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V34

Todd Boyle

Magazine Extract: Data Communications, Nov. 1994 V.34 Modems: watch the fine print

The V.34 standard for 28.8 Kbit/second modems was approved by the International Telecommunications Union's Telecom Standardization Sector (ITU-T) in September 1994. Until that date, there had been no official standard for modems since the V.32bis standard for 14.4 Kbits/second.

Previous high-speed modems such as V.32terbo, V.fc (V.fast class), etc. were either proprietary, or supported by a limited number of manufacturers for certain countries.

V.34 is a highly complex standard: the ITU-T standard is physically twice as long as the V.32bis spec, and it defines "adaptive intelligence" requiring the modems to adjust to line conditions at a much higher level. This standard requires at least 35 to 40 MIPS of processing power in the modem, and that assumes very careful programming to avoid wasting processing power. Approx. 20 MIPS were required for V.32.

Long delays by some chip manufacturers in releasing their V.34 chipsets suggests that they may be having problems implementing the standard. The combination of chip design, programming, and choosing which parts of the V.34 standard to support, is difficult for manufacturers. The V.34 standard allows manufacturers to choose from a wide range of design choices, and still be considered complying with the standard. Baud rates, auxiliary management channel, asymmetrical transmission, trellis codes, nonlinear coding, and precoding are listed as options above and beyond the standard.

All V.34 modems are likely to reach 28.8 kbit/s on good connections. However, problems are expected even between some modems of the same manufacturer when lines are bad. The problem is that some of the modems will drop back to unnecessarily slow speeds. As an example, the standard requires at least three baud rates be supported: 2400, 3000, and 3200. Other baud rates (2743, 2800, 3249) are optional but important in various line circumstances such as high frequency noise or baud limitations caused by pulse-code modulation technology used by some satellites. For example, if you have a minimally compliant modem it may fall back to 2400 baud when full V.34 modems may work at 2800 or 2743. The AT&T Comsphere modem hits 33.6 kbit/s on V.34 optional baud rate of 3249. Interesting parts of the technology are the way it works with the signal. An electrical signal can be charted as a voltage, over time. You may have seen this on an oscilloscope or automotive tuneup machine, etc.

After defining the baud rate, which is like slicing time into segments, the standard defines how to measure the squiggly line to yield nine different data points for each symbol or "constellation". The transmitter in the modem converts data into this standard, and the receiving chip measures the signal's voltage and phase to extract the 9 data points again. Thus there may be 3200 baud or symbols per second, and if the signal is clear, each symbol can carry 9 bits of data, giving the magic 28,800 kbits/second. (Therefore, don't ever say you have a 28,800 baud modem!)

Other technical areas in V.34 important for compatibility are:

  • Trellis coding - vendors must support one of these: 16, 32 and 64 bit coding on the receiver. This improves signal-to-noise by attempting to anticipate the next expected symbol.
  • Precoding - a form of equalization, perhaps like Dolby sound on a cassette. It cuts high frequency noise interference. Not mandatory.
  • Nonlinear Encoding - shifts the timing of the abovementioned 9 data points to nonlinear, unequal spacing to avoid certain types of interference caused by PCM analog-digital conversions done by telephone companies on voice lines. It is not mandatory under V.34.

The worst cases of V.34 to V.34 apparently are not all that bad: they will only occur under bad line conditions and apparently the worst you will get is 2400 baud times some number of data points.. and V.34 requires the modems to step down in smaller steps than under the old 14.4 standards (which often just cut the connection when quality deteriorated...)

The V.34 market is not a mature market. Most vendors will change and adapt their products. Buyers are recommended to look closely at the method promised by the Vendor to upgrade or update the modem:

  • EPROM memory - Eraseable Programmable Read only Memory - can be updated by a simple software download.
  • PROM - Upgrades are received in the form of chips. The chips are programmed by special equipment.

The "mail-back" modem. Some vendors actually sell this type--the ASIC's on the board must be replaced. ----In conclusion, and this is just my opinion, with V.34 modems on sale under $250 in magazines, it's hard to imagine buying a 14.4 modem now. However, now that the standard is very well known, it would be a shame to buy a V.terbo or something from old stock, when you'll run right into the upgrade problem as soon as you open the plastic!


© Algorithmica Japonica Copyright Notice: Copyright of material rests with the individual author. Articles may be reprinted by other user groups if the author and original publication are credited. Any other reproduction or use of material herein is prohibited without prior written permission from TPC. The mention of names of products without indication of Trademark or Registered Trademark status in no way implies that these products are not so protected by law.

Algorithmica Japonica

January, 1995

The Newsletter of the Tokyo PC Users Group

Submissions : Editor


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