What is ISDN?
DIVA T/A ISDN Modem Reference Guide ◆ Page 13
What is Data Compression?To establish communications 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, optional
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 character “@”, 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 Protocol (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 DIVAT/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 DIVAT/A compression negotiation, however, will assume control should no compression
be detected or the initial negotiation fail. No user configuration is required to enable the
DIVAT/A's compression functionality. Compression algorithms curre ntly supported by the
DIVAT/A include Stacker LZS (Hi/fn), MPPC (Microsoft), and Ascend.