How to implement ISDNs without user observability---Someremarks

In present day communication networks, the network operator or an intruder could easily observe when, how much and with whom the users communicate (traffic analysis), even if the users employ end-to-end encryption. With the increasing use of ISDNs, this becomes a severe threat.Therefore, we summarize basic concepts to keep the recipient and sender or at least their relationship unobservable, consider some possible implementations and necessary hierarchical extensions, and propose and evaluate some suitable performance and reliability enhancements.