Hardware Reference
In-Depth Information
receives signals from the correct frequency.
DOCSIS cable modems use 6MHz channels that can carry up to 38Mbps downstream or 27Mbps
upstream. DOCSIS 2.0 and earlier allow using only a single channel each way, meaning they are
limited to 38Mbps maximum downstream. DOCSIS 3.0 allows for bonding multiple channels, and
because most DOCSIS 3.0 modems use four or eight bonded channels down, that would allow up to
152Mbps or 304Mbps downstream speed. Although those are the maximum figures, what you really
get depends on your provisioned rate (that is, what the Internet service provider [ISP] agrees to give
you) and may be limited by other bottlenecks in the network or in your system.
Cable Pricing
Most cable providers offer a number of different connection speed plans providing various download
and upload speeds, ranging from as slow as 2Mbps to as fast as 100Mbps or higher. Some ISPs
require you to subscribe to their cable TV service or may provide a lower monthly rate if you bundle
cable Internet with other services, such as TV or phone.
Business-class cable pricing for comparable download/upload speeds tends to be higher because of
additional services such as static IP addresses, a greater number of email addresses provided, and
24/7 customer support.
Whether you are considering residential or business-class service, keep in mind that upload speeds
are typically much slower than download speeds, ranging from 10% to 20% of download speeds.
There may be equipment costs in addition to the monthly fees. For example, most ISPs provide cable
modem rental for a few dollars a month, or you can purchase your modem outright from the ISP or
from other vendors. Unless you need cable Internet service for just a few months, it makes more sense
to buy a cable modem than to rent one.
Digital Subscriber Line
The biggest rival to the cable modem in the broadband Internet business is the digital subscriber line
(DSL). DSL, like its predecessor Integrated Services Digital Network (ISDN), appeals to the
telephone companies who might be able to use the existing plain old telephone service (POTS)
wiring to provide high-speed Internet access. DSL is also appealing to businesses that don't have
access to cable modems but are looking for a high-performance, lower-cost alternative to ISDN or T-
1/T-3 leased line services.
Note
Some technical discussions of DSL refer to
xDSL
. The x stands for the various versions of
DSL being proposed and offered by local telephone companies and ISPs. DSL generally refers
to any type of digital subscriber line service.
How DSL Works
DSL takes advantage of the broadband nature of the telephone system, using the system's capability to
carry signals at multiple frequencies to allow both high-speed Internet traffic and phone calls at the
same time. Two methods for sending and receiving signals are used by the most common type of DSL,
Asymmetric DSL (ADSL):
• Carrierless Amplitude/Phase (CAP)
