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Link compression
(also known as per-interface compression) involves
compressing both the header and payload sections of a data stream.
Unlike header compression, link compression is protocol independent.
The link-compression algorithm uses Predictor
or STAC to compress the traffic in another link layer, such as PPP
or LAPB, to ensure error correction and packet sequencing (Cisco
HDLC uses STAC compression only):
- Predictor
--Predicts the next sequence of characters in the data
stream by using an index to look up a sequence in a compression
dictionary. It then examines the next sequence in the data
stream to see if it matches. If it does, that sequence replaces
the looked-up sequence in a maintained dictionary. If it does
not, the algorithm locates the next character sequence in the
index, and the process begins again. The index updates itself by
hashing a few of the most recent character sequences from the
input stream.
- STAC
-- Developed by STAC Electronics, STAC is a Lempel-Ziv (LZ)-based,
compression-based algorithm. It searches the input data stream
for redundant strings and replaces them with what is called a token,
which turns out to be shorter than the original redundant data
string.
Note: Predictor
is memory intensive and Stacker is CPU intensive.
If the data flow transverses across a
point-to-point connection, use link compression. In a
link-compression environment, the complete packet is compressed
and the switching information in the header is not available for
WAN-switching networks. Thus, the best applications for link
compression are point-to-point environments with a limited hop
path. Typical examples are leased lines or ISDN.
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