  This paper theoretically analyzes cable network disconnection due to randomly
occurring natural disasters, where the disaster-endurance (DE) levels of the
network are determined by a network entity such as the type of shielding method
used for a duct containing cables. The network operator can determine which
parts have a high DE level. When a part of a network can be protected, the
placement of that part can be specified to decrease the probability of
disconnecting two given nodes.
  The maximum lower bound of the probability of connecting two given nodes is
explicitly derived. Conditions decreasing (not decreasing) the probability of
connecting two given nodes with a partially protected network are provided.
