ProfitPress Mega CDROM2 …eeware (MSDOS)(1992)(Eng)/ProfitPress-MegaCDROM2.B6I/MAGAZINE/MISC/MNMAR19.ZIP/BUZZP2

Text File | 1991-02-18 | 11.5 KB | 237 lines

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The CCITT is the acronym for the Consultative Committee on International
Telephone and Telegraph. This is an international body of technical experts
responsible for developing data communications standards for the world. The
group falls under the organizational umbrella of the United Nations and its
members include representatives from major modem manufacturers, common carriers
(such as AT&T), and governmental bodies.

CCITT Modulation Standards -------------------------- The CCITT establishes
standards for modulation -- actual modem signaling methods. It also determines
standards for error correction and data compression (See part 1 of this series
for a full description of these modem layers). For this reason, it is possible
(and likely) that one modem might adhere to several CCITT standards, depending
on the various features and capabilities the modem offers.







All modems signal one another at a variety of speeds, so CCITT standards for
modulation are utilized by virtually every modem manufacturer. Some of the
standards which are primarily modulation do include some of the higher layers
(such as negotiation) as well. Multi-speed modems may use several of these
standards, which include:

V.21 ---- V.21 is a data transmission standard at 300 bps. This standard is
used primarily outside of the United States. (300 bps transmissions in the
United States primarily use the BELL 103 standard).

V.22 ---- V.22 is a data transmission standard at 1200 bps. This standard is
also used primarily outside of the United States. (1200 bps transmissions in
the United States primarily use the BELL 212A standard).

V.22bis ------- V.22bis is a data transmission standard at 2400 bps. This is
the international standard for 2400 bps, and is used both inside and outside
the United States.




V.23 ---- V.23 is a split data transmission standard, operating at 1200 bps in
one direction and 75 bps in the reverse direction. Therefore, the modem is only
"pseudo- full-duplex," meaning that it is capable of transmitting data in both
directions simultaneously, but not at the maximum data rate. This standard was
developed to lower the cost of 1200 bps modem technology, which was still very
costly in the early 1980s, when such modems were designed. This standard is
still in use, but primarily in Europe.

V.29 ---- V.29 is a data transmission standard at 9600 bps which defines a half
duplex (one-way) modulation technique. Although modems do exist which implement
this standard, it has generally only seen extensive use in Group III facsimile
(FAX) transmissions. Since it is a half-duplex method, it is substantially
easier to implement this high speed standard than it would be to implement a
high speed full-duplex standard. V.29 is not a complete standard for modems, so
V.29-capable modems from different manufacturers will not necessarily communi-
cate with one another.

V.32 ---- V.32 is also a data transmission standard at 9600 bps, but V.32
defines a full-duplex (two-way) modulation technique. It is a full modem
standard, and also includes forward error correcting and negotiation standards
as well. Many modem manufacturers already have or will be introducing
V.32-compatible modems. This is generally considered "the" standard for
high-speed modems today.

V.32 is expensive to implement, since the technology required for it is
complex. As this standard becomes more common and manufacturing techniques are
refined, the pricing for V.32 modems should go steadily downward. At this
writing, V.32 capable modems are selling for between $500 and $1000 each.

Some manufacturers have created modems that can use both their own proprietary
high speed standard and the V.32 standard, for compatibility with their older
non-V.32 modems. The new Hayes Ultra and U. S. Robotics HST Dual Standard are
examples of the new "dual personality" modems that are now on the market.

V.32bis ------- This is a developing high speed standard. When fully defined
(likely by early 1991), V.32bis will operate at 14,400 bps and, like V.32, will
be a full-duplex method. The CCITT has not yet defined this standard, so no
modems currently use it (although some new modems have implemented what is
expected to be the standard and may claim V.32bis compatibility).

Error Correcting and Data Compression ------------------------------------- The
CCITT also has adopted formal standards for the higher layers of Error Correc-
tion and Data compression (See Part 1 of this series for a full description of
these layers). In order for any error correction or data compression protocol
to work, modems on BOTH ends of the connection must support it. Once two modems
are connected, they automatically negotiate between themselves to determine the
best mutual protocols they both support.

V.42 ---- V.42 is a CCITT error-correction standard that's similar to MNP Class
4 (See "What is MNP" below). In fact, because the V.42 standard includes MNP
compatibility through Class 4, all MNP 4- compatible modems can establish
error-controlled connections with V.42 modems. This standard, however, prefers
to use its own better performing protocol -- LAPM (Link Access Procedure for
Modems). LAPM, like MNP, copes with phone line impairments by automatically
re-transmitting data that is corrupted during transmission assuring that only
error free data passes through the modems. Many modem manufacturers make MNP
Class 4-compatible modems, and some offer V.42-compatible modems as well.

