The “cost of {TAIL ~ survivability}” is defined as the ratio of the costs of network operation with and without {HEAD ~ survivability requirements} 
The “ cost of { TAIL ~ survivability } ” is defined as the ratio of the costs of network operation with and without { HEAD ~ survivability requirements }
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD