... fixed part contains pointers to the mandatory variable part and the optional part of the message. A pointer to the optional part is only included if the message type permits an optional part. ... the mandatory variable or optional part. The following sections describe these three parts in more detail. Mandatory Fixed Part (MF) The mandatory fixed part consists of those parameters that ... "ISDN User Part [ISUP]"). As with all other MTP users, SCCP messages are composed of three parts: a mandatory fixed part, mandatory variable part, and an optional part. All SCCP messages...