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 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
----
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 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 
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.