Cable Modems vs. Digital Subscriber Lines
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
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 systems;#3338;serve 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 to
handle two-way communication. The amplifiers that
strengthen the signal on its way to homes only operate in
one direction. These amplifiers must be upgraded to work in
both directions before cable modems will work.;#3338;Cable modems are a box that is external to your
computer. They are currently being produced by
manufacturers of TV top cable boxes, such as Scientific
Atlanta and General Instrument, as well as manufacturers of
computer components, such as 3Com, Intel and Motorola.
To connect to a computer, cable modems require an
Ethernet network interface card because the standard ports
on a computer are too slow to handle data at these speeds.
So, from the computer’s point of view it is on an Ethernet
LAN. This makes configuring the computer quite easy. The
network interface card will send Ethernet packets to the
cable modem. The modem will then modulate the data and send
it out over the coaxial cable as an RF signal, just as cable
TV signals are transmitted.
One small problem with this scheme is that typically
this data is carried via a low-frequency band that hasn’t
previously carried a TV channel. These low frequencies are
very susceptible to noise from common household electrical
Cable modems will be somewhat expensive when compared
to the analog modems being used by most computer users
today. According to Bruce Leichtman, broadband technologies
analyst for The Yankee Group “It’s a Lexus product and it’s
priced accordingly.” (Tedesco) Generally, there will be an
installation fee of about $150 which will pay for the
network interface card and the installation by the cable
operator. Users will then pay $34.95 to $59.95 per month;#3338;for the service, which will include the use of a modem
provided by the cable operator. This charge is sometimes
cheaper when coupled with cable television service, in some
cases as low as $19.95.
One of the biggest differences between cable modems and
digital subscriber lines is the number of lines available to
each. There are over 600 million telephone lines installed
in the U.S. today. No more than 12 million homes today can
handle cable modems. Although this number is growing
steadily, it is not expected to catch up with telephone
lines for many years.
Digital subscriber lines currently are not available
anywhere except in a very few test markets. Bell Atlantic
has been testing ADSL in Northern Virginia for the last few
months. Users have been paying sixty dollars per month,
which includes the use of an ADSL modem running at 1.5 mbps.
They plan on rolling out ADSL across their six-state mid-Atlantic
region in mid-1998, at which point the data rate
should be increased to 6 mbps. Customers will have to
provide their own modem. The price for this service has not
yet been determined. Pacific Bell is also about to launch
ADSL service in San Francisco and Los Angeles. They have
been testing the service in East San Francisco and Palo
U.S. West plans to debut HDSL in fourteen cities across
its service area very soon, reportedly during the fourth
quarter of 1997. It will be priced at $75 to $150,;#3338;depending on the amount of throughput the user requires.
The user will be required to supply the modem.
Telephone companies will also have to upgrade their
networks before digital subscriber lines can become
widespread. The first thing they must do is remove all
loading coils from their lines. Loading coils are
electronic devices that cut off all frequencies above 4khz.
They were installed years ago as a way of reducing noise
during voice calls. These higher frequencies are required
by digital modems. There are also several other problems
with telephone lines which must be resolved. These include
overlong loops which attenuate signals, bridged taps which
are essentially unterminated pairs and cross coupled
interference which is crosstalk between wires in feeder
cables that run into neighborhood distribution nodes.
XDSL modems will work much like cable modems, commonly
requiring a network interface card be installed in the PC.
Some vendors however, are developing modems that are
installed inside the PC and will not require the Ethernet
card. The modems work by adding two additional channels to
a phone line, giving a total of three channels. These
channels are divided by frequency. The first channel is
used for telephone service (POTS). It uses the spectrum
between 0khz and 4khz, as is common with telephones. The
other two channels are for upstream data and downstream
data. They use the spectrum from 4khz to 2.2mhz.;#3338;The allocation of frequencies to each channel helps
determine the throughput for the upstream channel and the
downstream channel. Several other factors will also play a
role in determining speed, such as the line length and loop
conditions. Therefore, the actual speed will vary greatly.
Generally speeds will range from 192 kbps to 9 mbps, with
downstream speeds being greater than upstream speeds because
more of the bandwidth is allocated to it. Downstream speeds
of about 1.5 mbps and upstream speeds of about 500 kbps seem
quite common. However, speeds as high as 55 mbps have been
reported using short loop distances.
Pricing for digital subscriber lines is expected to be
higher than for cable modems. Prices ranging from $75 to
$150 will probably be common. These prices will depend on
the throughput of the line and who supplies the modem. It
is expected that most telephone companies will require the
user to supply the modem. Modems are in the $300 to $400
Many people are questioning whether the Internet’s
backbone will be able to handle the traffic increase it will
see as a result of users having such high speed on ramps to
the information superhighway. These technologies will allow
users to transmit a lot of data to the Internet but once it
hits the net it is subject to the same traffic problems
that affect data from standard modems. The fact is that if
these technologies were widely available today the Internet
would not be able to handle it. However it is expected thatഊby the time they become widely available the net will have
undergone sufficient upgrades so that it will not become
saturated by this increase in data. There is currently a
great deal of work being done to upgrade the Internet’s
backbone. Work currently being done includes increasing
server access speeds, improving the backbone and NAP
bandwidth, increasing router speeds and introducing ATM into
the backbone for much lower latency. Also the size of the
Internet is constantly increasing. Telephone companies
installed an estimated 10 million kilometers of fiber in the
1980s. Only a fraction of the potential bandwidth has been
utilized (Digital Horizon).
Both of these technologies appear to be just what many
PC users are looking for – a way for them to surf the web at
increased speeds. Who is going to win the battle between
the cable operators and the telephone companies is yet to be
seen. Overall when comparing price and performance the
cable companies appear to have an advantage but whether they
can actually implement the technology is questionable. I
definitely wish them luck and hope that they implement the
necessary upgrades in my area soon.;#3338;Works Cited
The ADSL Forum’s Frequently Asked Questions. On-line.
Alsop, S. (1997, January 13). The Cable Industry’s Big
Dream. Fortune, 135, 147-8.
Andrews, D. and Ross, R. (1997, February). Cable
Modems: Fastest Internet Access In The East And West. PC
World, 15, 62-3.
Digital Horizon – Virtual Area Networks – Cable Modems.
On-line. Available: http://www.broadband-guide.
Goldberg, L. (1997, March 3). Interoperability Specs
For Cable Modems Released Amid Industry Fanfare. Electronic
Design, 45n5, 34-6.
Halfhill, T. (1996, September) Break the Bandwidth
Barrier. On-line. Available:
Internet Access: ADSL. On-line. Available:
Tedesco, R. (1997, August 11). Modems: Fast Bucks For
Cable?. Broadcasting ; Cable, 127n33, 43.