milar in operation to class 1 but can take advantage of the in- creased efficiency of character transmission where available. Retrain positive Class 3 provides for the transmission of characters and facsimile Training check data, either separately or combined in a mixed mode format. This is Transmitting subscriber identification a more complex terminal giving the possibility of keyboard and scanner input with the document architecture to support interoper- ation with ISO hierarchical models. The requirements of Classes 2 and 3 will continue to develop as necessary to track the evolution of document architecture standards. interface rate of 64kbit/s, to that recommended on the analogue Class 1 machines are already being implemented to provide higher network interface at between 14.4kbit/s and 2.4kbit/s. performance facsimile with direct digital access to the ISDN net- This will simplify implementation and provide the user with work, 3 seconds/page transmission and higher resolutions operation current G3 facilities at the faster and more efficient digital interface to 400 X 400 pels/inch. rate. G3-64kbit/s is proposed as an option to the basic G3 mode which is essential for interworking with the existing G3 machine 60.5.2 64kbit/s G3 type equipment population. In this machine the basic G3 operation is likely to be provided on the I series port by the implementation of the equivalent With the tremendous growth in G3 an enhancement is being dis- of a codec converting the G3 modem signals to the ISDN speech cussed in CCITT to provide a similar operation at the faster digital equivalent. 60/14 Facsimile transmission 60.6 G3 networks, switches, gateways and messages and controlled distribution of centrally received incoming PC fax. messages. By routing the facsimile traffic through an intelligent central switch, possibly associated with the company PBX, the The success of G3 facsimile has encouraged the provision of addi- required monitoring and control functions can be provided with cost tional equipment/facilities to 