DIVA T/A PC Card Reference Guide 13
What is Data Compression?To establish communicati ons over a PPP link, each end of the
connection must first send packets to configure and test the data link.
After the link has been established, optiona l facilities may be
negotiated as needed. One such facility is data compression. A wide
variety of compression methods may be negotiated, although typically
only one method is used for both directions of the link.
Data compression is a process where the effective throughput is
increased by encoding data in such a way that fewer bits are required
to represent it. For example, a text file might be compressed by
representing common words with single characters; thus if the word
“the” is represented by the char acter “@”, it will be transmitted three
times faster. Popular compression algorithms typically reduce the size
of the data by 50%, effectively doubling the rate at which it is
transmitted.
The Compression Control Protoc ol (CCP) works with MLPPP and is
responsible for configuring, enabling, and disabling data compression
algorithms on both ends of the point-to-point link. PC Compression
negotiation takes precedence over the DIVA T/A negotiation for
performance considerations (for example, the PC compression may
reduce the number of bytes that go through the serial interface, the PC
has the quicker CPU, etc.). The DIVA T/A compression negotiation,
however, will assume control should no compression be detected or
the initial negotiation fail. No user configur ation is required to enable
the DIVA T/A’s compression functionality.
Compression algorithms currently supported by the DIVA T/A
include Stacker LZS (Hi/fn), MPPC (Microsoft), and Ascend.