SUMMARY REPORT OF MEETING OF CCITT STUDY GROUP XVII
GENEVA, SWITZERLAND, 13-21 MARCH 1989

by Toby Nixon, Principal Engineer and Standards Committee
Representative, Hayes Microcomputer Products, Inc.

INTRODUCTION

CCITT Study Group XVII, which handles standards related to data
transmission on the telephone network, met in Geneva, Switzerland,
March 13-21, 1989. This is a summary of activity on items of
widespread interest, prepared by Toby Nixon, representative to the
CCITT from Hayes Microcomputer Products, Inc., and a member of the
USA delegation at the meeting.

V.42bis DATA COMPRESSION

The draft recommendation for data compression in V.42 modems reached
an advanced stage of development due to extensive and lengthy work by
the editor (Mr. Alan Clark of the UK) and the ad hoc editing group.
It was agreed to apply the accelerated approval procedures at the
September 25-29 meeting of Study Group XVII. Under the new CCITT
rules, this will result in a final, accepted recommendation (rather
than merely a "provisional" recommendation) shortly after the
meeting. Between now and then, manufacturers will be producing and
testing prototypes to verify the correctness of the draft standard.

INTERWORKING BETWEEN V.32 AND LOWER SPEED MODEMS

The proposed annex to V.32 specifying interworking with V.22bis
modems also received considerable attention, and a compromise was
reached between the competing positions that appears to solve all of
the outstanding issues. Again, between now and the fall meeting,
manufacturers will be producing prototypes to verify the draft, and
testing them with other multimode modems and with existing
lower-speed and V.32-only modems.

HIGHER SPEEDS IN V.32

British Telecomm presented very complete and persuasive test results
that indicate the feasibility of 12,000 bit/s and 14,400 bit/s
operation of echo- cancelling modems (V.32) over domestic and
international circuits. However, it does not appear that their
proposal is sufficiently detailed to permit adoption of an extended
V.32 recommendation incorporating their techniques before at least
the Spring 1990 meeting of the study group. Manufacturers will be
examining the BT contributions and trying to reproduce their test
results. The study group agreed that a half-duplex fast-turnaround
operating mode for V.32 (similar to that in the Hayes V-series
Smartmodem 9600) would be considered if the asymmetrical modem work
was discontinued. A liaison statement was sent to ISO requesting
their views on ways of expanding the V.24 (RS-232) speed
indication/selection circuits to accommodate modems with more than
two speeds.

CORRECTIONS TO V.42 AND STATISTICAL MULTIPLEXING

The proposed corrections to V.42 from the USA and Hayes and the V.42
statistical multiplexing extensions proposed by AT&T and Hayes were
accepted in principle and text drafted, but time constraints
precluded adoption of the final text. The information will be
annexed to the report of the meeting, and therefore available to
implementors and for comment at the next meeting (the corrections are
already incorporated in Hayes V.42 modems). AT&T committed to
perform a simulation of the performance of a V.42-based statistical
multiplexer versus an existing statistical multiplexing design which
packs data from several ports into a single information frame.

ASYMMETRICAL MODEM

The draft document which was jointly edited by USRobotics and Telebit
was reviewed once again, and additional editorial changes made. The
work is stalled awaiting test results. Unless comparative testing
shows that the asymmetrical modes provide significant performance or
cost/benefit advantages over V.32 (including the new higher-speed
V.32 proposals), the work on the standard will be discontinued. A
number of concerns continue to be raised regarding the effect of
forward-channel far-end echoes on the backward- channel transmission
in digital carrier systems (introduction of extreme quantization
noise), and the use of frequencies outside the normal voice band
(above 3100 Hz).

INTERNATIONAL STANDARD "AT" COMMAND SET, V.25bis ENHANCEMENTS

Toby Nixon was appointed as Special Rapporteur for Question 14/XVII,
which is concerned with enhancements to Recommendation V.25bis (the
current international standard for programmatic control of modems by
computers and terminals). Based on statements from the chairmen of
Study Group XVII, Mr. Klaus Kern of West Germany, and of Working
Party 1, Mr. Richard Brandt of AT&T, the clear intention of asking
Mr. Nixon to serve in this position was to get an AT command
set-based CCITT Recommendation as quickly as possible. Mr. Kern, in
fact, said that he strongly hoped that a version of the AT command
set standard being worked on in the USA could be presented as a
normal (white) contribution in advance of the September meeting of
the study group.

NEED FOR STANDARD "AUTOMATIC SPEED BUFFERING" MODE

The need was identified for development of enhancements to V.14
(async to sync conversion, without error-control) to incorporate
buffering and flow control. This is, of course, actually already
done by most manufacturers of error-control modems, but is not
provided for in any recommendation. Hayes will make a contribution
on how this is done, including Transparent XON/XOFF flow control
(already a feature of Hayes V-series modems). There are several
applications for Transparent XON/XOFF and recommendations that could
be affected: V.14, V.42 (error control), V.25bis (configuration),
V.120 (ISDN terminal adapters), X.3 (X.25 PADs), etc. Also, V.14 and
enhanced V.14 (with flow control and buffering) in non-error-control
half-duplex and asymmetrical modems will be studied.

FORWARD ERROR CONTROL FOR CELLULAR TELEPHONE MODEMS

OKI of Japan proposed extensions to V.42 to include redundant
encoding of various types to improve the performance on cellular
telephone channels, which have very high error rates. The proposals
are complex, and little work was done beyond the introduction of the
documents. Delegates will study them, and more discussion will take
place at the next meeting.

NETWORK MANAGEMENT

The activities which took place at previous Special Rapporteur's
meetings was reviewed. The topic which received the most discussion
was the current verbose encoding rules planned for management
protocols. Study Group XVII sent liaisons to other groups regarding
development of more compact encodings to allow efficient operation on
the very-low-speed side channels used for network management
communications between modems. The group also reviewed contributions
proposing object-oriented descriptions of modems and terminal
adapters, a necessary stepping-stone toward a physical-layer network
management standard.

ISDN TERMINAL ADAPTERS

A contribution from the USA on changes to Recommendation V.120 to
accommodate additional packet mode bearer services was reviewed and
largely accepted. Further work was deferred until the next meeting
because of the time devoted to other projects.
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