Radio Access Technology 9.4.12 Mw Reference Point (CSCF-CSCF) This interface enables the interrogating CSCF to direct mobile terminated calls to the serving CSCF. The protocol supported is SIP according to RFC 2543. However, some additions to SIP beyond what is defined in RFC 2543bis might be required to cope, e.g. with accounting, security or supplementary services requirements. 9.4.13 Nc Reference Point (MSC Server-GMSC Server) We perform the network-network based call control over the Nc reference point. Ex- amples of this include ISUP or an evolution of ISUP for bearer independent call control (BICC). In the ROO architecture we aim to have different options (including IP) for sig- nalling transport on Nc. 9.4.14 Nb Reference Point (MGW-MGW) We perform bearer control and transport over the Nb reference point. We may use RTP/UDP/IP or AAL2 to transport user data. In the ROO architecture we aim for differ- ent options to transport user data and bearer control, e.g. AAL2/Q.AAL2, STM/none, RTP/H.245. 9.4.15 CAP Based Interfaces This corresponds to the interfaces from the SGSN to the SCP, from the serving CSCF (and possibly the interrogating CSCF) to the SCP, from the MSC server to the SCP, and the GMSC server to the SCP. From Ref. [7], the interface from the SGSN to the SCP in the applications and services domain corresponds to the interface defined for UMTS GPRS to support charging ap- plication interworking. We require the interface from the CSCF to the SCP to allow the support of existing CAMEL based services. The interface from the MSC server to the SCP, and the GMSC server to the SCP corresponds to the standard interface defined for the CAMEL feature, which provides the mechanisms to support non-standard UMTS/GSM services of operators even when roaming outside the home PLMN. We can implement the CAP based interfaces by using CAP over IP, or CAP over SS7 as illustrated in Table 9.1. Table 9.1 Protocol Stack for CAP [7] MTP-3B Narrow-band SS7 STM 2 In IETF work is ongoing (e.g. SCTP/UDP/IP or directly SCTP/IP. 