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 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).

The ISDN physical layer uses a special communications protocol, 2B1Q (two Binary one Quaternary), to transmit data. This protocol samples the voltage level on the communications link 80,000 times per second. Each sample communicates two bits of data, for a potential effective throughput of 160Kbps. However, because of frame overhead and the D channel's use of 16Kbps of bandwidth, the effective throughput for the two B channels is 128Kbps, or 64Kbps each.

The major downside to ISDN, however, is not its implementation but its cost. Just last year, ISDN modems were rare and expensive, and only businesses with dedicated networking needs bothered to purchase them. Today, the ISDN market has changed considerably; ISDN modems are now available off the shelf at your local computer superstore.

The primary limiting factor today for ISDN is network usage costs. Depending on how your local public utilities commission structures your telephone company's tariffs, it may calculate ISDN costs based on a kilopacket (1024 packets) or message unit (usually about five minutes or less). Some telephone companies even base price on both kilopackets and message units, in addition to a basic monthly network access charge. Telephone companies usually charge different rates for a single B channel connection, with incremental costs for a 2B+D configuration. With hourly ISDN rates to a connection across town often exceeding hourly long-distance telephone rates across the country, no wonder many people refuse to switch to ISDN despite its many advantages.

Enter x2 Technology
People used to think 33.6Kbps v.34 was the best throughput modem manufacturers were going to achieve. Shannon's Law, a property of communications channels, defines the theoretical maximum throughput you can achieve through any communications channel. This law takes into account physical properties of the communications link, such as the amount of signal the channel can carry and the amount of noise or distortion on the channel. Based on typical values for a voice-grade, analog telephone circuit, 33.6Kbps is about the best throughput you can achieve.

The problem with Shannon's law is that it assumes both ends of the communications link are analog. The modulation scheme that v.34 communications links use is Pulse Code Modulation (PCM). In a process known as quantization, v.34 samples an incoming analog signal 8000 times per second and converts the voltage at each sample into an 8-bit binary value. However, because the telephone network's analog-to-digital conversions are not precise, a certain amount of error, quantization noise, enters the transmission during this conversion. This noise is what limits v.34 transmission throughput to 33.6Kbps.

x2 technology can function at higher throughput by exploiting a feature available in most Internet Service Provider (ISP) communications links: digital links (ISDN, frame relay, etc.) from their office to the telephone company's central office. ISPs have digital links because of the large amount of traffic those links can handle and because, for example, 1 PRI ISDN connection is more efficient than 23 individual analog telephone lines running into the office. The digital links to ISPs often terminate in a PBX or some other communications equipment that performs a digital-to-analog conversion and yields a series of telephone lines. The ISPs can place traditional analog modems on these lines to service user telephone calls.

If the downstream link (originating from the ISP) is digital, the ISP's equipment does not need to perform a digital-to-analog conversion before transmitting data. Instead, it can transmit a purely digital signal, one of 256 possible PCM codes, down the communications link 8000 times per second. To properly exploit this feature, ISPs must purchase special x2-enabled communications equipment to act as the modem pool and use all possible 256 PCM codes. Currently, only U.S. Robotics offers server-side communications equipment that lets ISPs provide this technology to their customers.

Furthermore, users accessing ISP resources must have a modem capable of understanding the x2 signals originating from the ISP. In other words, the modem must be able to discriminate among the 256 different PCM codes in use, and talk to the telephone network at the same PCM rate of 8000 cycles per second. Unfortunately, technical problems currently prevent the use of all 256 PCM codes, so x2-enabled modems use 128 of them to yield an effective throughput of 56Kbps.

