The CCITT 28.8 Kbps Standard: Some Background

The standard for 28.8 Kbps dial-up communications being developed by the CCITT will stretch the limits of dial-up telephone lines.  But don't expect to be sending files at top speeds immediately.

Dale Walsh,  vice president for advanced development at U.S. Robotics, Inc., cautions that most users won't be able to achieve the maximum speeds permitted under the standard right away. When the standard is adopted, modems that conform should be able to transmit at 19.2 Kbps on lines where a modem conforming to V.32bis (the current high-speed standard) can now transmit at 14.4 Kbs.  But Walsh,  a member of the CCITT committee developing the standard, says it is being formulated with the increasing digitization of  dial-up phone networks in mind.

The improved quality of phone lines, as much as any modem technology improvements, has made high speed dial-up communications possible" says Walsh. "We are designing the standard keeping in mind that phone networks will use more and more digital circuits and digital central office switches."

Consequently, speeds in the neighborhood of 28.8 Kbps will be the exception, rather than the rule--at least until the public switched telephone network becomes completely digital.

"I'd say when it's finished in a year or so, the standard will allow 19.2 Kbps transmissions on 80 percent of all lines, 24 Kbps on 50 percent of all lines, and 28.8 Kbps communications on 20 percent of all lines," says Walsh, who also helped develop the CCITT V.32bis standard for 14.4 Kbps dial-up communications.  "As the phone networks improve, top speeds will be more easily achieved, so I think it's more realistic to think of it as a 19.2 Kbps standard that is sometimes capable of higher speeds.

A common misconception is that the coming 28.8 Kbps standard will allow speeds of 115.2 Kbps, when combined with V.42bis data compression.  Walsh noted that such calculations are based on an assumption that V.42bis allows 4 to 1 data compression.  Outside the lab, V.42bis allows compression ratios between 2 to 1 and 3 to 1, meaning that under REAL LIFE conditions, the forthcoming standard will allow maximum throughput of about 86.4 Kbps.

"It's a mistake to use the 28.8 number and max everything out from that," said Walsh.

Not that the 28.8 Kbps standard doesn't have its advantages.  After all, 19.2 Kbps is an improvement over 14.4 Kbps.  And the forthcoming standard will adjust for line conditions, a critical factor in high-speed data communications--like no previous standard.  The standard under development will include a "probing" function that the modem can use to "sound out" the quality of the phone line.  That will allow the modem to optimize transmissions to take advantage of available bandwidth.

"We are still working on the training sequences," says Walsh.  "But before transmission starts, the standard will enable the modem to determine what bandwidth is available and how to best position the signal to match available bandwidth."

The standard achieves higher speeds through its ability to use more of a line's bandwidth, not just the center portion of the channel used under current standards.

"It will more closely match the modulation scheme to what's available,"  says Walsh.  "That way, the modem can shape the transmissions to adapt precisely to the channel, which is very important if you want to transmit at high speeds.

The most recent committee meeting was held last month.  Still to be agreed upon are such critical issues as training sequences, coding schemes, and signaling rates.  Walsh said he expects the committee to reach final agreement in 1993, with official CCITT adoption likely in 1994.

While some have taken to calling the standard under development V.Last, Walsh isn't convinced this will be the final modem standard.  After all, he notes, no one thought dial-up phone lines would ever be this noise-free, and further advances in that area could make even higher speed dial-up communication possible.

"I'm certain we'll have at least a fax version of this standard as the quality of phone lines keep improving," he said.  "We're trying to be sure that this standard will serve users into the year 2000.  But modem standards are like wars: you always think it's going to be the last one."


U. S. Robotics, Inc, (NASDAQ:USRX) (800)DIAL-USR voice for more information.