otherwise. For the control of supplemen- edge of the supplementary service invoked. No records of the tary services which are independent of the active call, the REGIS- supplementary service call state are held by the terminal and layer TER message is used. In addition, specific layer 3 messages have 3 messages are generated as a result of human action (e.g. pressing also been defined for the function of holding and retrieving a call. a button on the keypad). The operation of the terminal (e.g. display of messages, lighting of lamps etc.,) is controlled by the network via layer 3 messages. The network response to facility requests will be ISDN developments in the U.K. generally in the form of a sequence of IA5 characters in a DISPLAY information element or an in-band tone or announcement. The In common with most telecommunications administrations, BT arrangement of IA5 characters within a DISPLAY information wished to fully exploit the features of its emerging Integrated element follows similar rules to the coding of facility requests. Use Digital Network (IDN) in order to extend the range of services is made of the separators and "#" SO that sequences are machine offered to its customers. BT saw the early introduction of an ISDN readable and suitable for display to a human operator. pilot service as a means of gaining practical experience of develo- There are 2 types of stimulus procedures defined, the Keypad ping and operating an ISDN and stimulating customer interest, and Protocol and the Feature Key Management Protocol. Both proce- planning for this was started in 1979. In June of 1985 an ISDN pilot dures use the basic call control layer 3 messages, particularly the service was opened in the U.K., based on a System X local exchange INFORMATION message, as the transport mechanism for conve- in the City of London, under the marketing name IDA (Integrated ying the information between the terminals and the network. In the Digital Access). Early in 1986 it was extended to three further Keypad Protocol the user inv