3Com 3C421600A, Management Manual

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134 C
HAPTER
10: A
DVANCED
M
ODEM
C
ONFIGURATION WITH
CLI/AT
COMMANDS
Configuring Data
Compression
Settings
Data compression is a method by which the modem sending
(transmitting) compresses the data being sent as it transmits, and the
receiving modem decompresses the data as it is received. V.42
bis and the
Microcom Networking Protocol-5 (MNP-5) allows for this compression to
take place.
When data compression is enabled and an RAS 1500 makes an outgoing
call, and if it successfully establishes a V.42 error control connection with
the remote device, it negotiates for V.42
bis data compression. If
unsuccessful, the module then attempts to use MNP-5 data compression.
Compression does not occur unless the modems are able to establish an
error control (
Automatic Repeat Request [ ARQ ]) connection.
The type of compression used for a call, if any, is reported through the
ATI6 command and in the CONNECT message, if &A3 is configured as a
default setting.
V.42 bis versus MNP-5 Data Compression
Of the two data compression protocols, V.42 bis is considered to be the
better of the two in most cases because it dynamically deletes entries that
are no longer used.
In addition, it works better with files that are already compressed. These
include .ZIP files downloaded from many electronic bulletin board systems
and 8-bit binary files, which seem to modems to be compressed.
Sometimes the MNP-5 protocol attempts to compress an already
compressed file, which causes the file transfer to take longer. V.42
bis
only compresses data when compression yields an advantage.
To function correctly, V.42 bis requires V.42 error control to be enabled.
The maximum compression, based on a standard file, that can occur
using V.42
bis
is 4:1. MNP-5 requires the MNP-4 error control protocol to
be enabled and can only reach a maximum compression of 2:1.
When transferring compressed files, configure the RAS 1500 modems to
&K3 (See Configuring Data Compression Methods below). This allows
V.42
bis compression to work dynamically with the compressed data, but
disables MNP-5.