The descriptions of the various congestion-control functions for the common channel signaling system No. 7 (SS7), which are spread widely and piecemeal across the CCITT recommendations, are presented. The protocol architecture of SS7, structured according to Open Systems Interconnection (OSI) principles, is described. The message transfer part (MTP) provides the OSI Layer 1 and 2 services, plus part of the Layer 3 network service for the transmission of common channel signaling (CCS) message signal units (MSUs). The full Layer 3 service is provided by the addition of the signaling connection control part (SCCP) (designated Level 4 in the CCS functional model) in conjunction with the MTP. The SCCP provides an enhanced addressing capability over the MTP and enables connectionless services, and, if required, connection-oriented services. The user parts (UPs) make use of the MTP directly, although I5UP may make use of the SCCP for some services. Each UP defines a specific set of signal types to meet the signaling requirements of a user. >