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Recommendation Q.712
DEFINITION AND FUNCTION OF SCCP MESSAGES
1 Signalling connection control part messages
The signalling connection control part (SCCP) messages are used by the
peer-to-peer protocol. All messages are uniquely identified by means of a message
type code, which is to be found in all the messages. The meaning and definition
of the various parameter fields contained in these messages are specified in S 2.
The actual inclusion of these parameter fields in a given message depends on the
class of protocol and is specified in S 3.
1.1 Connection Confirm (CC)
A Connection Confirm message is sent by the called SCCP to indicate to the
calling SCCP that it has performed the setup of the signalling connection. On
reception of a Connection Confirm message, the calling SCCP completes the setup
of the signalling connection, if possible.
It is used during connection establishment phase by connection-oriented
protocol class 2 or 3.
1.2 Connection Request (CR)
A Connection Request message is sent by a calling SCCP to a called SCCP to
request the setting up of a signalling connection between the two entities. The
required characteristics of the signalling connection are carried in various
parameter fields. On reception of a Connection Request message, the called SCCP
initiates the setup of the signalling connection if possible.
It is used during connection establishment phase by connection-oriented
protocol class 2 or 3.
1.3 Connection Refused (CREF)
A Connection Refused message is sent by the called SCCP or an intermediate
node SCCP to indicate to the calling SCCP that the setup of the signalling
connection has been refused.
It is used during connection establishment phase by connection-oriented
protocol class 2 or 3.
1.4 Data Acknowledgement (AK)
A Data Acknowledgement message is used to control the window flow control
mechanism, which has been selected for the data transfer phase.
It is used during the data transfer phase in protocol class 3.
1.5 Data Form 1 (DT1)
A Data Form 1 message is sent by either end of a signalling connection to
pass transparently SCCP user data between two SCCP nodes.
It is used during the data transfer phase in protocol class 2 only.
1.6 Data Form 2 (DT2)
A Data Form 2 message is sent by either end of a signalling connection to
pass transparently SCCP user data between two SCCP nodes and to acknowledge
messages flowing in the other direction.
It is used during the data transfer phase in protocol class 3 only.
Fascicle VI.7 - Rec. Q.712 PAGE1
1.7 Expedited Data (ED)
An Expedited Data message functions as a Data Form 2 message but includes
the ability to bypass the flow control mechanism which has been selected for the
data transfer phase. It may be sent by either end of the signalling connection.
It is used during the data transfer phase in protocol class 3 only.
1.8 Expedited Data Acknowledgement (EA)
An Expedited Data Acknowledgement message is used to acknowledge an
Expedited Data message. Every ED message has to be acknowledged by an EA message
before another ED message may be sent.
It is used during the data transfer phase in protocol class 3 only.
1.9 Inactivity Test (IT)
An Inactivity Test message may be sent periodically by either end of a
signalling connection to check if this signalling connection is active at both
ends, and to audit the consistency of connection data at both ends.
It is used in protocol classes 2 and 3.
1.10 Protocol Data Unit Error (ERR)
A Protocol Data Unit Error message is sent on detection of any protocol
errors.
It is used during the data transfer phase in protocol classes 2 and 3.
1.11 Released (RLSD)
A Released message is sent, in the forward or backward direction, to
indicate that the sending SCCP wants to release a signalling connection and the
associated resources at the sending SCCP have been brought into the disconnect
pending condition. It also indicates that the receiving node should release the
connection and any other associated resources as well.
It is used during connection release phase in protocol classes 2 and 3.
1.12 Release Complete (RLC)
A Release Complete message is sent in response to the Released message
indicating that the Released message has been received, and the appropriate
procedures has been completed.
It is used during connection release phase in protocol classes 2 and 3.
1.13 Reset Confirm (RSC)
A Reset Confirm message is sent in response to a Reset Request message to
indicate that Reset Request has been received and the appropriate procedure has
been completed.
It is used during the data transfer phase in protocol class 3.
1.14 Reset Request (RSR)
A Reset Request message is sent to indicate that the sending SCCP wants to
initiate a reset procedure (re-initialization of sequence numbers) with the
receiving SCCP.
It is used during the data transfer phase in protocol class 3.
1.15 Subsystem-Allowed (SSA)
A Subsystem-Allowed message is sent to concerned destinations to inform
those destinations that a subsystem which was formerly prohibited is now allowed.
It is used for SCCP subsystem management.
PAGE6 Fascicle VI.7 - Rec. Q.712
1.16 Subsystem-Out-of-Service-Grant (SOG)
A Subsystem-Out-of-Service-Grant message is sent, in response to a
Subsystem-Out-of-Service-Request message, to the requesting SCCP if both the
requested SCCP and the backup of the affected subsystem agree to the request.
It is used for SCCP subsystem management.
1.17 Subsystem-Out-of-Service-Request (SOR)
A Subsystem-Out-of-Service message is used to allow subsystems to go
out-of-service without degrading performance of the network. When a subsystem
wishes to go out-of-service, the request is transferred by means of a
Subsystem-Out-of-Service-Request message between the SCCP at the subsystem's node
and the SCCP at the duplicate subsystem's node.
It is used for SCCP subsystem management.
1.18 Subsystem-Prohibited (SSP)
A Subsystem-Prohibited message is sent to concerned destinations to inform
SCCP Management (SCMG) at those destinations of the failure of a subsystem.
It is used for SCCP subsystem management.
1.19 Subsystem-Status-Test (SST)
A Subsystem-Status-Test message is sent to verify the status of a
subsystem marked prohibited.
It is used for SCCP subsystem management.
1.20 Unitdata (UDT)
A Unitdata message is used by a SCCP wanting to send data in a
connectionless mode.
