x2 vs. ISDN

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.

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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