Session Initiation Protocol (SIP) has been selected as the official end-to-end signalling protocol for establishing multimedia sessions in the IP-based Universal Mobile Telecommunication Systems (UMTS) network. Since a satellite component has been identified to play an integral role in UMTS, there is a need also to support SIP-based session establishment over Satellite-UMTS (S-UMTS) to achieve end-to-end seamless IP-based terrestrial/satellite network integration. However, the transport of SIP over the satellite radio interface is not efficient due to the request/response nature of SIP and its large message size, coupled with the unreliable wireless links and the larger propagation delay. With little work carried out in this area, this paper presents the research on the efficient solutions to transport SIP session set-up signalling over the S-UMTS radio interface based on the Radio Link Control acknowledged mode (RLC-AM) mechanisms. Two schemes are also proposed to overcome the inefficiency of the current RLC-AM procedure when applied for session set-up over S-UMTS. The proposed enhancements are easy to implement and are fully backward compatible with the existing implementation of RLC-AM in UNITS. (c) 2005 Elsevier B.V. All rights reserved.