Question: In the context of E-UTRAN, which direction(s) is/are applicable for the transmission of the EN-DC X2 Setup Response message to establish a TNL (Transport Network Layer) association between two nodes capable of interacting for EN-DC (Enhanced New Radio - Dual Connectivity)?
Option 1: eNB (Evolved Node B) to en-gNB (evolved Next Generation Node B) only
Option 2: en-gNB to eNB only
Option 3: Both eNB to en-gNB and en-gNB to eNB
Option 4: Neither eNB to en-gNB nor en-gNB to eNB, as it's sent within the same node type
Explanation: The EN-DC X2 Setup Response message initializes a TNL association between two nodes that support EN-DC. Given EN-DC's dual nature (involving LTE and NR technologies, represented by eNB and en-gNB), effective communication requires bidirectional initiation and response. Thus, this message can be sent from an eNB to an en-gNB and from an en-gNB to an eNB, facilitating interaction for setting up necessary associations.