error detecting means for detecting {HEAD ~ an error} in said {TAIL ~ feedback frame} processed by said substituting means 
error detecting means for detecting { HEAD ~ an error } in said { TAIL ~ feedback frame } processed by said substituting means
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH