The defensive fire plan adopted by the FSCC was separate from and not to be confused with the final protective fires employed by the defenders who manned the perimeter. The artillery batteries were used to prevent the enemy assault forces from reaching the wire and to cut off the lead elements from reinforcements. The fact that the North Vietnamese usually attacked with their battalions in column made it somewhat easier for the FSCC to isolate the assault elements from the reserves. When the enemy launched his attack, the FSCC placed a three-sided artillery box around the lead battalion; three batteries of 1/13 executed this mission. The fourth battery then closed the remaining side, which faced the friendly positions, with a barrage that rolled from one end of the box to the other--much like a piston within its cylinder. The NVA force in the box could not escape and could not avoid the rolling barrage. Those North Vietnamese who spilled out of the open end of the box were subjected to the final protective fires of the Marines along the perimeter. At the same time 1/13 worked over the assault force, the FSCC put a secondary box into effect for the benefit of the back-up units. The Army 175mm batteries were responsible for two sides which were about 500 meters outside the primary box. On order, the gunners rolled their barrages in toward the sides of the primary box and back out again. The third side was sealed by continuous flights of aircraft under the control of the TPQ-10 radar. Whenever B-52s could be diverted in time, Arc Light strikes were used to saturate the approach routes to the battle area.(157)