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