x2 vs. ISDN

Downloading information from the Internet today is often an exercise infrustration. Because large numbers of people and companies are competing foryour attention on the Web, Web sites have grown enormously with ever-increasingnumbers of image maps, animation, audio files, and full-motion videos. With asite's home page often consuming 100,000 bytes or more of information, nowonder people now call the Internet the "World Wide Wait" and punditspredict its imminent collapse.

Telephone companies have been offering several high-speed solutions toInternet access for users who demand high bandwidth. These solutions includeframe relay and ISDN communications links. For many users, ISDN is the logicalprogression from basic dial-up telephone modem networking to higher speeddemand-driven or dedicated links.

Now, a new kid has arrived in town. The latest technology entry on thecommunications front is x2 technology from U.S. Robotics. It lets your modemachieve throughputs of 56Kbps or higher over standard voice-grade telephonelines. U.S. Robotics now includes x2 technology in all its modems, even thelow-end Sportster modems.

What is ISDN? What exactly is x2 technology, and how does it work? How doesit differ from ISDN? What are its problems and benefits? Is it worth purchasinga USR modem if you don't already own one? Let's answer those questions.

ISDN
ISDN is an International Telecommunications Union TelecommunicationStandardization Sector (ITU-T) digital standard for transmitting data inpoint-to-point networks. ISDN has existed since the mid-1980s (although manytelephone companies did not offer it until the late 1980s or early 1990s) ISDNcommunications links are generally compatible with most of the existingPublic-Switched Telephone Network (PSTN) infrastructure. Customers can usein-place copper wire connections to their local telephone central office, andphone companies can use their existing telephone switching equipment to serviceISDN connections.

The typical ISDN connection is a 64Kbps point-to-point communications,digital channel, sometimes referred to as a B (for bearer) channel. B channelshandle the data interchange (whether the information exchanged is voice or datais 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 numberof B channels you receive. A BRI ISDN link consists of 1 D and 2 B channels; aPRI link consists of 1 D and 23 B channels.

Because ISDN makes a true digital connection from your computer to theremote computer, ISDN implementation has limitations. For instance, a typicalISDN BRI connection (the most common type of connection in residential and smallbusiness environments) must be no farther than 18,000 feet from the phonecompany's central office; otherwise, you need special line conditioning. Thislimitation is not a problem for 90 percent of the existing telephone lines inthe US.

Once set up, ISDN communications links for data throughput act just likeordinary telephone links, only faster. You dial a telephone number to place acall through ISDN circuits, the same as you do with a modem. The differencebetween ISDN and traditional modems is the digital nature of the connection.Computers speak in digital, and traditional telephone lines are analog; themodem usually converts between digital and analog. With ISDN, nodigital-to-analog conversion is necessary.

For an ISDN link, call setup and protocol negotiation take as little as twoseconds; for a typical v.34 modem communications link, they require 45 secondsto 60 seconds. In ISDN, information travels through the D channel,identifying the source, destination, and type of connection you want. Then, ISDNtelephone equipment can intelligently decide how to handle the call.

Furthermore, in ISDN, because the signal doesn't need conversion, theeffective throughput rate is much higher. To achieve even higher throughput, youcan use the process of bonding, combining two or more B channels througha channel aggregation protocol such as Multilink Point-to-Point Protocol (PPP).

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

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

The primary limiting factor today for ISDN is network usage costs.Depending on how your local public utilities commission structures yourtelephone company's tariffs, it may calculate ISDN costs based on a kilopacket(1024 packets) or message unit (usually about five minutes or less). Sometelephone companies even base price on both kilopackets and message units, inaddition to a basic monthly network access charge. Telephone companies usuallycharge different rates for a single B channel connection, with incremental costsfor a 2B+D configuration. With hourly ISDN rates to a connection across townoften exceeding hourly long-distance telephone rates across the country, nowonder 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 modemmanufacturers were going to achieve. Shannon's Law, a property of communicationschannels, defines the theoretical maximum throughput you can achieve through anycommunications channel. This law takes into account physical properties of thecommunications link, such as the amount of signal the channel can carry and theamount of noise or distortion on the channel. Based on typical values for avoice-grade, analog telephone circuit, 33.6Kbps is about the best throughput youcan achieve.

