, there is still an X2 interface between two eNBs, which enables a fast resume after the UE transitions to the idle state. In the control-plane CloT EPS optimization, the data exchange between the NB-IoT termi- nal and the eNB is performed at RRC level. In the downlink and uplink, the data packets may be piggybacked in the RRCConnectionSetup and RRCConnectionSetupComplete messages, respectively. For larger payloads, data transfer may continue using DLInformationTransfer and ULInformationTransfer messages. In all of these messages, there is a byte array containing non-access stratum (NAS) information, which in this case corresponds to the NB-IoT data packets. As a result, this procedure is transparent to the eNB, and the UE's RRC sublayer forwards the content of the received DLInformationTransfer directly to its upper layer. The dedicatedInfoNAS message is exchanged between the eNB and the MME via the S1-MME interface. For this data transfer mechanism, security on access stratum (AS) level is not applied. Since there is no RRC con- nection reconfiguration, it may immediately start after or during the RRC connection setup or resume procedure. The RRC connection has to be terminated afterwards with the RRC connection release. In the user-plane CloT EPS optimization, data is transferred over the conventional user plane through the network; that is, the eNB forwards the data to the S-GW or receives it from this node. In order to minimize the UE complexity, only one or two dedicated radio bearers may be simultaneously configured. One needs to distinguish between two cases: 1. If the previous RRC connection was released with a possible resume operation indi- cated, the connection may be requested as a resume procedure. If the resume procedure is successful, then the security is established with updated keys and the radio bearer is set up similar to the previous connection. there was no previous RRC connection release with a resume indication, or if the resume request was not accepted by the eNB, the security and radio b