Handsets and Session Continuity

Session continuity can be effected by handsets, which can be dual-mode (3G+WiFi and 3G+WiMAX, for example). Such a handset has two receivers or radios to initiate calls simultaneously. An LTE handset has only one receiver and is able to attach to a single LTE or 3G network at a given time.

The question of session continuity appears in 3GPP standards Release 8, 9, and 10—each building on the previous. Release 8 defines Single Radio Voice Call Continuity (SRVCC) as the mechanism for moving active voice sessions between LTE and existing 2G or 3G circuit networks. In moving over sessions such as these, it is key to keep latency as low as possible to increase the possibility of successful handovers.

Release 9, because of variable signaling latencies within the core network, could not guarantee smooth handovers to circuit networks. And though IMS provides great flexibility in locating application servers remotely from the UE, that flexibility actually increases the latency in signaling media changes to the access network. Due to the high total signaling latency and the difficulty of successfully coordinating handover timing between 3G and 4G call legs, the possibility of call drops increased.