A private network firewall 225 is treated as if it were a media gateway network entity. The current values for each phase are processed in three successive processing operations, using in turn the current values from each terminal as local current values and the combination of the other two terminal current values in each case as the remote current values. The physical structure of the contact scheme is broken down into three components, a) the GaN, b) an interface material and c) the metallic conductor.