The telecommunications industry is rapidly deploying packet based data networks to augment or replace the existing circuit switched telephone network. This trend is driven by the dramatic growth in data and new types of media, leaving voice traffic consuming a small fraction of the total bandwidth.

Recent programming languages have attempted to provide support for concurrency and for modular programming based on abstract interfaces. Building on experience of adding monitors to CLU, a language oriented towards data abstraction, it is explained how these two goals conflict. In particular, the clash between conventional views on interface abstraction and the programming style required for avoiding monitor deadlock is discussed. It is argued that the best compromise between these goals is a combination of a fine-grain locking mechanism together with a method for explicitly defining concurrency properties for selected interfaces.>