Problems in x2 Paradise
Despite the apparent instant success that x2 technology has found in the consumer marketplace, several problems associated with the technology will impede its deployment and ultimate adoption by the market as a whole. First, no ITU communications standard exists for 56Kbps dial-up modem technology. Rockwell International and other manufacturers are working on alternative 56Kbps dial-up modem technology, known as K56Flex. Communications industry leaders such as Motorola and Lucent Technologies (formerly Bell Labs) have adopted this competing technology. These companies have submitted K56Flex to the ITU-T for adoption as the 56Kbps standard in dial-up modems. Naturally, the market is hesitant to purchase modems that use technology that may eventually go the route of mass-market nonacceptance, like the old v.FC 28.8Kbps modem standard did.

Adding a new variable to the standards equation is 3Com's acquisition of U.S. Robotics in February of 1997. According to several 3Com Web documents, 3Com intends to fully support and extend x2 technology in existing U.S. Robotics and 3Com products. This support will supplement the K56Flex support 3Com already intends to build into several of its product lines. As 3Com is a major player in the communications industry, it's difficult to imagine that 3Com's decision will not have an effect in the long term.

x2 technology is beginning to receive wider acceptance in the marketplace. Several other modem manufacturers such as Practical Peripherals and Cardinal Technologies either currently ship or will soon ship x2-capable modems. Major computer mail-order companies such as Gateway 2000 and Dell are starting to include x2-capable modems in order configurations that require high-speed Internet access. However, if you think you can get a couple of x2 Sportster modems from your local mail-order house and set up a 56Kbps link between you and a friend, think again! The client desktop modems aren't capable of an outbound 56Kbps link; they operate at a standard v.34 33.6Kbps speed for outbound transmissions.

In addition, because the x2 server-side equipment is available only from U.S. Robotics, the server-side components are expensive to upgrade to, especially if the ISP doesn't already have U.S. Robotics equipment. In areas that relatively small ISPs service, some time might pass before those ISPs replace their existing equipment to adopt x2 technology.

As to whether x2-capable modems really provide the speed boost they claim, reports from professional testing sources and end users show mixed results. At the outset, the best transfer speed you'll see from your ISP is 53Kbps, because of FCC regulations on how communications equipment can exploit the telephone circuit. The news only gets worse from there.

Assume that your local ISP supports x2 technology--here in Rhode Island, no ISPs support x2. If you use your Internet connection to upload a great deal of data, you'll see no improvement over v.34, because x2 modems support extended transfer speeds only on the download link from your ISP. Upload transfers still use the 28.8Kbps v.34 communications protocol.

Furthermore, x2 technology appears to be very temperamental when it comes to the quality of the circuit your telephone company gives you. Spend some time reading the modem-related newsgroups on the Internet, and you'll find numerous postings that mention frequent disconnects and transfer speeds that are well under the supposed speed-doubling that the technology's name implies. Naturally, if you call your telephone company's service representatives to complain about poor line quality--if you can get a technician to test the quality of your line--they are likely to dismiss your complaint. They'll say that you're paying for a voice-grade telephone circuit; if you want a data-quality circuit, you need to pay for one.

Good Buy or Good Bye?
If you are a consumer who uses the Internet for email, usenet, and Web surfing, x2 technology definitely has a benefit you can capitalize on, but only if your ISP supports x2, if your phone line can handle the technology, and if your usage is primarily download-link oriented. U.S. Robotics lists ISPs that have x2 support available on their Web sites, and the company has a method of determining whether your phone line can support x2. Point your browsers to for additional information.

If you are a commercial Internet user who uploads large amounts of data or who uses dedicated PPP connections for Web-hosting services, you probably won't benefit from x2 at all. In fact, you probably want to avoid it. Because the technology is new and still needs refining, existing v.34 modems or ISDN connections are still best for most commercial users, depending on how you use the Internet.

If your business uses the Internet to download large amounts of data (e.g., patches from vendor support sites, the latest news about your software products, or articles from Microsoft's Knowledge Base), you will probably benefit from x2. The telephone company usually assesses tariffs differently for business users than for residential users (business users pay telephone bills based on message units, whereas many residential users have unlimited local-calling-area usage). Thus, if you download significant amounts of data, x2 might help you significantly reduce your telephone bills.