Cable Modems Vs. Digital Subscriber Lines

Cable Modems vs. Digital Subscriber Lines

Bob Fager

Econ 235ഊThe Internet has grabbed on to the world and it isn't

letting go. Nearly 36 million U.S. homes currently have PCs

and everyone is dying to jump on the information

superhighway. The Internet, which started as a group of

government agencies and universities, has grown to include

almost anyone, from home users to large companies and

everyone in between. It makes sense then that providing

Internet service is big business. The service which used to

be dominated by groups of nerdy computer whizzes using

equipment in someone's basement is now being provided by

many telephone companies, large on-line services and may

soon be available from you local cable company.

Computer users are an impatient group. They are

starving for a faster way of connecting to the 'net. Until

now home users have had to suffer with the slow connections

available with analog modems or spend a relatively large

amount on having a digital line, such as ISDN, installed and

then continue paying a lot for the monthly charges

associated with such lines.

Standard analog modems have always been hindered by the

bandwidth they are allowed to use. Standard voice grade

phone lines use the frequency spectrum between 0khz and 4khz

to transmit their signal. 33.6 kbps modems packed nearly 11

bits of data per hertz, a remarkable feat, which is very

near the theoretical limit. To allow faster connections

modems must use a wider bandwidth.ഊTwo new competing technologies are now being developed

which use this broadband idea to give computer users the

speed they crave. Telephone companies are working on

developing a way to use the standard twisted pair copper

wires that now connect nearly every home in America to

transmit data at high speeds. These technologies,

collectively called xDSL, come in two main flavors. ADSL,

which is an acronym for asymmetric digital subscriber line,

is the most common. This name was coined by Bellcore in

1989. The other main type of digital subscriber line is

called HDSL. It stands for high-bit-rate digital subscriber

line. These two technologies are essentially the same,

except they apportion a different bandwidth to upstream

(user to network) and downstream (network to user) data

transmission.

Concurrently, cable television providers are working on

technologies to allow them to connect computers to their

network and allow users to connect to the Internet at speeds

just as high. Such equipment is being called a cable modem.

Cable modems offer the possibility of transferring data

at rates up to ten megabits per second, a speed nearly ten

times faster than that of ISDN and about twenty times faster

than today's fastest analog modems. This number is somewhat

misleading however. The truth is that in order to actually

achieve that speed you must be the only user on the network.

The reason for this is that this throughput is shared by

everyone connected to a given line. Typical cable systemsഊserve 500 to 2500 homes on one line. Therefore the actual

throughput will depend upon how many other people in your

neighborhood are also trying to access the 'net. Actual

speeds vary greatly.

Cable modems are already being used in several limited

areas, mostly large metropolitan areas, especially in

Southern California. According to research by The Yankee

Group there are approximately 25,000 people already using

cable modems. They expect this number to grow to around

275,000 by the end of 1998 (Tedesco).

The reason cable modems are not already widespread is

that they present a bigger technical challenge to cable

operators than anything they've ever faced. Cable companies

do not have a very good track record. They've given us lots

of unfilled promises - 500 channels of television,

interactive television and low priced telephone service.

Before cable operators can offer service to cable modems

they must upgrade their network. Only about fifteen to

twenty percent of existing cable networks are modem-ready.

The rest will need to spend a great deal of money upgrading

connectors, transmitters and sometimes wires. The biggest

problem is that most cable networks were not designed