e 4.23 Signalling interfaces and protocols To create a control plane for each interface requires the creation of: signalling association sets for Sigtran narrowband signalling linksets broadband signalling linksets for Iu-CS interface Signalling Transport over IP (SIGTRAN) SCTP Associations and Association Sets The Stream control transport protocol (SCTP) was developed as the basic lower layer protocol to serve different signalling adaptation layers. The SCTP runs directly over the IP. It is similar to the TCP but has some enhancements. It features: reliable data transfer (continuous monitoring on reachability); connection oriented with multiple streams; ordered or unordered delivery; congestion and flow control; protection against known attacks (in IP networks). SCTP associations are logical 'point-to-point' signalling channels between two nodes (see Figure 4.24). There is no physical resource allocation per association. Associations are created between a pair of signalling units. SCTP Multihoming Each signalling unit has a pair of interfaces each with a logical IP address. During the establishment of an SCTP association the two peer signalling units exchange their IP addresses. Either IP address may be used as source or destination addresses but one of the IP addresses is the primary address and is normally used while the other is the secondary IP address and is only used when the primary is unavailable. An Core Network Planning and Optimisation association MSC (server) HLR (client) CCSU 0 CCSU 0 IP network CCSU 1 CCSU 1 CCSU 2 CCSU 2 association set / SS7 signalling link set Figure 4.24 SCTP association/association set (CCSU, common channel signalling unit) SCTP association is said to be multihomed when more than one address/path to the peer signalling unit exists for use in failure situations. An association set is a set of SCTP associations connecting two signalling points. The bit rate of signalling traffic in an association is not limited but it is recommended that as many associations as there are signall