At the check nodes, {TAIL ~ each} check node determines the modulo 2 sum of {HEAD ~ the incoming bits} in a similar fashion shown in the sign component of FIG 17 and checks if the sum is zero or not 
At the check nodes , { TAIL ~ each } check node determines the modulo 2 sum of { HEAD ~ the incoming bits } in a similar fashion shown in the sign component of FIG 17 and checks if the sum is zero or not
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH