Downloading information from the Internet today is often an exercise in
frustration. Because large numbers of people and companies are competing for
your attention on the Web, Web sites have grown enormously with ever-increasing
numbers of image maps, animation, audio files, and full-motion videos. With a
site's home page often consuming 100,000 bytes or more of information, no
wonder people now call the Internet the "World Wide Wait" and pundits
predict its imminent collapse.
Telephone companies have been offering several high-speed solutions to
Internet access for users who demand high bandwidth. These solutions include
frame relay and ISDN communications links. For many users, ISDN is the logical
progression from basic dial-up telephone modem networking to higher speed
demand-driven or dedicated links.
Now, a new kid has arrived in town. The latest technology entry on the
communications front is x2 technology from U.S. Robotics. It lets your modem
achieve throughputs of 56Kbps or higher over standard voice-grade telephone
lines. U.S. Robotics now includes x2 technology in all its modems, even the
low-end Sportster modems.
| 56Kbps voice-grade lines or 64Kbps
digital lines may sound like an easy choice, but is it? |
What is ISDN? What exactly is x2 technology, and how does it work? How does
it differ from ISDN? What are its problems and benefits? Is it worth purchasing
a USR modem if you don't already own one? Let's answer those questions.
ISDN
ISDN is an International Telecommunications Union Telecommunication
Standardization Sector (ITU-T) digital standard for transmitting data in
point-to-point networks. ISDN has existed since the mid-1980s (although many
telephone companies did not offer it until the late 1980s or early 1990s) ISDN
communications links are generally compatible with most of the existing
Public-Switched Telephone Network (PSTN) infrastructure. Customers can use
in-place copper wire connections to their local telephone central office, and
phone companies can use their existing telephone switching equipment to service
ISDN connections.
The typical ISDN connection is a 64Kbps point-to-point communications,
digital channel, sometimes referred to as a B (for bearer) channel. B channels
handle the data interchange (whether the information exchanged is voice or data
is irrelevant). The ISDN configuration also includes a D (for delta) channel.
Signaling and X.25 packet networking use the D channel.
ISDN is usually available in two configurations--Basic Rate Interface (BRI)
and Primary Rate Interface (PRI). The difference between the two is the number
of B channels you receive. A BRI ISDN link consists of 1 D and 2 B channels; a
PRI link consists of 1 D and 23 B channels.
Because ISDN makes a true digital connection from your computer to the
remote computer, ISDN implementation has limitations. For instance, a typical
ISDN BRI connection (the most common type of connection in residential and small
business environments) must be no farther than 18,000 feet from the phone
company's central office; otherwise, you need special line conditioning. This
limitation is not a problem for 90 percent of the existing telephone lines in
the US.
Once set up, ISDN communications links for data throughput act just like
ordinary telephone links, only faster. You dial a telephone number to place a
call through ISDN circuits, the same as you do with a modem. The difference
between ISDN and traditional modems is the digital nature of the connection.
Computers speak in digital, and traditional telephone lines are analog; the
modem usually converts between digital and analog. With ISDN, no
digital-to-analog conversion is necessary.
For an ISDN link, call setup and protocol negotiation take as little as two
seconds; for a typical v.34 modem communications link, they require 45 seconds
to 60 seconds. In ISDN, information travels through the D channel,
identifying the source, destination, and type of connection you want. Then, ISDN
telephone equipment can intelligently decide how to handle the call.
Furthermore, in ISDN, because the signal doesn't need conversion, the
effective throughput rate is much higher. To achieve even higher throughput, you
can use the process of bonding, combining two or more B channels through
a channel aggregation protocol such as Multilink Point-to-Point Protocol (PPP).
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