                 THE CCITT 28.8 KBPS STANDARD: SOME BACKGROUND
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     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 band-
width is available and how to best position the signal to match available band-
width."
     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 infor-
mation.

