Date: Fri,  9 Oct 1992 09:23:26 -0400 (EDT)
From: Tom Holodnik <tjh+@CMU.EDU>
Subject: Re: Dial-up gripe

[ quite a bit deleted ]

On the subject of standards,  these things are a moving target.  V.32bis
and V.42bis are second iterations of the earlier V.32 and V.42 standards
(hence the term "bis").  Prior to that, many modems were advertised with
MNP levels 3,4, and 5.  In 1993, it's epected that another standard,
currently called V.fast will come to maturity at the standards committee
level, and will reach the market in 1994.  This will provide for speeds
of up to 115 kilobits per second over a modem link.  It's easy to get
confused, so I wrote a summary... 

Here's a summary of common standards and jargon:

V.32:
        A standard for modulation, which provides for an effective data
rate over the telephone system of 9600 baud. This allows for echo
cancellation over the telepone line, so that the line is full duplex.
Echo cancellation is done on the modem by allowing the its receiver to
know what's being transmitted, so that it can filter that signal off
of the incoming data.

V.32bis:
        Another standard for modulation which provides for an
effective data rate over the telephone system of 14.4 kilobits per
second. While the data rate between modems is 14.4 kilobits per
second, the speed of either serial line on each end of the pair of
modems is settable; one end may operate at 9600 bps, and the other end
may operate at 19.2 kbps. This is known in some modem manuals as
"speed conversion." Some modems allow for "baud rate adjustment," to match
the speed of data on the remote serial link.
        For our configuration, your host may not be capable of receiving
characters faster than 19.2 kbps, so you set the serial line between
you machine and your modem to operate at 19.2. You can then set the
modem to operate at 14.4 kbps on the telephone line to the remote
modem on campus.

MNP levels 3 and 4:
        This is a protocol for performing error detection and
correction. This is done in MNP 3 by taking the bits passing over your
asynchronous connection and framing them into a packet for
transmitting over a synchrnous link.  In MNP 4, the size of the packet
is variable, so that if your line is well behaved, it will send data
in larger chunks.

MNP Level 5:
        This provides everything in levels 3 and 4, but also provides
for data compression. Best case, the effect of compression under MNP 5
is a factor of two over the data rate. On a 2400 bps  MNP 5 link, under
ideal circumstances, you can transfer data at 4800 bps.


V.42:
This standard provides for error detection and correction of a modem link.
V.42 begins by dividing the transmitted data into blocks and sending them
time-aligned, performing async to sync conversion.  Error detection is
accomplished by checking each packets CRC, and correction is accomplished by
retransmission (called ARQ, or Automatic Repeat reQuest).  V.42 provides for
interoperability between error-correcting and non-error-correcting modems.
The relationship between V.42 and MNP Level 4 is as follows:  V.42 uses
LAP-M (Link Access Procedure for Modems), an HDLC format, or MNP-4 as its
error-correction algorithms.  The V.42 standard defines hand-shaking
procedure for selection and negotiation of error correction, optional modem
procedures and loop-back testing of the modem link.

V.42bis:
        V.42bis compression is built on V.42, which uses MNP 3,4 or
LAP-M, and provides for interoperability for modems using MNP-5
compression.  A modem that uses V.42bis compression will encapsulate
the asynchronous traffic into packets for transmision over a
synchronous link over the telephone line. V.42bis allows for the
transmitting modem to send 8 frames before getting an ACK, and
proceeding with more frames. The sending modem will calculate a 16 bit
cyclic redundancy check, and send it in the packet to the other modem.
If the CRC doesn't match what the receiving modem calculates in the
6th packet in the 8 packet stream, it will will send a signal to the
other modem that it received an error in packet 6. The transmitting
modem will then retransmit packets 6, 7, and 8.

"bis":
        Since the "V." series of standards were formed by the ISO
(International Standards Organization), there's a significant European
influence. "bis" comes from the latin term meaning, "second try" or
"second iteration."  V.32bis is the second version of the V.32
protocol.


Hope this helps...

Tom Holodnik
Network Development / Data Communications