V.32 ---- V.32 is also a data transmission standard at 9600 bps, but V.32 defines a full-d
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
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.
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 Correction 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 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 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 compression. 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 -- compression 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
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
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.
E-Mail Fredric L. Rice / The Skeptic Tank