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'otocols
and Compressio
9.2
Compression
Compression is used to increase the effective transfer speed over
the line. Since compression schemes are dependent on error free
data transfer, they are always used in conjunction with a protocol.
The modem supports two different compression schemes, MNP5
which uses run length and adaptive frequency encoding, and V.42bis
which uses Lempel-Ziv encoding.
The %Cn command controls the use of compression (both MNP5
and V.42bis) as follows :
AT#COp |
ATHC gs
AT%C3e%].
The increase in speed using compression depends on several
factors :
Compression is disabled.
. | Compression is enabled.
Compression of transmitted data only (V.42bis).
| Compression of received data only (V.42bis).
Scheme :
MNP5 gives a theoretical increase of 200% and V.42bis of 400%.
Data :
The compressibility of the transmitted data. Plain text files are
normally most compressible and binary files least compressible. Pre-
compressed files (ZIP, ARC and others) are not compressible at all.
Efficiency :
The efficiency of the data processing unit internally in the modem.
9.2.1
Preferred compression scheme
The preferred compression scheme its V.42bis. The reasons for
this are increased compression, less dependency on transferred
data and the auto compression ability.
9.2.2
Auto compression
When connected in V.42bis the modem continuously monitors
the compression efficiency. When the compression starts to be
inefficient it turns itself off. It will still monitor the compression
efficiency and restart itself when the efficiency rises. This means
that you don't have to disable compression to transfer pre-
compressed files since the compression disables itself when
necessary to allow the highest possible transfer speed.
LZTR104006 R1.00
9-3
Zat 14.4/14.4-7 and 19.2/19.2-7
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