call to exchange u201c x u201d and exchange u201c b u201d thinks it is receiving the call from exchange u201c y u201d however both exchange u201c a u201d and exchange u201c b u201d are actually dealing with the same exchange namely the exchange of the invention regarding the exchange of the invention the selected outgoing trunk route could have been on the same node as the incoming trunk route in this case the same node would have played both an incoming role and an outgoing role the selected outgoing trunk route could have had the same node end point code value as the incoming trunk route or a different node end point code value both cases would have worked equally well fig5 shows how the exchange of the invention can have many nodes and yet use as few as two point codes fig5 shows how the same pair of node end point codes u201c x u201d and u201c y u201d can be re used each of the four neighbouring exchanges u201c a u201d u201c b u201d u201c c u201d and u201c d u201d thinks it is connected to an exchange u201c x u201d and to an exchange u201c y u201d however they are all connected to the same exchange of the invention exchanges u201c a u201d u201c b u201d and u201c d u201d are each connected to a different pair of nodes exchanges u201c b u201d and u201c c u201d are connected to the same pair of nodes a neighbouring exchange u201c a u201d say can choose whether to make the trunk routes u201c a x u201d and u201c a y u201d have equal priority or whether to make trunk route u201c a x u201d be its first choice route and trunk route u201c a y.