The Hidden Fine-grained State Machine in Cellular Network for Simultaneous Voice and Data Services

The Radio Resource Control (RRC) state machine has been widely studied to improve application performance and reduce the energy consumption of smart phones. We observe that even a smart phone remains in the RRC state, there may still exist some significant performance change if it simultaneously carry on voice and data services. For example, the Round Trip Time (RTT) will get larger when a voice call comes upon, and may get even larger after finishing the voice call. We conduct in-depth study and find out the primary cause of those phenomena - smart phones may get different physical channels even in the same RRC state. In this paper, we define new states according to which physical channel is allocated to smart phones, then discover a fine-grained state machine which is hidden in the physical layer of user plane, which can explain those phenomena very well. We construct the hidden fine-grained state machine for two carriers, one only supports 3G network in most area and the other supports 4G network. The state machine can help us analyze the interference of voice to data. Using the fine-grained state machine, we also offer some suggestions on how to avoid those performance loss both from the perspective of carriers and users.