At the check nodes, {TAIL ~ each} check {HEAD ~ node} determines the modulo 2 sum of 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 { HEAD ~ node } determines the modulo 2 sum of 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 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH