New Telecom Quarterly xDSL: The Solution for Today’s Bandwidth Demands? David Smith hen asked about the challenges primary focus will be on Asymmetrical facing the information-intensive Digital Subscriber Line (ADSL). W society we live in, Bill Gates said, “Bandwidth bottleneck. No question, that’s xDSL Technologies 1 the biggest obstacle.” The Internet users cry POTS lines had previously been deemed more, more, more. Small businesses want unsuitable for broadband communications, digital access. Large firms are demanding yet these ordinary twisted copper pairs, more and faster access, and service provid- when equipped with xDSL modems, can ers are searching for ways to meet these Mr. David Smith is Vice transmit movies, television signals, graphics, President of Consulting, needs without having to replace the existing and high-speed data. xDSL is being pro- Alliances, and Education network infrastructure. posed in several versions that vary in both at Technology Futures, Inc. His background is in Digital subscriber line (xDSL) is one bandwidth and reach, as illustrated in Table such emerging technology under evaluation the management, design, 1. The most popular version, ADSL, offers a and implementation of that, while not a panacea to bandwidth 200-fold increase in throughput over today’s information-based problems, can provide a very timely and fastest analog modems. Some versions are operations in both the public and private beneficial short- to medium-term solution. asymmetrical with differing data rates xDSL provides an alternative to the existing sectors. He has held running in each direction, i.e., in the down- positions in management POTS or ISDN line between the subscriber stream and upstream directions. Others are at the Microelectronics and the local access switch. It typically symmetrical, providing the same data rate and Computer Technology provides: Corporation (MCC) and both upstream and downstream. With almost SEMATECH. David was a 600 million telephone lines in the world principal in creating the • More bandwidth from the switch to the today, there is certainly ample market SIA National Technology subscriber than vice versa. opportunities for xDSL technologies. These Roadmap. • The capability to route data calls on the technologies also offer an advantage over access side of the switch to a packet fiber- or coax-based technologies, which are network before they enter the public both very costly to deploy. telephone network. ADSL Technologies Figure 1 illustrates a typical xDSL Because it can quickly and cost effec- architecture. tively meet the needs of users who want xDSL has received a lot of press and is more bandwidth on demand now, ADSL is in many trials, but does it have a future, and extremely promising as an interim solution. what will it take to make it a market suc- According to a story in PC Week (August 26, cess? This article will consider the future of 1996), there will be five million ADSL lines xDSL technologies and potential market installed by the year 2000. With 600 million opportunities. A number of different xDSL telephone lines worldwide, this will hardly technologies will be discussed, but the 2Q97 1070-3683/97/$5 © 1997 Technology Futures, Inc. Page 33 Figure 1 • Commercial roll-outs of ADSL will drive down the cost of xDSL systems. xDSL Architecture • ADSL is moving from a point-to-point 1.5-6.1 kbps Computer type of system to a multipoint system whereby the service provider can have 16-640 kbps Broadband central office type equipment which Network Existing facilitates more efficient utilization of the Line ADSL ADSL resource. Narrowband POTS Splitters Network Rate Adaptive ADSL (RADSL) POTS Video Some providers, like Bell Atlantic and Decoder NYNEX, are experimenting with a more Source: Technology Futures, Inc. advanced form of ADSL—Rate Adaptive ADSL (RADSL)—which saves the expense of having to qualify the local loop for the higher speeds. This qualification could make a dent in ever-growing demands for include, for example, the quality of lines, greater, faster bandwith. subscriber distance from the central office, ADSL is moving Several key points to remember about load coils on the lines, bridge taps, and the from a point-to- ADSL are: number of existing splices. The speed of point type of RADSL is determined by the capabilities of • The technology has been accepted the copper pair which adjusts to line quality system to a internationally. much like a data modem does today. When multipoint • The American National Standards Institute you look at the other digital options such as has approved a standard protocol that system whereby T1/E1 and PRI-ISDN, they either work or sets up interfaces for ADSL (ANSI T1.413). they don’t work. In contrast, the RADSL the service pro- • Several telephone companies and Internet technology can function on less than vider can have service providers are conducting tests of optimum lines. However, there is a tradeoff central office the