Hardware Reference
In-Depth Information
• Use download managers to schedule large downloads overnight.
• Instead of using Windows Update to download large files such as service packs or programs
such as Windows Live Essentials, download a single installation program that you can
distribute via your network to other users.
• Use the service's usage checking feature to determine whether your usage is getting close to the
limits set by the service.
• If you consistently get close to or exceed the bandwidth limits, consider upgrading to another
plan that offers higher limits.
ISDN
The connection speed of dial-up modems is limited by Shannon's Law. (See the section “56Kbps
Modems,” later in this chapter.) To surpass the speed limitations of dial-up modems, you need to use
a digital signal. ISDN was the first step in the move to digital telecommunications. With ISDN, you
can connect to the Internet at speeds of up to 128Kbps. Because the telephone companies developed
ISDN, you can purchase a variety of service plans. Depending on the ISDN service you choose, you
can use it strictly for Internet service or to service multiple telephony applications such as voice, fax,
and teleconferencing.
Depending on where you live, you might find that ISDN service is available for Internet uses, or your
local telco might offer faster DSL service as an alternative. Because ISDN was not originally
designed for Internet use, its speed is much lower than other broadband options. Also, ISDN costs
about twice what a typical ADSL or cable modem connection costs per month. To determine whether
a particular Internet service is using ISDN, you might need to ask the ISP. For example, AT&T uses
the term “Digital Enhancer” for its ISDN BRI offering in Connecticut, and “Digiline Service” in
several plains states.
ISDN doesn't require as high a line quality as DSL, so it can be offered in areas where DSL can't
work without a major upgrade of the telephone system.
Leased Lines
For users with high bandwidth requirements (and deep pockets), dedicated leased lines provide
digital service between two locations at broadband speeds. A
leased line
is a permanent 24-hour
connection to a particular location that only the telephone company can change. Businesses use leased
lines to connect local area networks (LANs) in remote locations or to connect to the Internet through a
service provider. Leased lines are available at various speeds, as described in this section.
To connect networks in distant locations, networks that must support a large number of Internet users,
or especially organizations that will be hosting their own Internet services, a T-1 connection might be
a wise investment. A
T-1
is a digital connection running at about 1.5Mbps. A T-1 can be split (or
fractioned
), depending on how it is to be used. It can be split into 24 individual 64Kbps lines or left
as a single high-capacity pipeline. Some ISPs allow you to lease any portion of a T-1 connection that
you want in 64Kbps increments (or
fractions
).
Figure 16.4
shows how a T-1 line is fractioned.
