At the check nodes, each check {HEAD ~ node} determines {TAIL ~ 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 
determines
At the check nodes , each check { HEAD ~ node } determines { TAIL ~ 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 OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL 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