High Speed Connections to the Home: ADSL and Cable Modems

June 29, 1998

The emergence of the World Wide Web has precipitated an increased demand for higher speed connections to the Internet. Advanced multimedia services, CD-quality audio, interactive gaming, digital video and other on-demand services, video conferencing, and effective home-shopping are some of the many applications driving the requirement for increased bandwidth into the home.

Currently, the most popular method of accessing electronic services from the home is through dial-up, modem-based services. Typical speeds of home modems range from 9 600 bps (bits per second) or 14 400 bps (14.4 kbps) to the most recent 56 000 bps (56 kbps) standard. ISDN (Integrated Services Digital Network) technologies are used primarily by businesses and a small number of households to provide higher speed dedicated access to network services ranging from 64 000 to 128 000 bps. These speeds are normally useful for electronic communications and Web surfing but are largely inadequate when large software downloads are required or when the materials being accessed are large image, video or sound files. For example, a relatively small video file of 8 Mbytes takes over 70 minutes to download on a 14.4 kbps modem or over 20 minutes on a 56 kbps modem.

Two technologies are now available as the primary contenders for a home-based solution to the bandwidth problem: Asymmetric Digital Subscriber Line (ADSL) and Cable Modems. ADSL and cable modem technologies provide speeds greater than 25 times as fast as a 56 kbps modem.

DSL (Digital Subscriber Line) technologies are the replacement technology for ISDN, which is now considered a "transition" or legacy technology. There are a number of "flavours" of DSL (digital subscriber line) technology such as ADSL, SDSL, HDSL, and VDSL. ¹ All of these deliver data over copper telephone lines using advanced data compression varying only in the speeds of bandwidth being provided. ADSL, in particular, is an improved transmission technology that provides reliable transport of data/video services and regular telephone services over existing copper telephone lines. ADSL is uniquely suited for home subscribers' use of Internet services.

On average, ADSL technologies can deliver content about 50 times faster than a 28.8 dial-up modem. Typically, ADSL services provide speeds of greater than 2.2 Mbps downstream (from the network to the personal computer) and up to 1.1 Mbps upstream (from your computer to the network). The service does not require a separate phone line and is always on. This means a user can be on the phone and access the Internet at the same time on the same line. ADSL requires the use of an ADSL modem, an Ethernet card, setup software and an existing telephone line. The phone line distance from the central office and condition of lines are issues for ADSL deployment and generally can be used only when line distances are less than 12 000-15 000 feet. The service provider is usually required to check the line for suitability before beginning the installation.

The original idea behind ADSL was to deliver video-on-demand and home shopping services and, therefore, to compete with the cable companies. "Asymmetric" refers to the transmission capacity to the customer from the network being greater than that from the customer to the network. An ADSL line delivers a high-speed data/video channel to the subscriber, while a lower speed, bi-directional "control" channel is used for uploading data to the network. It also simultaneously provides POTS (Plain Old Telephone Service). ADSL was never totally bi-directional and the bi-directional channels that exist were added to allow program selection and interactive services. As most consumer uses are downloading of data from the network, the slower speed for upstream data is less of a problem, and the asymmetry is appropriate for home use. Businesses, however, provide information to the network and require the upstream bandwidth to serve their customers, which makes ADSL inappropriate for business uses. Other forms of DSL technologies are designed to provide greater upstream bandwidth.

Cable modem technology uses the existing infrastructure of coaxial wire used to deliver cable television. However, cable television systems were originally designed to transfer signals one-way, that is, to deliver channels into the home. To deliver data services, the cable companies have begun to upgrade their network with the digital switches necessary to provide bi-directional communications.

The cable company advantage over the phone companies is that the cable network is largely fibre optic with coaxial cables running into subscriber homes. Coaxial cable is much thicker and more amenable to very high bandwidth uses than the thin copper cables used in phone systems. The current generation of cable modems provides downstream bandwidth ranging from 10-30 Mbps¾ roughly 350 times faster than today's modems¾ and over 100 Mbps modems have been tested. By building switches into their networks, cable companies are hoping to compete with the phone companies in the provision of data and telephone services.

Cable modems are also asymmetric in that they receive data at a higher speed than they send them. Current cable modems receive data at about 10 Mbps and send at about 800 kbps. The theoretical maximums for coaxial cable are much greater than for copper wire and with some expectation that gigabit (1000 Mbps) cable modems will become available in the future. As with ADSL modems, the cable company is usually required to complete the initial installation and setup.

Pricing of ADSL services in Canada is currently being challenged by ISPs (Internet Service Providers) who claim that the monopoly position of the telephone companies has provided unfair advantage to their Internet service provider subsidiary, Sympatico. The current monthly rates for ADSL services provided by Sympatico are about $70 while ISPs claim that they cannot offer the service for less than $200/month. It is likely that increased competition for ADSL services and decreasing prices for modem hardware will keep prices low.

If cable modem technology takes off, the ISPs will similarly find themselves in a bad competitive position, since the cable modem technology is tied to the specific technologies and services of the cable company. It is unlikely that ISPs will be able to resell cable bandwidth because the infrastructure is closely tied to cable television operations. Do these developments sound the death knell for ISPs? ISPs will be required to provide higher value services by offering better customer services, commercial domain name/web hosting services and other forms of value-added consulting.

Standardization for cable modem and ADSL also remains an issue. Over 50 different companies are developing cable and ADSL modems and the number is constantly growing. Currently, it is impossible to guarantee that a cable or ADSL modem purchased from one vendor will work with the modem of another. Working groups within the cable and telephone industries are developing standards for the interoperability of devices and data communications. As the market evolves for these high-speed modems, it is likely that increased competition will encourage standardization and reduce costs.

All high-bandwidth systems, whether cable modem or ADSL, are only as good as the slowest link. If a user with a high-speed connection is trying to reach a site with only a small amount of bandwidth, the transmission speed will drop to that of the site with the lower speed. One solution emerging to deal with this problem is the use of proxies and caching¾ facilities for storing files locally and increasing performance. The benefits of higher speed connections to even slow sites will become more and more apparent as these services are implemented.

Prices for cable and ADSL hardware have been a barrier to access and will continue to drop as both technologies gain acceptance. The cost of providing both cable and ADSL services is considerable, which will impede the entry of all but the largest ISPs. Any increased competition for ADSL services from ISPs that can overcome the cost barriers in these early days of rollout may be able to challenge the phone companies’ current monopoly position of this technology. The lack of effective competition among cable companies hinders cooperation amongst them and slows the deployment of cable modem technology. ADSL is the major competitor to the cable companies attempts to enter the data services industry. Aggressive marketing to provide service to homes will probably continue to force down prices for both ADSL and cable modem services.

High Bandwidth Web Page
URL: http://www.specialty.com/hiband/

Dan Kegel's ADSL Page
URL: http://www.alumni.caltech.edu/~dank/isdn/adsl.html

The TeleChoice Report on ADSL
URL: http://www.telechoice.com/

ADSL Forum
URL: http://www.adsl.com/

ADSL Tutorial (Kimmo Saarela)
URL: http://www.cs.tut.fi/tlt/stuff/adsl/pt_adsl.html

GTE. Digital Subscriber Line (DSL)
URL: http://www.gte.com/dsl/

CableLabs. "Cable Data Modems: A Primer for Non-Technical Readers."
URL: http://www.cablelabs.com/Cablemodem.pdf

Cable Modem Resources on the Web.
URL: http://rpcp.mit.edu/~gingold/cable/

CATV CyberLab.
URL: http://www.catv.org/

Gillett, S. E. "Connecting Homes to the Internet: An Engineering Cost Model of Cable vs. ISDN."
URL: http://www.tns.lcs.mit.edu/publications/mitlcstr654.html

¹ ADSL is described in this Network Notes. SDSL is a single-line digital subscriber line that delivers 1.544 Mbps upstream and downstream over a single copper twisted pair wire. HDSL is high-data-rate DSL that is the same as SDSL except that it uses two copper twisted pair wires. Both HDSL and SDSL have operating ranges limited to about 12 000 feet. Very-high-date-rate DSL is aymmetric but provides 13-52 Mbps upstream and 1.5-2.4 Mbps downstream but only over very short distances of about 1 500 to 4 500 feet.

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