July 15, 1992 -- CURRENT STATUS OF THE V.FAST MODEM STANDARD

An interview with Donald O'Connor. Chairman of TIA TR 30.1, the American
committee of approximately 35 persons representing about 25 companies
developing U.S.A. positions on core modem data pump issues for CCITT Study
Group XVII.

CCITT (Consultative Committee International for Telegraphy and Telephony)
is the Geneva-based advisory committee established by the United Nations
to recommend worldwide transmission standards within the International
Telecommunications Union. CCITT Study Group XVII consists of about 50-60
scientific and industrial organizations (such as manufacturers) and user
groups such as national telephone administrations. Study Group XVII is
developing the "V.Fast" Recommendation for two-wire duplex modems with
data rates above 14,400 bits per second (bps). O'Connor is Fellow of
Engineering, Racal-Datacom, Central Engineering Division.

QUESTION: There is growing interest in what has been nicknamed the "V.Fast"
standard for modem speeds to reach and surpass 19,200 bits per second. How
close are we to obtaining the "V.Fast" standard during the Summer of
1992?

O'CONNOR: It's more than a year in the future. "V.Fast" is just a nickname.
There is no standard V.Fast modem.

When the CCITT adopts a V series standard, a number is assigned, such as
the V.32 standard for modems with a data rate of 9,600 bits per second or
V.32bis for modems capable of operating as fast as 14,400 bits per second.
Since we are still using the nickname of "V.Fast" for the standard now
being studied, you can tell it is still in the discussion stage.

In fact, very little has been decided about the V.Fast standard. What has
been accomplished so far is that the new standard will use single carrier
modulation for data rates between 9,600 to 24,000 bps in 2,400 bps
increments. There may be data rates of 26,400 and 28,800 bps, but
performance at these speeds need to be substantiated. Data rates below
9,600 bps are also possible.

V.Fast is the most complicated of the V standards to date. Issues still to
be decided include the modulation scheme, how to achieve fractional bits
per symbol, the channel probing strategy, the startup handshake sequence,
automode capability with the installed base of modems, issues such as
transmit power control, constellation pre-warping, transmit shaping and
many other technical issues. Almost all of the technical issues are still
to be decided. Even symbol rates and carrier frequencies are not yet
decided.

QUESTION: How are these issues decided?

O'CONNOR: Manufacturing companies have positions on each of these issues
and they prepare engineering papers advocating their positions. Then the
Rapporteur Group (a study group which is assigned to a specific issue such
as V.Fast) comprised of persons from a number of manufacturers and
administrations (national PTTs) come together to evaluate these positions.
The competitors thrash out conflicting opinions to reach recommendations
that will best serve the needs of customers. Then we make our
recommendations to Study Group XVII. When the Study Group tackles the
issue, there are further discussions that involve additional users (PTTs).
When there is consensus by all those concerned, then a standard can be
issued.

QUESTION: When can we expect to have the V.Fast standard?

O'CONNOR: There's a Rapporteur's meeting scheduled for September 1992,
possibly another one in late 1992, a Study Group XVII meeting in January
1993 and another Rapporteur's meeting in the Spring. Then there will be a
Study Group XVII meeting in September 1993. The earliest that V.Fast can
be approved is at this September 1993 meeting. But considering the
significant number of open issues, the first quarter of 1994 is much more
likely.

QUESTION: Some companies. such as Racal-Datacom, have already announced
plans to ship V.Fast modems this year. Will we have to wait until sometime
in 1994 to obtain CCITT standard V.Fast modems?

O'CONNOR: Yes. There are no standard V.Fast modems today. There can't be a
true standard V.Fast modem until agreement is reached in CCITT which may
take place in early 1994. Any announced V.Fast modem today is really a
proprietary two-wire duplex unit with data rates greater than the V.32bis
standard.

Most of the details and alternatives for the eventual V.Fast standard are
already on the discussion table. The menu is there with major choices
before us. I don't expect any new significant technical details will be
added to the discussion. Although there are many characteristics before
us, the menu has been presented for our selecting. The Study Group has to
sort out the best technology. There may also be testing of existing
proposals. Meanwhile vendors may produce new modems with speeds greater
than V.32bis and the user should consider whether these proprietary modems
are flexible enough to meet the eventual V.Fast standard. Since there are
still so many outstanding issues, users need to protect their investment
via the modem's flexibility to meet the standard.

In conclusion, V.Fast today is only a nickname and there are no standard
V.Fast modems. Most of the technical details are on the discussion table
in CCITT, but it will be a long time before a standard is finalized
(September 1993 best case, Spring 1994 more realistic). With regard to
proprietary "pre-standard" V.Fast modems, the user, in order to protect
his investment, should consider the "pre-standard" modem's flexibility to
meet the ultimate standard.

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