In this section, we describe the functions of the different protocol layers and their location in the LTE architecture. Figures 2 and 3 show the control plane and the user plane protocol stacks, respectively [4]. In the control-plane, the NAS protocol, which runs between the MME and the UE, is used for control-purposes such as network attach, authentication, setting up of bearers, and mobility management.
All NAS messages are ciphered and integrity protected by the MME and UE. The RRC layer in the eNB makes handover decisions
based on neighbor cell measurements sent by the UE, pages for
the UEs over the air, broadcasts system information, controls UE
measurement reporting such as the periodicity of Channel Quality
Information (CQI) reports and allocates cell-level temporary identifiers
to active UEs. It also executes transfer of UE context from the
source eNB to the target eNB during handover, and does integrity
protection of RRC messages. The RRC layer is responsible for the
setting up and maintenance of radio bearers.