technology. type equipment which facilitates Table 1 more efficient xDSL Technologies utilization of the resource. Name Meaning Data Rate Mode DSL Digital Subscriber Line 160 Kb/s Duplex HDSL High Data Rate Digital Subscriber Line 1.544 Mb/s Duplex 2.048 Mb/s ADSL Asymmetrical Digital Subscriber Line 1.5 to 9 Mb/s Down 16 to 640 Kb/s Up VDSL* Very High Data Rate Digital Subscriber Line 13 to 52 Mb/s Down 1.5 to 2.3 Mb/s Up IDSL ISDN Digital Subscriber Line 128 Kb/s Duplex * Also called BDSL and VADSL. Source: Technology Futures, Inc. Page 34 2Q97 New Telecom Quarterly in that there will be some loss in signal The use of IDSL enables users to bypass the speed. CO switch entirely which, in turn, allows service providers to conserve switch re- ISDN Digital Subscriber Line (IDSL) sources and reduce network congestion. ADSL technology is also being adapted Carriers face two choices: to work with existing technologies. For example, Ascend Communications has (1) Upgrade the existing network infrastruc- developed an ISDN Digital Subscriber Line ture to handle the increasing demand. The Ascend IDSL or IDSL. To the home or business user, it (2) Move data traffic to a separate packet- technology looks exactly like a 128 Kb/s ISDN connec- switched network designed specifically addresses one of tion. However, unlike BRI-ISDN, it is a data- for data communications. only connection, so it does not need to the largest connect through a central office voice Some carriers have estimated a 10-fold impediments to switch. MFS Communications is currently increase in costs to upgrade the existing wide-scale data introducing this service to its customer base. public switched network to handle the Ascend believes that running IDSL over increasing demand from both voice and data deployment over existing ADSL technology will enable users users. Consequently, offloading data traffic is the public tele- to deploy ADSL a lot cheaper and faster the only solution that really makes sense. phone network because there already exists a fair amount of xDSL does just that (see Figure 2). software which supports ISDN standards. today—it creates The Ascend IDSL technology addresses ADSL Networks a packet net- one of the largest impediments to wide-scale ADSL installations, in which the technol- work, separate data deployment over the public telephone ogy supports a wider network of users, are network today—it creates a packet network, also beginning to emerge. This type of usage from voice calls, separate from voice calls, for more efficient enables xDSL technologies to compete head- for more efficient transport. The average data call typically on with other wideband technologies such transport. lasts much longer than the voice calls for as T-1, Primary Rate ISDN, and private which the central office switches were networks. For example, at this year’s Global designed. Internet access has inflated the Networking 1997 trade show in Calgary, average call duration from approximately Netspeed and TELUS demonstrated an ADSL three minutes to about 20 minutes, with network that used speed routers which some calls lasting several hours according to supported both IP and IPX routing, Rate Bell Communications Research (Bellcore). Adaptive xDSL configuration, as well as Figure 2 Packet Switched Network for Data Services Telephone Company Central Office Customer Premises Public Switched Telephone Interface MDF with Computer Network with ADSL Splitter Splitter Voice Switch Interface DS, HSS FDDI Packet Network Ethernet (IP, ATM, Frame Relay) Data Unit POTS Source: Technology Futures, Inc. 2Q97 Page 35 firewall support. They demonstrated a Ascend Pipeline 85, but at a higher data connection speed that ran at 7 Mb/s down- speed. stream and 1 Mb/s upstream. Long distance carriers and competitive access providers are also beginning to DSL Access Multiplexers (DSLAMS) deploy xDSL technologies. Symmetrical The other big change in ADSL technol- high-speed services such as HDSL (sym- ogy is the evolution from ADSL modems metrical, 768 Kb/s) and SDSL can be provi- between the subscriber and the local access sioned and priced lower than comparable switch to DSL Access Multiplexers (DSLAMs) T1/E1 or fractional services. Because these which reside between central offices. One of services are tariffed at lower prices for equal the drivers for this is the influx of smaller bandwidth, they could serve as the access carriers including competitive local access ramp for businesses to their local and long carriers and Internet service providers distance calling and Internet data services. entering this marketplace. AG Communica- Digital subscriber line is also creeping PairGain has a tions, PairGain Technology, and ALCATEL into the public network and is likely to special access are just some of the companies beginning to become more pervasive over the next gateway for its offer these products. DSLAMs are also couple of years as volume drives the prices beginning to interface with other protocols.