This standard defines the Keypad signalling protocol for the purpose of supplementary service control at an interface at the S reference point between a Terminal Equipment (TE) and a Private Telecommunication Network (PTN). The Keypad protocol operates in conjunction with the signalling protocol specified in ETS 300 192. It is based on use of the Keypad facility and Display information elements. While the procedures associated with keypad invocation are specified in this standard@ the allocation of the access codes used to request or indicate a supplementary service is outside the scope of this standard. This standard is applicable to TEs which are intended for connection to PTNs@ and is also applicable to the user accesses of PTNs.