V.34: The next-generation modem

Network World
Dale Walsh

March 7, 1994

IN THE WORLD of data delivery, engineers are continuously trying to push the envelope on modem speed. Their latest attempt at increasing the number of bits a modem pushes across dial-up facilities each second is the V.34 proposal, the latest in the V series of modem standards.

Originally nicknamed V.fast, V.34 will enable the speed of next-generation modems to top out at 28.8K bit/sec. The proposal is now being studied by the the International Standards Union's Telecommunication Standardization Sector (TSS), the international body that sets standards for data communications devices.

Formerly called the CCITT, the TSS is expected to finalize the V.34 standard in June. However, field testing of prototype modems meeting the draft specification is underway.

V.34's top speed of 28.8K bit/sec is attained in direct modem-to-modem communications. Factoring in data compression, a procedure for reducing the size of computer files being transmitted, V.34 modems will be able to send asynchronous data at double or triple the 28.8K bit/sec speed.

To justify development of another V series modem standard, the TSS felt it had to call for a speed that would be at least twice as fast as the 14.4K bit/sec V.32bis, the prevailing fastest standard. At the outset of development, most TSS participants believed 28.8K bit/sec was a stretch for dial-up phone connections. Considerable study and testing revealed, however, that the 28.8K bit/sec speed could be achieved on a high percentage of calls.

Three major changes make it possible for modems to attain 28.89 bit/sec speeds over dial-up lines. The equipment in carrier networks has drastically improved over the years, resulting in fewer line impediments and paving the way for data to be transmitted faster.

At the same time, modem technology has dramatically improved. In essence, modems are much more intelligent and flexible than they used to be. For instance, modems today can sense the quality of dial-up circuits and adjust their speeds up or down match line conditions.

Finally, the availability of low-cost, high-power silicon chip technology, coupled with the development of new computation-intensive technologies, makes it possible for modems to process signals faster.

For example, V.34 modems will be able to process between 1,000 and 2,000 computations for each bit sent and received. Compare that with the 200 to 300 calculations the 9.6K bit/sec modems in Group III facsimile machines perform on each bit.

Yet increasing both the complexity and speed of a modem doesn't come easy. There are definite boundaries for both, and V.34 pushes those limits. A number of innovations enable V.34 to transmit data faster.


One of these innovations is called precoding. The need for precoding actually came from an earlier modem innovation called adaptive equalization, which compensates for amplitude and phase distortions of signals transmitted across a dial-up line. However, adaptive equalization sometimes magnifies channel noise. Precoding allows the transmitting modem to use a coding procedure to change the transmitted signal in ways that prevent noise multiplication in the receiving modem.

Multidimensional coding and constellation shaping are other innovations that V.34 will use to give the data greater immunity to noise in the channel. Noise can make the modem receiver confuse a transmitted symbol for another symbol.

Each symbol a modem sends represents a certain number of bits. The more computation-intensive the modem's technology, the more bits per symbol the modem can send.

A V.22bis, or 2,400 bit/sec, modem sends four bits per symbol and 600 symbols per second to achieve its speed of 2,400 bit/sec. The V.32 modem sends four bits per symbol and 2,400 symbols per second reach its speed of 9.6K bit/sec. V.32bis sends six bits per symbol and 2,400 symbols per second to achieve 14.4K bit/sec. But V.34 will send up to nine bits per symbol and 3,200 symbols per second to achieve 28.8K bit/sec, twice as fast as V.32bis.

Sending modems assemble a number of bits and convert them into a corresponding symbol, roughly like bits can be converted into an alphanumeric character. The result is an alphabet of symbols, which are electronic waveforms. However, because each symbol carries several bits of information, it is easy to confuse one symbol for another. It's the receiving modem's job to correctly detect these symbols and convert them back into bits.

In modem terms, the difference between one legitimate symbol and another is called the distance between the two symbols. The greater the intersymbol distance, the more noise is required to span that distance. The coding and shaping called for in the V.34 standard increases the distance between transmitted symbols.

But the quest for greater intersymbol distance leads to high-performance codes that rapidly increase receiver implementation complexity. Reduced complexity decoding is an innovation that will make it possible for a V.34 modem to select a high-performance code without paying the complexity penalty. In other words, V.34 uses a code that, without reduced complexity decoding, would be impossible to use.

Another innovation V.34 will tap is nonlinear coding, which addresses the problem of signal peaks being distorted due to nonlinear circuit elements.

Normally, there is uniform distance between signals. Nonlinear coding increases distance between signs to make them less susceptible to distortion.

A final innovation, line probing, will enable V.34 modems to quickly examine the dial-up line during the modem initialization procedure for distortions or noise. A V.34 modem will then select the best transmission strategy to optimize the data transmission rate over that circuit.

To measure the amount of noise and other unwanted signals on dial-up lines and extrapolate how fast the modem should run, V.34 calls for the use of test tones to determine data speed capability.


With all these innovations, anyone looking inside a V.34 modem would see more sophisticated hardware elements than are used in current modems.

For example, V.34 modems use higher resolution analog-to-digital as well as digital-to-analog converters called sigma-delta converters to reduce noise generated by the modem itself as opposed to noise generated by the dial-up line.

In addition, because innovations such as multidimensional coding chew up memory, V.34 modems will have larger and faster memory chips than other modems. Finally, V.34 modems will use a new generation of higher speed digital signal processors, which enable them to support higher computational loads.

Low bit rate video and audio graphic conferencing are new applications made possible through the increased bandwidth used by V.34. Low bit rate video allows two computers to communicate using video, voice and data. Through one phone line, for example, a sales manager can hold a meeting with his sales force -- speaking to and seeing the sales representatives on his computer, in addition pulling up a spreadsheet for use in the meeting.

Obviously, the more sophisticated, state-of-the-art hardware pushing the technology will add to the cost of V.34 modems. However, as the manufacturing learning curve declines and vendors recover their research and development costs, consumers will see the price of V.34 modems drop to the price of today's V.32bis modems.

Although V.34 is on the threshold of being completed and adopted, a number of modem manufacturers have introduced modems that already work at speeds of up to 28.8K bit/sec. Those modems are based on proprietary schemes, rather than the TSS V.34 standard.

These early-to-market modems only communicate with other modems from the same vendor or a small group of vendors that use the same proprietary scheme and will likely not comply with the V.34 standard when it comes out.

Most modem manufacturers are going to offer an upgrade path to the V.34 standard. They will provide 14.4K bit/sec V.32bis products that can later be upgraded to the V.34 standard for a fee. This strategy ensures compatibility between their V.34 modems and those of other vendors.

There are two V.34 upgrade strategies that most vendors will likely follow: software and hardware. Software-upgradable modems require that the V.32bis modem processors initially have enough additional computational horsepower to run V.34. Hardware-upgradable modems require that both the computational engine and software be upgraded later, when the standard comes out.

It can be assumed that all V.34 modems will talk with most of the earlier generation V series modems, automatically falling back to run at the highest common speed.

But when two V.34 modems communicate, a special handshake will quickly exchange a capabilities menu, test the channel conditions, negotiate transmission speeds and complete an initialization sequence -- all within five seconds.

Just another technical advancement in the world of data communications.

Dale Walsh is vice president of advanced development for U.S. Robotics, Inc., in Skokie, Ill., and a member of the TSS committee developing the V.34 standard. He can be reached via the Internet at dwalsh(at)usr.com.


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