LTE, an acronym for Long-Term Evolution. 4G LTE is a standard for wireless broadband technology designed to support roaming Internet access via cell phones and handheld devices. Due to the requirements on high data rate and low latency for broadband wireless communication systems, LTE air-interface protocol stacks to be designed and implemented with high data processing efficiency. LTE User Equipment (UE), this paper analyzes the main challenges in the link-layer protocol stack design and application, including the data handling efficiency, synchronic, flexibility and portability. Authentication service is one of the most essential services in LTE networks.