The user signalling bearer service (USBS) allows the user to send and receive information on a D-channel. This information is passed transparently (i.e. without modification of contents) through the network. SDUs are expected to be delivered, in sequence, with a high probability: and probability of misdelivery, mutilation and duplication is expected to be very low. Normally the network will not interpret or act upon this information.
ITU-T I.232.3 FRENCH-1993 history
1993ITU-T I.232.3 FRENCH-1993 USER SIGNALLING BEARER SERVICE CATEGORY (USBS) [Superseded: ITU-T I.232]