SS7/IP Interworking Tutorial
-Transporting SCCP over IP

SUA (SCCP User Adaptation Layer) is a protocol defined by the IETF sigtran Working Group for transporting SS7 SCCP (Signaling Connection Control Part) user part signaling messages (e.g., TCAP and RANAP) over IP using the Stream Control Transmission Protocol (SCTP). SUA is used between a signaling gateway and an IP signaling endpoint and between IP signaling endpoints. SUA supports both SCCP unordered and in-sequence connectionless services and bi-directional connection-oriented services with or without flow control and detection of message loss and out-of-sequence errors (i.e., SCCP protocol classes 0 through 3).

For connectionless transport, SCCP and SUA interface at the signaling gateway. From the perspective of an SS7 signaling point, the SCCP user is located at the signaling gateway. SS7 messages are routed to the signaling gateway based on point code and SCCP subsystem number. The signaling gateway then routes SCCP messages to the remote IP endpoint. If redundant IP endpoints exist, the signaling gateway(s) can load share amongst active IP endpoints using a round-robin approach. Note that load sharing of TCAP messages occurs only for the first message in a TCAP dialogue; subsequent TCAP messages in the same dialogue are always sent to the IP endpoint selected for the first message, unless endpoints share state information and the signaling gateway is aware of the message allocation policy of the IP endpoints. The signaling gateway may also perform Global Title Translation (GTT) to determine the destination of an SCCP message. The signaling gateway routes on global title, i.e. digits present in the incoming message, such as called party number or mobile subscriber identification number.

For connection-oriented transport, SCCP and SUA interface at the signaling gateway to associate the two connection sections needed for connection-oriented data transfer between an SS7 signaling end point and an IP endpoint. Messages are routed by the signaling gateway to SS7 signaling points based on the destination point code (in the MTP-3 address field) and to IP endpoints based on IP address (in the SCTP header).

SUA can also be used to transport SCCP user information between IP endpoints directly rather than via the signaling gateway. The signaling gateway is needed only to enable interoperability with SS7 signaling in the switched circuit network.

If an IP resident application is connected to multiple signaling gateways, multiple routes may exist to a destination in the SS7 network. In this case, the IP endpoint must monitor the status of remote signaling gateways before initiating a message transfer.

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