teadily over the destination lines. This allows either fewer lines NETWORK STRUCTURES 95 Network control: billing reports and North and South undeliverable message American traffic sent here Dial-up leased line links Singapore London Far East European and Traffic Middle East traffic Fig. 6-9. Typical message-switch network. to each site, or lines of a lower speed to be used to handle the same traffic loads. Perhaps the most powerful and useful capability found in message switching is the ability to access a variety of different services. A good message switch can control leased lines in addition to interfac- ing to the Telex, TWX, DDD, international Telex, and international DDD networks. Users can therefore access any of these services through proper addressing of their messages. This capability makes it possible for private networks to communicate compatibly with ex- listing public networks. 6.5 INTEGRATED VOICE AND DATA It is wise to plan for the future. Even the corporation that has no identified near-term need should ensure that its telecommunication network can accommodate high speed data switching and transmis- TELECOMMUNICATIONS sion. The present level of office automation already demands effi- cient, low-cost means of interconnecting terminals at the local level. Electronic mail, high quality facsimile, and teleconferencing are ex- amples of emerging applications that require long-distance switched data communication between local areas. The reasons for integrating voice and data are straightforward. Sharing switch capacity and transmission bandwidth significantly re- duces cost. A single 56 kbps voice channel, for example, can carry the multiplexed output of tens of 1200 bps data terminals. The ul- timate design objectives for voice and data networks are the same: timely delivery of an accurate replica of the transmitted message at minimum cost. Implementation techniques differ to the extent that they derive from different backgrounds-telephone and computers. Integration of digital voice and data, on the