Optimized Macro Mobility within the 3GPP IP Multimedia Subsystem

In the 3GPP IMS, macro mobility (changing IP-address) is not supported, if the user changes access technology the user equipment (UE) will automatically get a new IP-address. After this reassignment of the IP-address the user has to re-register to the IMS and send new invites to all corresponding nodes (CNs) before the sessions can continue. This will potentially introduce a long interruption of ongoing sessions. In this paper a solution is introduced that reduces the handover delay by sharing the registration information and call states. The full register and invite flows are not necessary in that case, since the servers in the IMS already have state information about the user and sessions from the context transfer. The benefit with respect to reduced handover delay is quantitatively analyzed using an experimental implementation of the improved SIP mobility solution for IMS.