# Evaluating the Impact of Pre-Configured Uplink Resources in Narrowband IoT
## 2 NB-IoT
### Operational States
#### 2.1.1 Early Data Transmission  
To improve the power efficiency of UE, particularly those that require infrequent small data transmissions in the UL, EDT has been introduced in 3 GPP release 15 [11; 23]. Thisapproach reduces the power consumption for UE sending sporadic small data packets by enabling data transmission during the RAP. EDT applies to UP and CP solutions, with differences elaborated later in this section.  
To implement EDT, UE need to signal their intent to engage in data transmission during Msg 3. Compared to release 13, where the eNodeB lacked prior knowledge of the intentions of the UE data transmission, the EDT mandates using a specific (N)PRACH preamble dedicated to the EDT signaled in Msg 1. Furthermore, the network disseminates the maximum Transport Block Size (TBS) that Msg 3 in EDT can utilize. Over-the-air security poses another consideration. In release 13, data bearer security is established after RAP completion, but for EDT, security must be established during Msg 3 transmission. Mechanisms to achieve this are detailed subsequently. In the RRC_IDLE state, if a UE does not have more data than the broadcast maximum, it may employ EDT, as shown in Figures 3 and 4.  
In the UP solution for EDT within release 15, the UE application-specific (AS) context facilitates data resumption and signals radio bearers and security context for user plane data transmission. Compared to the default RAP, where these procedures took place after  
Figure 4: The sequence of messages for establishing connections and transmitting packets, detailing the default RAP from Msg 1 to Msg 6, EDT from Msg 1 to Msg 4, and PURs from Msg 3 to Msg 4.  
Figure 3: The NB-IoT Uplink Early Data Transmission procedure according to 3 GPP release 15, highlighting the transmission of data in Msg 3 and the reception of acknowledgment by the UE in Msg 4.  
Msg 4 reception, the UP solution for EDT entails the UE activating suspended data bearers for Msg 3 transmissions, resuming security, and allowing early DL data transmission.  
The CP solution for EDT allows UL data within the initial NAS message \"CONTROL PLANE SERVICE REQUEST\", transmitted via the RRCConnectionSetupComplete. The objective is to reduce signaling overhead, latency, and UE power consumption. In the CP EDT mechanism, Msg 3 and Msg 4 accommodate NAS messages containing UL and DL user data, ensuring data delivery before Msg 5 while keeping the UE in RRC_IDLE mode.  
However, the CP EDT mechanism faces challenges. Unlike the UP EDT approach, it lacks AS security. It employs NAS messages piggybacked on RRC messages through an SRB0 (signaling radio bearer), which can cause issues, especially for Msg 4 with DL data, potentially leading to UE transitioning to RRC_IDLE mode without guaranteeing successful DL data delivery.  
In summary, EDT reduces the need to enter the connected state and, therefore, improves the power efficiency of the UE for small, infrequent data transmissions. The UP and CP solutions differ in handling data transmission during the RAP, with considerations for security and UE state transitions.