Payload compression, also known as per-virtual-circuit compression, compresses only the payload,
or data portion, of the packet. The header of the packet is not touched.
Link Compression
Link compression, also known as per-interface compression, compresses both the header
and payload section of a data stream. Unlike header compression, link compression is protocol
independent.
The link compression algorithm uses Stac or Predictor to compress the traffic in another link
layer such as PPP or LAPB, ensuring error correction and packet sequencing. Cisco proprietary
HDLC protocol is capable of using Stac compression only.
Predictor Use this approach to solve bottleneck problems caused by a heavy load on the
router. The Predictor algorithm learns data patterns and “predicts” the next character by using
an index to look up a sequence in a compression dictionary. This is sometimes referred to as
lossless because no data will be lost during the compression and decompression process.
Stac This method is best used when bottlenecks are related to bandwidth issues. The Stac
method searches the input data stream for redundant strings and replaces them with a token
that is shorter than the original redundant data string.
If the data flow traverses 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. Typical examples are
leased lines or ISDN.
If you use payload compression, you should not use header compression. This
is redundant, and you should configure payload compression only.
In the following example, we turned on LAPB encapsulation with Predictor compression and
set the maximum transmission unit (MTU) and the LAPB N1 parameters:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface serial0
Router(config-if)#encapsulation lapb
Router(config-if)#compress ?
predictor predictor compression type
stac stac compression algorithm
Router(config-if)#compress predictor
Router(config-if)#mtu 1510
Router(config-if)#lapb n1 12096
Compression 937
The LAPB N1 represents the number of bits in an LAPB frame, which holds an X.25 packet.
It is set to eight times the MTU size, plus any overhead when using LAPB over leased lines. For
instance, the N1 is specified at 12,080 (that is, 1,510 × 8) plus 16 bits for protocol overhead. The
LAPB N1 parameter can cause major problems if it’s not configured correctly, and most often it
should be left at its default value. Even so, it can be really valuable if you need to set the MTU size.