Constraints in Eq. (15) maintain the flow conservation from each primary ISCW to its backup ISCWs. Equation (16) imposes that the flow does not exist if there is no wireless path from ISCW at potential location x to ISCW at potential location y ; in other words, ISCW at potential location y is not the backup ISCW of ISCW at potential location x . Equation (17) and (18) indicate that ISCWs should be placed at the potential locations along the wireless path from ISCW x to ISCW y . Equation (19) maintains the total number of ISCWs needed in this Fi Wi access network.