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The original inputs of voice, data, and image information are anticipated to be carried across the WAN via an ATM backbone network, ideally based on fiber optics. The video signals may also travel across the ATM WAN, and may be received at the central office via broadcast satellite, or the programming may be stored on tape or disk in a video server at the CO. The voice input is handled at each end of the connection through a splitter which ensures that it is unaffected by the presence or absence of digital data. The video and data inputs are time division multiplexed into an ADSL signal which originates in an ATU-C (ADSL Transmission Unit-Central Office). The signal then travels the local loop cable plant, terminating in one or more ATU-Rs (ADSL Transmission Units-Remote) located on customer premises. The ATU-Rs act as modems, splitting the total bandwidth into three channelsdownstream channel, bidirectional channel, and voice channel. The ATU-Rs can be in the form of standalone units, or printed circuit boards in PCs or other host computers. They can also be contained within set-top boxes in a video or TV application. The bit error rate (BER) of ADSL is 10-9, which compares favorably even with traditional T1 at 10-7.
ADSL Applications
ADSL is intended for applications requiring a high-speed downstream channel, and a relatively low-speed upstream channel (see Figure 10.1). While the technology is not appropriate for many bandwidth-intensive, conversational data communications applications, it appears to be ideal for entertainment TV (CATV), business TV and educational TV. Telecommuters also can be well served by ADSL for remote LAN access. In a SOHO (Small Office Home Office) environment, ADSL provides plenty of bandwidth for most applications, including Internet access, and file and image transfer. Into the future, ADSL promises to serve Video-on-Demand (VOD) applications, providing sufficient bandwidth for multiple downstream video channels, as well as the upstream video selection.
Figure 10.1 ADSL configuration in example residential application.
The primary interest in ADSL is on the part of the incumbent telephone companies, which are facing competition from CATV companies for the residential voice and data market. As the CATV providers prepare to compete for that business, they are making plans to upgrade existing coaxial cable plant to add bidirectional, switched voice and data capabilities. The CAPs and IXCs have similar plans, although they must build new local loop facilities to reach that market. In the face of that pressure, the telephone companies must find a means to increase bandwidth in the local loop in order to compete effectively. Their options are to lay new cable plant (fiber optic or coax), make use of wireless technology, or increase the capacity of existing twisted-pair plant. ADSL offers a relatively inexpensive solution, accommodating entertainment TV, videoconferencing, and higher-speed data transfer over existing cable plant of varying gauges. Most carriers view ADSL as a short-term solution, filling the bandwidth gap until more substantial cable plant is deployed.
ADSL Status
While ADSL has performed well under laboratory conditions, it has yet to be proven in the field. Bell Atlantic was a primary force behind ADSL development, and continues to champion the technology. Pacific Bell and some other RBOCs, have rejected ADSL, preferring a hybrid local loop technology of fiber and coax.
Bell Atlantic has been trialing ADSL since March 1993 to deliver video-on-demand in Fairfax County, VA and Dover Township, NJ. US West plans to trial ADSL for Internet access and remote LAN access in Minneapolis/St. Paul, MN and Boulder/Denver, CO. The monthly rate for the US West offering (1.544 Mbps) will vary from $60 to $100 per month, compared to ISDN (128 Kbps) charges of $32 per month plus usage or $100 per month on a flat rate basis; US West anticipates dropping the monthly rate to as little as $35 by 1998 [10-2]. GTE is also offering ADSL in Irving, TX for Internet access and remote LAN access [10-3] and [10-4]. Southwestern Bell intends technical trials of ADSL at both 1.544 Mbps and 6 Mbps [10-5]. UUNET Canada announced in June 1996 the beginning of a trial that ultimately will involve as many as 400 users. This first application of ADSL by an Internet Service Provider (ISP) will operate at 1.544 Mbps from the network to the user, with a 64 Kbps upstream channel [10-6].
In addition to the strong interest of U.S. LECs, ADSL has gained the attention of foreign carriers. British Telecom (UK) began a field trial in February 1994, and Telefonica de Espana (Spain) followed a month later. Trials also are being conducted by Mercury Communications (UK) and Telecom Australia, and are planned for the networks of Italy, France, Korea, The Netherlands, Hong Kong, Brazil, Finland, Norway and Germany.
The costs of implementing ADSL technology are high in absolute terms, but quite reasonable when compared to the alternative of laying new cable plant, such as hybrid fiber/coax. While the ATU-R currently sells for about $1,000, costs are expected to drop considerably as support for ADSL increases [10-7]. The total cost to the carrier is in the neighborhood of $6,000 per line, although it is expected to fall to $3,000 by 1997 and $750 in the future [10-8]. Brooktree Corp. (CA) recently (May, 1996) announced a transceiver chip which will deliver 384 Kbps on a bidirectional basis for $30; the interface card built around it is expected to cost as little as $100 [10-9].
There also exist nonstandard versions being trialed by GTE and other carriers. One such version is known as SDSL (Symmetrical Digital Subscriber Line), operating at 1.544 Mbps on a symmetrical basis. SDSL is touted by AT&T Paradyne as a solution for videoconferencing applications, that require symmetrical bandwidth [10-10] and [10-11]. The higher-speed versions of ADSL also are known as VDSL (Very High Bit-Rate Subscriber Line). In competition with the DMT compression technique is Carrierless Amplitude/Phase modulation (CAP), which is supported by AT&T Paradyne and other manufacturers. Using the less-expensive CAP technique, AT&T Paradyne announced in June 1996 another version of ADSL, operating downstream at 7 Mbps. Paradyne plans to request that ANSI make CAP a second standard [10-4]. In fact, the only ADSL products working in the field are based on CAP, rather than DMT. Pioneer manufacturers of ADSL equipment include Amati Corporation, AT&T Paradyne, Orckit, and PairGain Technologies.
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