Just Follow the Copper Wire RoadNew technologies have a way of muddying what once seemed to be a clear stream of progress. To meet Japan's expanding telecommunications bandwidth needs, NTT has committed itself to a decade-plus effort to install a nationwide fiber-optic network. But now comes ADSL - an innovative digital transmission technology that promises multi-megabit per second data communications speeds over the existing copper wire phone network.
by Noriko Takezaki "To stick with fiber optics, or not to stick with fiber optics?" That is the question that now confronts Japanese telecommunications giant NTT (Nippon Telegram and Telephone). In the face of faster-than-expected growth of Internet use, NTT is struggling with the challenge of being flexible enough to meet the demand for high-speed data communications. Until about a year ago, Japan never imagined not spending the time and money to install a nationwide network of fiber-optic cables for improvement of telecommunications services. With the support of the Ministry of Posts and Telecommunications (MPT), NTT has been promoting nationwide optical Fiber-to-the-Home (FTTH) coverage by 2010. Now, however, another advanced technological concept that doesn't need optical fiber has recently caused NTT some embarrassment by jumping into the fray: asymmetric digital subscriber line (ADSL).
A solution in search of a problem?
"The most attractive point [of ADSL] is that we can now realize higher speed communications by using the existing infrastructure," observes a spokesman for US Robotics KK. This Japanese subsidiary of US Robotics is developing ADSL modems for introduction to the Japanese market. "NTT has been installing an optical fiber network to realize high-speed communications, but its completion still requires some time and cost. By employing ADSL, users can enjoy benefits of higher speed communications earlier, such as a further expansion of the already successful business of karaoke with moving images." ADSL technology was originally developed by BellCore (the research arm of the Bell Operating Companies) in the US in 1989. Since then, ADSL trials have been carried out in the US and Europe. Unfortunately, early trials of video-on-demand - initially regarded as one of the more promising applications for ADSL - ended with unfavorable results. Because of this, ADSL technology has failed to attract much public attention - until now. Because of the drastic growth in use of the Internet, ADSL technology is at last gaining popularity in the industry. In the US and Canada, several telecommunications operators are currently conducting service trials, and some have even started ADSL-based commercial services this year. So, why not in Japan?
NTT not convinced
According to an NTT spokesperson, the company has been evaluating ADSL technology as part of its research on advanced technology planning for various digital subscriber line services. These - including HDSL (High-data-rate Digital Subscriber Line), RADSL (Rate Adaptive Digital Subscriber Line), SDSL (Symmetric Digital Subscriber Line), VDSL (Very-high-data-rate Digital Subscriber Line) - are often collectively referred to as xDSL. What NTT found regarding ADSL in its primary evaluation of xDSL, according to the company spokesman, were more disadvantages than advantages when comparing the use of copper lines to the use of fiber-optic lines. The point about which NTT is most skeptical is whether ADSL technology can be successfully applied to the existing access network in Japan. There are, after all, some technological differences between the network structures in Japan and the US. NTT points, for example, to the possibility of "crosstalk" (audio interference between adjacent lines) if ADSL is applied to the existing Japanese telephone network. Rather than using plastic insulation - the major insulation method used in the US and Europe - about 60% of the underground feeder cables in Japan are more than 35 years old and utilize old-fashioned paper-wrapping insulation. This makes them more vulnerable to crosstalk. To solve the crosstalk problem, rather than laying new copper lines, NTT has been working to replace its paper-wrapped copper lines with new fiber-optic cables. Also, because of the difference in the cable unit structure in Japan, an ADSL signal here is more likely to experience signal interference from an adjacent ISDN line than in the US and Europe. In the US and Europe, the cable unit is a pair of copper lines (total two); in Japan the cable unit is a "quad," consisting of two pairs of copper lines (total four). Therefore, there is a possibility in Japan that an ISDN signal may be carried on the other lines in the same quad in which an ADSL signal is being transmitted. If this occurs, the ADSL signals are likely to experience significant signal interference from the ISDN signals. And even when the ISDN signals are in an adjoining quad, or the next-adjoining quad, interference by ISDN signals can cause uneven transmission of ADSL signals. The maximum effective transmission distance of an ADSL signal can be reduced by as much as 30% to 40% through the interference of a nearby ISDN signal. Further, the transmission signal spectrum of ISDN in Japan overlaps some of the spectrum of ADSL. This increases the likelihood that ADSL signals will suffer interference from ISDN signals. ISDN in Japan uses TCM (Time Compression Multiplexing) with a 320-kHz frequency bandwidth, which overlaps with the entire ADSL upstream bandwidth and some of the ADSL downstream bandwidth. In the US and Europe, ISDN uses the echo canceler method instead of TCM with an 80-kHz bandwidth, which does not overlap with the ADSL bandwidth. Therefore, for NTT - which has built its reputation on providing universal service with high and stable quality to its subscribers - the quick adoption of ADSL was far from a given. "Considering the recent drastic drop in the price of fiber-optic cables, we have rather considered the replacement of old copper cables with fiber-optic ones to be more cost-effective overall," says Ken-ichi Narumiya, executive manager of NTT's technology department, advanced technology planning section.
Competing standards
The CAP standard is a version of QAM (Quadrature Amplitude Modulation) in which incoming data modulates a single carrier and then is transmitted down a telephone line. CAP supporters include Bell Atlantic, Lucent Technologies, Northern Telecom, Nynex, Oki America, Rockwell, and US West. DMT, on the other hand, is a version of multicarrier modulation; incoming data is collected and then distributed over a large number of small individual carriers, each of which uses a form of QAM modulation. DTM standard supporters include Alcatel Telecom, Ericsson Australia, and NEC America. In terms of standardization, the DMT method is already supported by ANSI (the American National Standards Institute). In the market, however, more products utilizing the CAP method have been introduced because of their simple circuit configuration and lower power dissipation. NTT has evaluated both standards, but the company says that it would be very difficult to decide which one to adopt. Instead, NTT would prefer to wait until one of the methods becomes the dominant player in the market.
What does the future hold in store?
The MPT set up a study group for investigation of advanced and diversified networks early this year. In June, that study group compiled a report regarding the possibility of using xDSL in Japan's subscriber network. The report (after considering the results of NTT's primary analysis of the use of ADSL in Japan) calls for the early adoption of xDSL in Japan, "since xDSL is a technology that will enable megabit-class access speed even before FTTH is realized, and since the development of new service applications is expected to advance through use of the technology." Although the report itself does not have any enforcement power, there is no doubt that it has put some pressure on NTT to adopt ADSL. The fundamental discussion required to resolve this issue is what kinds of technologies are necessary, and what kinds of services are expected in today's changing telecommunications field. "The conventional telephone technology is a dying technology, in a long-term view," says Shumpei Kumon, executive director of the Center for Global Communications, International University of Japan, and a key member of the MPT study group. "But, do we still think it is necessary to spend much money for renovation of a dying technology? Rather, we need to think more seriously about the use of the Internet." NTT, and the whole Japanese telecommunications industry, now faces a drastic change of business environment brought about by growing Internet use. What users, particularly Internet users, want is not an ideal solution that will take another decade to achieve, like FTTH. What they want is something that can be made available as early as possible, so that they can enjoy the benefits of the Internet and high-speed data communications as much as possible. And, for now, that something looks like ADSL.
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