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Cable Versus Dsl 53-10-60 DATA COMMUNICATIONS MANAGEMENT CABLE VERSUS DSL John R. Vacca INSIDE DSL; Cable Modems; ADSL; CDSL; G.Lite; HDSL; IDSL; RADSL; SDSL; VDSL; POTS; DSL and Cable Modem Rollouts; High-Speed Data Entry; Buying DSL Service; Installing DSL; Security Problems, Residential Users, Telecommuters, DSL System Components; DSL Network; DSL Hubs INTRODUCTION Internet access via cable modem has become available in many residen- tial areas over the past few years. Cable has the capacity to transmit data at speeds as fast as Digital Subscriber Line (DSL) when configured prop- erly and under optimal conditions. Due to the fact that cable lines are not available in the vast majority of commercial districts, cable does not com- pete with DSL in the enterprise market at all, in most cases. Cable was designed for residential use, and in some cases may be a cost-effective solution for residential high-bandwidth Internet access. Therefore, the challenge of cable versus DSL is primarily in the residential and telecom- muter markets. With that in mind, and before continuing with the theme of this article (cable vs. DSL), one can take a look at the technology issues first, and then some basic terminology. TECHNOLOGY ISSUES What is DSL? How does it work? What are the types of DSL? These are some of the questions this article will surely answer; as well as some of the pros and cons of the use of cable modems versus DSL. PAYOFF IDEA The article discusses the current state of cable DSL: What Is It? modem access versus DSL. It also examines how In essence, by using the existing tele- prevalent cable modem and DSL services are in major U.S. markets. A comparison of the two phone cabling infrastructure, DSL is technologies with regard to speed, cost, etc., are a technology backed by telephone presented. The article also covers the planning enterprises that provides high-band- implications or considerations for the enterprise width services to the home and en- network manager (e.g., to support telecommut- terprise. Because DSL utilizes a ing employees, etc.). Finally, the article discusses future directions for cable modems and DSL. Auerbach Publications © 2000 CRC Press LLC DATA COMMUNICATIONS MANAGEMENT greater range of frequencies than ordinary dial-up services (allowing for a super-fast connection), this high bandwidth is possible. For most pro- viders, this technology is still in the early stages of rollout. How Does It Work? The general idea behind DSL technology is relatively easy to grasp, al- though it is rather sophisticated. As previously mentioned, DSL utilizes a large range of frequencies, which means a higher bandwidth and a fast connection. For example, consider this: only a small fraction of one’s telephone line capacity (bandwidth) is being used (that being only the low frequencies) when making an ordinary telephone call. By transport- ing data in the higher frequencies, DSL takes advantage of this idle band- width. This results in making it possible to talk on the phone and be on the Internet simultaneously. DSL Types As shown in Exhibit 1, there are several competing forms of DSL, each adapted to specific needs in the marketplace. Some forms of DSL are widely used standards, some are proprietary, and some are simply theo- retical models. They can best be categorized within the modulation methods used to encode data. Exhibit 1 shows different types of DSL technologies. These technologies are sometimes collectively referred to as xDSL. As explained in Exhibit 1, Asymmetric Digital Subscriber Line (or ADSL) is the most popular form of DSL technology. The fact that the up- stream and downstream bandwidth is asymmetric, or uneven, is the key to ADSL. In practice, the higher-speed path will be the bandwidth from the ISP to the user (downstream). This is mainly due to the desire to ac- commodate the typical Internet usage pattern, where the majority of data is being sent to the user (Web pages, graphics, programs, and video) with minimal upload capacity required (keystrokes and mouse clicks). Speeds typically range from 144 Kbps to 1.1 Mbps downstream, and range from 144 Kbps to 1.5 Mbps upstream. Exhibit 1 also shows that there are other forms of DSL as well: ADSL Lite; Consumer Digital Subscriber Line (CDSL is a proprietary technology trademarked by Rockwell International), G.Lite, HDSL, IDSL, RADSL, SD- SL, and VDSL. Many of these forms are just starting to become available through the telephone enterprises, and some of them have just complet- ed the testing/development stages. DSL versus Cable Modems Cable modems, although capable of high potential access speeds, also have drawbacks. Primarily, the signal is shared between the subscribers Auerbach Publications © 2000 CRC Press LLC CABLE VERSUS DSL EXHIBIT 1 — The Many Flavors of DSL Type Description ADSL Asymmetric digital subscriber line. The most common standard, it theoretically offers 1.5 Mbps to 8 Mbps downstream and 16 Mbps to 640 Kbps upstream speeds. HDSL, HDSL2 These two high data rate DSLs are symmetric services capable of 1.5 Mbps and 2.048 Mbps speeds, respectively. HDSL requires two or three wire pairs; HDSL2, just one pair. IDSL ISDN DSL. Similar to ISDN, it allows you to use existing ISDN equipment. But the maximum speed in both directions is 144 Kbps. RADSL An asymmetric service, rate adaptive DSL promises to provide between 600 Kbps to 8 Mbps downstream and 128 Kbps to 1 Mbps upstream speeds, while offering simultaneous voice service. RADSL can dynamically adjust to line conditions. SDSL Symmetric DSL, a popular alternative to ADSL, is offered by NorthPoint and various ISPs. The service promises two-way 768 Kbps access. G.Lite G.Lite is user installable and provides speeds of 1.544 Mbps downstream (UADSL/ and 512K bps upstream. Backed by many hardware vendors and the DSL-Lite) Universal ADSL Working Group. VDSL The fastest and newest DSL on the block, very high speed DSL is an asymmetric service, offering speeds of 12.9 Mbps to 52.8 Mbps. in a specific area, and the technology is broadcast oriented. Less band- width is available to each subscriber as the number of subscribers in- creases in that area. Moreover, cable access offers the user no choice regarding providers, and is only available in a limited area. It is available only through the cable enterprise, which has little experience with Inter- net services. CURRENT STATE OF CABLE MODEM ACCESS VERSUS DSL So what is holding up DSL and cable modem access? If one has been told that the lack of widely accepted standards is the main factor delaying ca- ble modem and xDSL access, think again. Lack of standards is, at most, a minor factor in the delay of widespread access. Because solid standards have not been established, xDSL and cable mo- dem access are being held up. So, one needs to put things in perspective although it is certainly true that the lack of standards is having some effect. Cable modem and xDSL services are dedicated subscriber services. One uses a cable modem or xDSL box to connect to one place — the ser- vice provider. One does not carry it around like a modem. Where you expect to move the device from place to place or to a different service provider (as with a 56 K or 33.6 K modem), standards only affect the end user in cases. A cable modem or xDSL box is like a television cable box (which is never moved — one buys or leases it with the service and it stays with the service) — not like a modem. By allowing vendor compe- tition, it is true that standards will lower hardware costs a bit. This, in turn, will lower equipment costs, but not by much. The important thing Auerbach Publications © 2000 CRC Press LLC DATA COMMUNICATIONS MANAGEMENT is that if the provider can offer service at a price one likes, that is what matters most. The fact that the provider will be able to buy the box, now being leased for $10 a month, for $100 less next year is simply not that much of an issue. Anyway, it is probably a small fraction of the overall cost of providing the service. The history of cable modem access by Cablevision on western Long Island over the last several years tends to support this. The original beta access used a specific brand of cable modem (which was provided with the service). When the beta program ended, the original equipment was discarded in favor of a newer, higher performance product made by Lan- City. This did not affect end users much — they just returned the original modems and installed the new ones. The current service provides a 10 Mbps Ethernet connection for under $70 per month (this includes the lease of the cable modem unit, connection, and ISP service) for residen- tial customers. This is three to five times faster than a full T1. The LanCity cable modem follows the only real standard that matters so far — it con- nects to the user’s PC using standard 10 Mbps Ethernet, like most other xDSL units and cable modems. One has to look elsewhere than the lack of widely accepted standards if one wants to know why xDSL and cable modem service is slow in be- coming pervasive. Maybe one should wonder if the cable enterprises are simply afraid of moving into a new technology they do not understand; and if the telcos are not that eager to access a DSL service in direct com- petition to their data T1 and T3 services (and with a lower price tag); and whether anybody has enough backbone capacity to support the large numbers of high-speed customers the new service would attract.
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