 can be grouped in pairs such that for each pair
 can be grouped in pairs such that for each pair  and
 and  , the relation
 , the relation  holds where
 holds where  . A new concept of a characteristic circle of a rational function is introduced. It is shown that its existence is necessary and sufficient for MIB network realization.
 . A new concept of a characteristic circle of a rational function is introduced. It is shown that its existence is necessary and sufficient for MIB network realization.