The problem with Shannon's law is that it assumes both ends of thecommunications link are analog. The modulation scheme that v.34 communicationslinks 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 thevoltage at each sample into an 8-bit binary value. However, because thetelephone network's analog-to-digital conversions are not precise, a certainamount of error, quantization noise, enters the transmission during thisconversion. This noise is what limits v.34 transmission throughput to 33.6Kbps.

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

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

Furthermore, users accessing ISP resources must have a modem capable ofunderstanding the x2 signals originating from the ISP. In other words, the modemmust be able to discriminate among the 256 different PCM codes in use, and talkto the telephone network at the same PCM rate of 8000 cycles per second.Unfortunately, technical problems currently prevent the use of all 256 PCMcodes, so x2-enabled modems use 128 of them to yield an effective throughput of56Kbps.

Problems in x2 Paradise
Despite the apparent instant success that x2 technology has found in theconsumer marketplace, several problems associated with the technology willimpede its deployment and ultimate adoption by the market as a whole. First, noITU communications standard exists for 56Kbps dial-up modem technology. RockwellInternational and other manufacturers are working on alternative 56Kbps dial-upmodem technology, known as K56Flex. Communications industry leaders such asMotorola and Lucent Technologies (formerly Bell Labs) have adopted thiscompeting technology. These companies have submitted K56Flex to the ITU-T foradoption as the 56Kbps standard in dial-up modems. Naturally, the market ishesitant to purchase modems that use technology that may eventually go the routeof 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 ofU.S. Robotics in February of 1997. According to several 3Com Web documents, 3Comintends to fully support and extend x2 technology in existing U.S. Robotics and3Com products. This support will supplement the K56Flex support 3Com alreadyintends to build into several of its product lines. As 3Com is a major player inthe communications industry, it's difficult to imagine that 3Com's decision willnot 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 CardinalTechnologies either currently ship or will soon ship x2-capable modems. Majorcomputer mail-order companies such as Gateway 2000 and Dell are starting toinclude x2-capable modems in order configurations that require high-speedInternet access. However, if you think you can get a couple of x2 Sportstermodems from your local mail-order house and set up a 56Kbps link between you anda friend, think again! The client desktop modems aren't capable of an outbound56Kbps link; they operate at a standard v.34 33.6Kbps speed for outboundtransmissions.

In addition, because the x2 server-side equipment is available only fromU.S. Robotics, the server-side components are expensive to upgrade to,especially if the ISP doesn't already have U.S. Robotics equipment. In areasthat relatively small ISPs service, some time might pass before those ISPsreplace 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. Atthe outset, the best transfer speed you'll see from your ISP is 53Kbps, becauseof FCC regulations on how communications equipment can exploit the telephonecircuit. The news only gets worse from there.

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

Furthermore, x2 technology appears to be very temperamental when it comesto the quality of the circuit your telephone company gives you. Spend some timereading the modem-related newsgroups on the Internet, and you'll find numerouspostings that mention frequent disconnects and transfer speeds that are wellunder the supposed speed-doubling that the technology's name implies. Naturally,if you call your telephone company's service representatives to complain aboutpoor line quality--if you can get a technician to test the quality ofyour line--they are likely to dismiss your complaint. They'll say that you'repaying 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 Websurfing, x2 technology definitely has a benefit you can capitalize on, but onlyif your ISP supports x2, if your phone line can handle the technology, and ifyour usage is primarily download-link oriented. U.S. Robotics lists ISPs thathave x2 support available on their Web sites, and the company has a method ofdetermining whether your phone line can support x2. Point your browsers tohttp://x2.usr.com for additional information.

If you are a commercial Internet user who uploads large amounts of data orwho uses dedicated PPP connections for Web-hosting services, you probably won'tbenefit from x2 at all. In fact, you probably want to avoid it. Because thetechnology is new and still needs refining, existing v.34 modems or ISDNconnections are still best for most commercial users, depending on how you usethe 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 usersthan for residential users (business users pay telephone bills based on messageunits, whereas many residential users have unlimited local-calling-area usage).Thus, if you download significant amounts of data, x2 might help yousignificantly reduce your telephone bills.