It is used in connectionless protocol classes 0 and 1.
1.21 Unitdata Service (UDTS)
A Unitdata Service message is used to indicate to the originating SCCP
that a UDT it sent cannot be delivered to its destination. A UDTS message is sent
only when the option field in that UDT is set to "return on error".
It is used in connectionless protocol classes 0 and 1.
2 SCCP parameter
2.1 affected point code
The "affected point code" identifies a signalling point where the affected
subsystem is located.
2.2 affected subsystem number
The "affected subystem number" parameter field identifies a subsystem
which is failed, withdrawn, congested or allowed. In the case of SST messages, it
also identifies the subsystem being audited. In the case of SOR or SOG messages,
it identifies a subsystem requesting to go out of service.
2.3 calling/called party address
The "calling/called party address" parameter field contains enough
information to uniquely identify the origination/destination signalling point
and/or the SCCP service access point.
Fascicle VI.7 - Rec. Q.712 PAGE1
It can be any combination of a global title (dialled digits for example),
a signalling point code, and a subsystem number. The subsystem number (SSN)
identifies a SCCP user function when provided.
In order to allow the interpretation of this address, it begins with an
address indicator indicating which information elements are present. The address
indicator also includes a routing indicator specifying if translation is
required, and a global title indicator specifying global title format.
The "calling/called party address" parameter field has two different
meanings depending on whether it is included in a connection-oriented or
connectionless message.
For a connection-oriented message, the significance of these fields is
related to the direction of the connection setup (i.e. independent of the
direction the message is going.
For a connectionless message, the significance of these fields is
dependent on the direction the message is going (just as for OPC and DPC).
2.4 credit
The "credit" parameter field is used in the acknowledgements to indicate
to the sender how many messages it may send, i.e., window size. It is also used
in the CR and CC message to indicate the proposed and selected credit, and in the
IT message to audit the consistency of this connection data at both ends of a
connection section.
2.5 data
The "data" parameter field contains information coming from upper layers
or from SCCP management.
In connectionless and connection-oriented messages the data parameter
field contains information coming from the upper layers.
Information coming from SCCP management will be contained in the data
parameter field of a UDT message. In this case the data parameter field of the
UDP message will only contain the SCCP management message.
2.6 diagnostic
The "diagnostic" parameter field is for further study.
2.7 error cause
The "error cause" parameter field is used in the Protocol Data Unit Error
message in order to indicate what is the exact protocol error.
2.8 end of optional parameters
The "end of optional parameters" parameter field is used in any message
containing optional parameters to indicate where the part allocated to these
optional parameters ends.
2.9 local reference number (source/destination)
The "local reference number (source/destination)" parameter field uniquely
identifies in a node a signalling connection. It is an internal working number
chosen by each node independently from the destination node. At least one local
reference number is to be found in any message exchanged on a signalling
connection section.
Note - Remote reference number is used to reflect the local reference
number at the remote end of a connection section.
2.10 protocol class
For connection-oriented protocol classes, the "protocol class" parameter
field is used during the connection establishment phase; it is negotiated between
the two end SCCP. It is also used during data transfer phase to audit the
consistency of this connection data at both ends of a connection section.
For connectionless protocol classes the "protocol class" parameter field
is used to indicate whether or not a message should be returned on error
occurence.
PAGE6 Fascicle VI.7 - Rec. Q.712
2.11 receive sequence number
The "receive sequence number" parameter field P(R) is used in the data
acknowledgement message to indicate the lower edge of the receiving window.
It also indicates that at least all messages numbered up to and including
P(R) - 1 are accepted.
2.12 refusal cause
The "refusal cause" parameter field is used in a Connection Refused
message to indicate the reason why the connection setup request was refused.
2.13 release cause
The "release cause" parameter field is used in a Released message to
indicate the reason of the release.
2.14 reset cause
The "reset cause" parameter field is used in a Reset Request message to
indicate the reason why a reset procedure is invoked.
2.15 return cause
For connectionless protocol classes, the "return cause" parameter field is
used to indicate the reason why a message was returned.
2.16 segmenting/reassembling
The "segmenting/reassembling" parameter field is used in the data message
for the segmenting and reassembling function. It is the more data indicator
(M-bit). This is used only in connection-oriented messages.
It is set to one in a data message to indicate that more data will follow
in a subsequent message.
It is set to zero in a data message to indicate that the data in this
message forms the end of a complete data sequence.
2.17 sequencing/segmenting
The "sequencing/segmenting" parameter field contains the information
necessary for the following functions: sequence numbering, flow control,
segmenting and reassembling.
2.18 subsystem multiplicity indicator
The "subsystem multiplicity indicator" is used in SCCP management messages
to indicate the number of associated replicated subsystems.
3 Inclusion of fields in the messages
The inclusion of the information elements specified in S 2 in the various
messages specified in S 1 according to their type depends on the class of
protocol. SCCP messages are specified in Table 1/Q.712 and SCCP management
messages are specified in Table 2/Q.712.
All SCCP management messages are embedded in the "data" parameter of the
Unitdata message.
The following applies to Tables 1/Q.712 and 2/Q.712:
m mandatory field
o optional field (which is included in a message when needed)
Fascicle VI.7 - Rec. Q.712 PAGE1
Table 1/Q.712 - Landscape. File: 712T1E.DOC
PAGE6 Fascicle VI.7 - Rec. Q.712
TABLE 2/Q.712
SCCP management messages
Messages
Parameter fields SSA SSP SST SOR SOG
SCMG format ID m m m m m
Affected SSN m m m m m
Affected PC m m m m m
Subsystem multiplicity m m m m m
indicator
Fascicle VI.7 - Rec. Q.712 PAGE1