Configure header on HDLC link:
R2(config)#interface serial1/0
R2(config-if)#compress ?
stac stac compression algorithm
R2(config-if)#compress stacR2(config-if)#compress ?
stac stac compression algorithm
R2(config-if)#end
R2#
This is for header compression, not for data compression.
Must be configured on both sides.