In {TAIL ~ addition}, links can be added to {HEAD ~ the network topology model} to reduce the cost of network operation 
In { TAIL ~ addition } , links can be added to { HEAD ~ the network topology model } to reduce the cost of network operation
OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH