committee about two years to finalize the specifications.
ú Based on this standard, several companies have developed and tested DMT-based ADSL systems in real networks and field trials. Among those developing and proposing chips for DMT-based ADSL systems are companies such as: Alcatel, Analog Devices, Motorola and Texas Instruments. Among the companies having deployed ADSL DMT systems are companies such as Alcatel, Amati, Aware, Ericsson, and Orckit.
ú These companies and others have affirmed their commitment to make fully-interoperable chips and systems by creating, in July 1996, an interoperability forum to guarantee full ATU-C (LT) and ATU-R (NT) interoperability among several vendors. This interoperability will help realize a business model in which consumers can buy their ADSL modems at a wholesale or retail facility and connect it to the operator's (e.g., telephone company, Internet service provider) modem multiplexer, or DSL access multiplexer (DSLAM). This scenario also promotes full competition among the industrial partners, ensuring the best price for the operator and the consumer. Four operators in the U.S. known as the Joint Procurement Consortium (JPC -- Ameritech, BellSouth, Pacific Bell, and SBC Communications) considered these facts when they selected the DMT standard for their ADSL commercial services, starting in mid-1997 in some areas.
ú In July and September some CAP proponents have proposed an alternative ADSL standard based on CAP in T1. This proposal has been rejected by T1 twice for the following reasons:
1) DMT has proven available and has been successfully deployed by several operators.
2) DMT has the proven capabilities to cope with multiple sources of noise interference.
3) A new standard would require around two years before final acceptance and would dissipate valuable resources needed for the commercial deployment of ADSL.
4) A second standard would result in compatibility confusion of modem systems installed at customer premises and carrier facilities.
5) Nothing about the CAP proposal dispels the validity of the evaluation conducted by the committee in 1992-1993.
6) At the November 1996 T1E1 meeting an ad hoc group was formed for the purpose of developing a rate adaptive ADSL CAP/QAM specification. This was formed outside of the technical subcommittee due to a lack of consensus with in the subcommittee, and any resulting standard is more than a year away.
ú At this time, the industry has only one viable supplier of CAP chips, i.e. Globespan Technologies, the former AT&T Paradyne unit. A single supplier is not conducive to industry competition that would ensure the lowest prices for operators and consumers.
ú Therefore, Alcatel holds that DMT represents the more viable option to achieve a successful large-scale deployment of ADSL. DMT offers the better solution for the ultimate success of high-speed Internet services and, later, enhanced video services. The DMT ADSL standard ensures a business model with full interoperability between modems from several vendors, to the benefit of both operators and consumers.
ANNEX
DMT: THE BETTER CHOICE FOR ADSL
Viable ADSL architectures will have to function in many different environments, each with varying line and noise conditions and other factors affecting performance. The transmission technique has to offer maximum flexibility and robustness.
ú Performance: DMT assures outstanding performance, even on loops in poor condition, because the available transmit bandwidth is divided into a set of independent subchannels. The quality of each subchannel is measured and bits are assigned to each accordingly. During operation, the quality of each tone is monitored constantly and adjustments are made to the bit distribution to maintain the desired performance. The automatic bandwidth optimization and rate adaptation of DMT ensure that virtually any line can accommodate communication at the highest available rate.
ú Bit Rate Flexibility: DMT's multitone functionality offers bit rate flexibility. This feature is intrinsically offered for upstream and downstream connections with low granularity. With DMT, it is easy to vary the bit per carrier allocation scheme, even on-line; CAP systems available today do not provide such flexibility. More flexibility in CAP would require extra costs and processing power.
ú Transmit Power Spectrum flexibility, masking and shaping: The Power Spectral Density (PSD) of DMT is fully configurable for SNR (signal to noise ratio) optimization, for limiting egress noise (noise generated by ADSL towards the external world), or for compatibility with other services (for instance by preventing the modem to send power on particular frequency bands= spectral masking). The multiple carriers of DMT allow operators to achieve this required flexibility. CAP systems available today only offer a flat PSD.
ú Equalizing requirements to handle complex twisted pair loops: ADSL promises maximum performance from loop to loop. Because CAP systems are unable to adapt to spectral characteristics of the different loops, equalizers need to be designed for the worst case loops to be encountered in the field. This results in a suboptimal performance in all but worst case loops (and this may be a large majority). DMT equalization is inherently more powerful benefiting from its inherent spectrally adaptive characteristics for time and frequency domain equalization . The equalization is therefore optimized for each particular loop.
ú Resistance to impulse noise: DMT is less sensitive to impulse noise because of its longer symbol duration. A DMT receiver can even recognize symbol degradation under impulse noise. CAP requires larger interleaving depths and special attention for CAP equalizer adaptation (run-away) during impulses. CAP also has a demonstrated tendency towards error propagation.
ú Complexity and integration: Alcatel has integrated the ADSL DMT modem in two chips. This implementation shows that DMT can be realized with acceptable cost, power and complexity. Alcatel's ATU-R (NT) is an example of compact industrial design ready for mass deployment.
ú ATM transport: DMT is ideally suited for ATM transport. The fine bit rate granularity of DMT perfectly supports the flexibility that can be offered by ATM. The Alcatel ADSL system provides ATM transport over ADSL. This offers a flexible transport system for the provision of existing and new medium- to high-bit rate services, while facilitating a gradual evolution of B-ISDN services to the residential or business subscriber.
Disclaimer: This document represents the work and opinions of Alcatel Telecom only. It does not imply endorsements from ANSI, ETSI, the JPC, or any other companies named therein. |
