Data Compression (&Kn) - US Robotics COURIER User Manual

COURIER HIGH SPEED MODEMS
DATA COMPRESSION (&Kn)
When data compression is enabled, the transmitting modem
detects redundant bits of data and recodes them into shorter
units of fewer bits. The receiving modem decompresses the
redundant data units before passing them to the receiving DTE.
Compression does not occur unless the modems are able to
establish an error control connection. In addition, the modem
should be set for Transmit Data flow control (&Hl, 2 or 3), as
described earlier in this chapter.
&KO Data compression disabled.
&Kl Auto enable/ disable. This is the default. The modem
enables compression if the DTE rate is fixed, &Bl, and
disables compression if the DTE rate follows the link rate,
&BO. The reason is that compression offers no
throughput advantage when the DTE and link rates are
equal: compression may even degrade throughput.
&K2 Data compression enabled. Use this setting to keep the
modem from disabling compression.
&K3 Selective data compression. The modem negotiates only
for V .42 bis compression, and disables MNP Level 5
(MNPS) compression. Use this setting to transfer 8-bit
binary files, .ARC files, and other files that are already
compressed. See the note below.
If the Courier is connecting with a V.42 or HST modem, it
negotiates V.42 bis compression. If V.42 bis is not feasible, the
connection remains under error control, but without data
compression.
If the Courier is connecting with an MNP modem, it negotiates
for MNP Level 5 (MNPS) data compression. If the remote
modem does not have MNPS, the connection remains under
MNP error control, but without compression.
NOTE: MNPS compression is not useful when transferring files
that are already compressed, such as the .ARC files downloaded
from many Bulletin Boards and 8-bit binary files, which appear
to the modem to be compressed. MNPS tends to add data to the
transmission so that throughput over the link degrades. V.42 bis
compression dynamically detects when data is already com-
4-12 Interface Controls
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