Appendix C. SCCP Messages (ANSI/ETSI/ITU-T)

The table in this appendix summarizes Signaling Connection Control Part (SCCP) messages and the purpose of each. For an introduction to SCCP, refer to Chapter 9, "Signaling Connection Control Part (SCCP)."

Table C-1. SCCP Messages

MESSAGE/CODE	FULL MESSAGE NAME	PURPOSE
CR 0 0 0 0 0 0 0 1	Connection Request	Sent by SCCP to another SCCP peer to request a setup of a logical signaling connection between them so that data transfer can take place in a connection-orientated fashion.
CC 0 0 0 0 0 0 1 0	Connection Confirm	Sent in response to a CR message to indicate that the node has performed the setup of the requested logical signaling connection.
CREF 0 0 0 0 0 0 1 1	Connection Refused	Sent by the destination or an intermediate SCCP node in response to a CR message to indicate a refusal to set up a logical signaling connection.
AK (NE) 0 0 0 0 1 0 0 0	Data Acknowledgment	May be sent when using protocol class 3 to control the window flow.
DT1 0 0 0 0 0 1 1 0	Data Form 1	Sent by either end of a logical signaling connection to pass SCCP user data transparently between two SCCP nodes. DT1 is only used in protocol class 2.
DT2 (NE) 0 0 0 0 0 1 1 1	Data Form 2	Sent by either end of a logical signaling connection to pass SCCP user data transparently between two SCCP nodes. DT2 is only used in protocol class 3.
ED (NE) 0 0 0 0 1 0 1 1	Expedited Data	Performs the same function as the DT2 message, but includes the capability to bypass the flow control mechanism and is, therefore, only used in protocol class 3.
EA (NE) 0 0 0 0 1 1 0 0	Expedited Data Acknowledgment	Used to acknowledge an ED message. Each ED message must be acknowledged before another is sent.
XUDT 0 0 0 1 0 0 0 1	Extended Unitdata	Used by SCCP to transmit data with optional parameters, using connectionless classes 0 and 1.
XUDTS 0 0 0 1 0 0 1 0	Extended Unitdata Service	Sent back in response to a XUDT message if the XUDT message cannot be delivered to its destination. Only used when the optional field in XUDT is set to "return on error." Protocol class indeterminate due to absence of protocol class parameter.
IT 0 0 01 0 0 0 0	Inactivity Test	May be sent periodically by either end of a logical signaling connection to make sure the logical signaling connection is active and to audit the consistency of connection data at both ends. Used in connection-orientated classes 2 and 3.
LUDT 0 0 0 1 0 0 1 1	Long Unitdata	Used by SCCP to transmit data with optional parameters, using connectionless 0 and 1. If ATM is the underlying network, it allows sending of Network Service Data Unit (NSDU) sizes up to 3952 octets without segmentation.
LUDTS 0 0 0 1 0 1 0 0	Long Unitdata Service	Sent back in response to a LUDT message if the LUDT message cannot be delivered to its destination. Only used when the optional field in LUDT is set to "return on error." Protocol class indeterminate due to absence of protocol class parameter.
ERR 0 0 0 0 1 1 1 1	Protocol Data Unit Error	Sent on detection of any protocol errors. Used during the data transfer phase in connection-orientated classes 2 and 3.
RLC 0 0 0 0 0 1 0 1	Release Complete	Sent in response to the Released (RLSD) message to indicate that the RLSD message was received and that the necessary procedures have been performed. Used during connection release phase in connection-orientated classes 2 and 3.
RLSD 0 0 0 0 0 1 0 0	Released	Sent to indicate that the sending SCCP wishes to release a logical signaling connection and that the associated resources have been brought into the disconnect pending condition. Also indicates that the receiving node should release the logical signaling connection and its associated resources. Used during connection release phase in connection-orientated classes 2 and 3.
RSC (NE) 0 0 0 0 1 1 1 0	Reset Confirm	Sent in response to a Reset Request (RSR) message to indicate that RSR has been received and that the necessary procedure has been performed. Used during the data transfer phase in connection-orientated class 3.
RSR (NE) 0 0 0 0 1 1 0 1	Reset Request	Sent to indicate that the sending SCCP wishes to initiate a reset procedure (re-initialization of sequence numbers) with the receiving SCCP. Used during the data transfer phase in protocol class 3.
SBR (M)(A) 1 1 1 1 1 1 0 1	Subsystem Backup Routing	Optional message sent before rerouting traffic to the backup subsystem. Provides more connectivity information so the end node can determine the traffic mix received for a subsystem.
SNR (M)(A) 1 1 1 1 1 1 1 1 0	Subsystem Normal Routing	Optional message sent prior to rerouting traffic to the primary subsystem, to the backup of the subsystem that is now allowed. Allows the end node to update the traffic mix information that the subsystem is receiving.
SRT (A) 1 1 1 1 1 1 1 1	Subsystem Routing Status Test	Optional message sent to verify the routing status of a subsystem marked as under backup routing.
SSA (M) 0 0 0 0 0 0 0 1	Subsystem Allowed	Used by SCCP subsystem management (SCMG) to inform SCMG at concerned destinations that a formerly prohibited subsystem (such as VLR/HLR) is now available, or that a previously unavailable SCCP is now available. As a result, the node receiving the SSA updates its translation tables.
SSP (M) 0 0 0 0 0 0 1 0	Subsystem Prohibited	Used by SCCP subsystem management (SCMG) to inform SCMG at concerned destinations that a subsystem (such as VLR/HLR) has failed. The receiving end of an SSP message updates its translation tables; as a result, traffic could be re-routed to a backup subsystem, if available.
SST (M) 0 0 0 0 0 0 1 1	Subsystem Status Test	Used by SCCP subsystem management (SCMG) to verify the status of a subsystem marked prohibited or the status of an SCCP marked unavailable. The receiving node checks the status of the named subsystem and, if the subsystem is allowed, sends an SSA message in response. If the subsystem is prohibited, no reply is sent.
SOR (M) 0 0 0 0 0 1 0 0	Subsystem Out-of-Service Request	Used by SCCP subsystem management (SCMG) to allow subsystems to go out-of-service without degrading performance of the network.
SOG (M) 0 0 0 0 0 1 0 1	Subsystem Out-of-service-grant	Used by SCCP subsystem management (SCMG) in response to a Subsystem Out-of-Service Request (SOR) message to the requesting SCCP if both the requested SCCP and the backup of the affected subsystem agree to the request.
SSC (M)(I) 0 0 0 0 0 0 1 0	SCCP/subsystem-congested	Sent when an SCCP node experiences congestion.
UDT 0 0 0 0 1 0 0 1	Unitdata	Used by SCCP to transmit data, using connectionless classes 0 and 1.
UDTS 0 0 0 0 1 0 1 0	Unitdata Service	Sent in response to a UDT message if the UDT message cannot be delivered to its destination. Only used when the optional field in UDT is set to "return on error." Used in connectionless protocol classes 0 and 1.
KEY: (A)—Messages supported in ANSI SCCP Messages not supported in ETSI SCCP [].cellular network, does not seem place to define them as reader should know them only [2]. (I)—Messages supported in ITU-T SCCP only [60]. (NE)—Messages not supported in ETSI SCCP [10]. (M)—SCCP subsystem management (SCMG). These are transmitted within the data parameter a UDT, XUDT or LUDT message.