If a change to line switching data transfer is to take place, then {HEAD ~ a line} to another switch is switched through {TAIL ~ the exchange} 4 and the data are transferred to this by line switching 
If a change to line switching data transfer is to take place , then { HEAD ~ a line } to another switch is switched through { TAIL ~ the exchange } 4 and the data are transferred to this by line switching
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH