28
TOTE-A-FAX 542
In other words, if you happen to get
a poor quality connection (and it is
totally random), you will not
quadruple your data throughput
simply because the local and remote
modems both support V.42 and V.42
bis. You
will
get a higher effective
data throughput working with two
modems that support V.42 and V.42
bis. But you will get a higher
effective data throughput with a
good “clear” connection, and a
correspondingly lower effective data
throughput speed with a “noisy”
connection.
4.7.3 V.42 & V.42
BIS IN
D
ETAIL
The Tote-A-Fax 542 modem
supports V.42 and V.42 bis. An
internal controller regulates data
compression. Efficient block size
adjustments help to compensate for
situations where the signal-to-noise
ratio is poor.
A V.42 bis modem combines two
error correction protocols: Link
Access Procedure–M (LAPM), and
MNP 1–4. This increases the the
possibility of an error-free trans-
mission. Data is sent in frames with
a “start” bit and a “stop bit” on either
end of the data frame. The start and
stop bits are “stripped” from the data
being transmitted. This means only
eight (8) bits per character are sent
from the local terminal device (your
PC), rather than 10 bits per
character. This represents a 20%
gain in data throughput. However,
this gain is partially off-set by the
framing characters and Cyclic
Redunancy (CRC) data used by
MNP.
Encoded information is added to the
data frame for the receiver to
decode. It then determines if the
block was error free. That determ-
ination is made through the CRC
process. The receiver checks the
data-encoded information to:
• Indicate if the data is correct.
• Recognize and identify inperfect
data frames/
• Determine if the data has failed
CRC. If so, retransmission is
automatically spread.
The V.42 bis modem works by a
simple process. Its initial action is to
seek LAPM error correction
compatibility with the receiving
modem.
• If the compatibility exists, it
employs the LAPM error
correction protocol.
• If the V.42 bis modem cannot
find LAPM compatibility, it tries
to connect using the MNP-5
protocol.
• If it can’t establish connection
according to either protocol, it
will still connect withthe remote
modem in normal mode; error
correction is not applied.
4.7.4 S
UMMARY
The Tote-A-Fax 542 modem is a
2400 bps modem. It has the
potential to allow effective data
throughput of up to 7200 bps when
it uses MNP Level 5 data
compression, or up to 9600 bps
when it uses the V.42 bis data
compression protocol.
