To connect Radio Network Controller nodes to form a Radio Access Network, there is a need to protect the physical lines connecting these nodes against failure.  A common architecture to achieve line protection in an SDH based network uses two physical lines to connect two nodes of the network.  The node that is sending the data traffic, sends the data on two lines to the receiving node.  If one of the lines breaks, the receiving node can extract the traffic from the other line. 
The RNC node includes two line termination boards, of which one is active and the other is in standby mode.  A link supervision block supervises the function of the lines and termination boards, and performs a switchover from the active to the standby board should a fault occur on one of the connections. 
In the AXE system manufactured by Telefonaktiebolaget LM Ericsson such a supervision system is implemented.  Faults detected on the active board are sent towards the supervision block spontaneously when they are detected.  The supervision block then reads out state information from the standby board.  The results are correlated to achieve information about a fault cause (failure in the active board or the connected line, failure in the standby board or the connected line, failure in the internal link between the boards, no failure). 
Such a system is prone to some problems:
We can imagine that an identical fault is detected on both boards.  Due to varying detection and signal transmission times, it is undetermined which fault (i.e. from which board) that will be reported first.  Then the correlation result might not reflect the true physical situation, and the supervision block may misunderstand the situation.  Varying detection times is caused by the standby board not reporting faults immediately, but instead checks the situation at fixed time intervals. 
Even a single short disturbance can lead to a failure being reported without being justified. 
In addition, the active and standby boards have two rather different functions.  In case of failure, the functions must be swapped, without loosing information on the faults that has occurred.  This might be complicated to implement in a computer program.