# Narrowband Internet of Things (NB-IoT):
## 3 Narrowband-IoT Protocol Stack
### Control and User Plane Optimization  
To support massive end-to-end device connectivity with extremely low complexity and reduce the transmission signaling, NB-IoT implements new small data transmission procedures based on Cellular IoT (CIoT) Evolved Packet System on both Control Plane (CP) and User Plane (UP). These transmission procedures support small bursts of data efficiently while guaranteeing the long-range coverage as compared to legacy GPRS [85; 86]. In this regard, NB-IoT is can support more than one data path in CP for the transmission of user data which is carried by the signaling messages managed by the Mobile Mobility Entity (MME) as shown in Figure 5. The procedures are optimized to efficiently support the small data transfer as follows:  
* Mandatory CP CioT EPS;
* Optional UP CioT EPS. CP CIoT EPS optimization encapsulates the data packets in Non-Access Stratum (NAS) by using control plane signaling messages. In this regard, this procedure is mandatory. Compared to the conventional SR procedure, the NB-IoT UE skips some steps required for each data transfer hence this optimization procedure best fits the short data transmission or reception. On the other hand, UP CIoT EPS optimization requires the RRC connected mode to get the scheduled radio resources as well as Access Stratum (AS) between the UE and the network. This mode uses the newly introduced connection to Suspend and Resume procedures. Connection suspend procedure helps to retain the network context so that the UE can resume the connection when traffic is available. Retaining the context helps the UE and the network to skip the AS and RRC reconfiguration in each data transfer. Since it uses user plane, the UP CIoT EPS is suitable for both small and large transactions. Furthermore, the UE in Service Request procedure (an LTE procedure used by the UE and base station to transmit or receive data in RRC idle state) is required to be in a connected state in order for base station to allocate the radio resources. For NB-IoT this SR is optional; however, NB-IoT UE that supports UP optimization needs also to support SR. For example; if the NB-IoT UE wants to transmit the uplink data in idle state, it will send the random access preamble through which the base station and UE will establish RRC connection and UE will be allocated with the radio resources for data transfer. After a certain period of inactivity, the base station initiates the release procedure. Similarly, for UE downlink data reception, if the UE is in DRX mode, the UE regularly listens to downlink signaling and if the UE notices the paging message, it will perform the SR procedure as described in uplink data transmission. Additionally, if the UE is in PSM mode, it will be completely inaccessible until it initiates the same SR procedure for the uplink grant or by using Tracking Area Update (TAU). There are works that are addressing the upper layers such as [77], where the authors proposed an efficient small data transmission scheme by using CP procedure. The proposed scheme enables the devices to transmit data packets through the RRC connection setup procedure when the device is in idle mode. This process reduces the signaling overhead caused by the security setup process and data radio bearer setup process. However, a suggestion could be to analyze the power consumption during this small data transmission and compare its effectiveness to when the same data is transmitted during the UP procedure. **Summary:** This section discussed PHY layer features, highlighting the corresponding enhancements on cell acquisition procedure, random access channel estimation, and interference mitigation. It then addressed the MAC layer enhancements regarding resource allocation, link adaptation, coverage and  
Figure 5: Representation of NB-IoT IP and Non-IP data path: Blue line displays the IP data path in UP mode (as Legacy LTE), Red line displays the non-IP data path in CP mode, and dashed-line displays the IP data path in CP mode. capacity, and power management.