All SCCP connectionless data requests and connection establishment requests contain a mandatory called and calling party address. The calling address is optional for the connection request message. These addresses are passed to and from user applications by the SccpAddr data structure, which is a C-language structure representation of the actual address passed in the SCCP protocol message.
SCCP addresses can take several forms, containing various combinations of point code, subsystem number, and global title. The combination of the address and routing indicator constructed by applications (or received from the SS7 network) together with the SCCP configuration allow these messages to be routed to the correct destination or local application.
For outgoing messages from applications, the called party address in the unitdata request or connection request message is used to route the message. The routing method is chosen based on the value specified in the routing indicator field of the called party address. The routing methods are:
Point code and subsystem number (ROUTE_PC_SSN)
Global title (ROUTE_GLT)
When ROUTE_PC_SSN is chosen, the message is routed to the destination point code/subsystem number (DPC/SSN) specified in the called party address. The subsystem number must be present.
If the DPC is present, a route must be configured for the point code and the subsystem must be configured for that route unless the default routing configuration option is selected.
If the DPC is absent, the message is routed to the point code associated with the first (and typically only) route in the SCCP configuration file, but that point code is not included in the outgoing message. This option is typically used only on point-to-point SS7 links, such as the link between a mobile switching center (MSC) and base station controller (BSC) in a wireless network, where the destination point code is not needed for routing.
If a global title is present in the called party address, it is copied to the outgoing message but the routing indicator remains ROUTE_PC_SSN.
When ROUTE_GLT is chosen, an address translation must be configured that, when combined with the address mask configured for the application SAP, matches the global title specified in the called party address. The message is routed to the point code and subsystem number from the configured address translation entry. If no subsystem number is configured for that address translation but one is supplied by the application, it is copied to the outgoing message.
If the global title must be further translated at another node, the routing indicator configured in the address translation entry specifies ROUTE_GLT and the point code in the address translation entry specifies the next translator node. If this is the final translation, the address translation entry specifies ROUTE_PC_SSN and the DPC in the address translation entry specifies the final destination point code.
When a global title is specified by the user, the global title is translated if possible by the SCCP task, and the global title is included in the outgoing SCCP address along with up to four parameters.
The glTransType, encoding, numPlan, and natAddrInd fields are only used with a global title. Based on the value of the glTitleInd field, some or all of these values are included with a global title in an outgoing SCCP address.
If swType is set to SW_ANSI, two combinations can be included with a global title. The glTitleInd field determines which is selected:
global title + translation type (glTitleInd = GLT_TT)
global title + translation type + numbering plan + encoding (GLT_TT_NP_E)
If swType is set to SW_ITU, four combinations can be included with a global title. The glTitleInd field determines which is selected:
global title + encoding + nature of address
(glTitleInd = GLT_ITU_FMT1)
global title + translation type (GLT_ITU_FMT2)
global title + translation type + numbering plan + encoding (GLT_ITU_FMT3)
global title + translation type + numbering plan + encoding + nature of address (GLT_ITU_FMT4)
If the user wants the global title to be passed to another node for translation, the ROUTING_IND field can be defined in one of the ADDR sections in the SCCP configuration file. If ROUTING_IND is set to GLT, the global title is passed along with its routing field set to route by global title. The point code field is included, but the subsystem field is not included in the outgoing message.
Refer to Global title translation for more information.