implementation is still occurring throughout the world-wide based on the PTT core service. PTT networks, nevertheless, in many countries the bulk of the Modem networks grew phenomenally during the 1970s both do- infrastructure is now based on digital connections both leased and mestically and internationally. The SNA networks offered efficient switching. data connection based on synchronous data connection, however, Wide Area Network users realised the same benefits as PTTs in many computer systems also adopted different protocols for com- digital connections and in the earlier 1980s the first service became munication, including asynchronous type connections. Syn- available from PTTs for digital leased services. AT&T, the deregu- chronous connections essentially rely on the accurate timing of the lated long distance carrier offered T1 services to Government of- two elements trying to communicate. This allows fast exchange of fices in North America. information, given that both elements are synchronised. Asynchronous connection on the other hand assumed poor timing between the elements. Therefore data was surrounded by error and 56.3 Digital networks start/stop information SO enabling its accuracy when received. Synchronous connection is much more efficient than asyn- The early 1980s T1 services were the start of an explosion in growth chronous since large amounts of data can be transmitted faster. To of T1 services throughout North America, with 11 million digital improve the efficiency of asynchronous connections statistical connections in place today. In Europe a different standard for digital multiplexing methods were developed during the late 1970s, both connection was offered by British Telecom in the mid 1980s called for local and wide area connectivity. 2Mbit/s or E1. The principles of statistical multiplexing was to take advantage of Both T1 and E1 utilise the same basic building blocks, but are slow data connections associated with asynchronous connections. constructed in a different way. The building b