V.42bis ------- V.42bis is a CCITT data compression standard similar to MNP
Class 5, but providing about 35% better compression. Of course, this also means
it provides better throughput. V.42bis only compresses data that needs compres-
sion. Each block of data is analyzed, and if it can benefit from compression,
compression is enabled. Files on bulletin board systems are often compressed
already (using ARC, PKZIP, and similar programs). While MNP Class 5 can
actually decrease throughput on this type of data, V.42bis will not -- compres-
sion is only added when a benefit will be realized.

To negotiate a standard connection using V.42bis, V.42 must also be present.
Thus, a modem with V.42bis data compression is assumed to include V.42 error
correction. Some modem manufacturers already make V.42bis compatible modems,
and more are on the way.



V.42bis is NOT compatible with MNP Class 5. A V.42bis modem will establish an
error-free connection with MNP-capable modems (since V.42bis includes V.42),
but only up to MNP Class 4.

What is MNP? ------------ MNP stands for "Microcom Networking Protocol" and was
created by Microcom, Inc., a modem manufacturer. MNP offers end-to-end error
correction, meaning that the modems are capable of detecting transmission
errors and requesting re-transmission of corrupted data. Some levels of MNP
also provide data compression.

As MNP evolved over time, different classes of the standard were defined,
describing the extent that a given MNP implementation supports the protocol.
Most current implementations support Classes 1 through 5. There are higher
classes, but are usually unique to modems manufactured by Microcom, Inc. since
they are still proprietary.

MNP is generally used for its error correction capabilities, but MNP Classes 4
and 5 also provide performance increases, with Class 5 offering real-time data
compression. The lower classes of MNP are not usually important to you as a
modem user, but they are included here for completeness.


MNP Class 1 ----------- MNP Class 1 is referred to as Block Mode. It uses
asynchronous, byte- oriented, half-duplex (one way) transmission. This method
provides only about 70% efficiency. It provides error correction only, and is
rarely used today.

MNP Class 2 ----------- MNP Class 2 is called Stream Mode, and uses
asynchronous, byte- oriented, full- duplex (two way) transmission. This class
also provides error correction only. Because of protocol overhead (the time it
takes to establish the protocol and operate it), throughput at Class 2 is
actually only about 84% of that for a connection without MNP, delivering about
202 cps (characters per second) at 2400 bps (240 cps is the theoretical
maximum). Class 2 is rarely used today.

MNP Class 3 ----------- MNP Class 3 incorporates Class 2, and is more effi-
cient. It uses a synchronous, bit-oriented, full-duplex method. The improved
procedure yields throughput about 108% of that of a modem without MNP, deliver-
ing about 254 cps at 2400 bps.

MNP Class 4 ----------- MNP Class 4 is a performance enhancement class that
uses Adaptive Packet Assembly(tm) and Optimized Data Phase(tm) techniques.
Class 4 improves throughput and performance by about 5%, although actual
increases depend on the type of call (local or long- distance, noisy or clean
connection), and can be as high as 25% to 50% on some links.


MNP Class 5 ----------- MNP Class 5 is a Data Compression protocol which uses a
real- time adaptive algorithm. It can give an increase of up to 50% in through-
put, but the actual performance of Class 5 is very dependent on the type of
data being sent. Raw text files will allow the highest increase, while program
files cannot be compressed as much and the increase will be less. On pre-
compressed data (files already compressed with ARC, PKZIP, etc.), MNP 5 can
actually EXPAND the data and performance can actually decrease. For this
reason, MNP 5 is often disabled on BBS systems.

MNP Class 7 ----------- MNP Class 7 is the other major MNP protocol you are
likely to encounter. MNP 7 provides Enhanced Data Compression. When combined
with Class 4, it can obtain about a 300% improvement in performance. It is
designed primarily for use with V.22bis (2400 bps) modem. This class is
currently unique to Microcom modems.  Since it requires much more hardware and
is usually inferior to V.42bis, it is not likely to proliferate.

What does it all mean? ---------------------- Despite the fact that they can
seem quite confusing, all of these standards exist to benefit you the modem
user. You want to be able to compare modems on price, reliability, performance,
and support. You also want to be able to know that modems from different
manufacturers will communicate with each other.








The past couple of years in the high speed modem arena has shown what happens
when market demand occurs faster than associated standards. You are forced to
pick a single manufacturer and become locked in to gain the capabilities you
want. The purpose of standards is to prevent this situation.

When standards are widely adopted, you get the best of technology and competi-
tion. However, you need to know what the standards mean to be able to be an
informed consumer.

Next month we'll wrap up this discussion with explanations of most of the rest
of the various terminology common to the modem world, but not always fully
understood. Don't miss it!

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