Question: What is the primary purpose of a Tunnel Endpoint Identifier (TEID) in GPRS tunnelling protocol between network elements like SGSNs, S-GWs, P-GWs, and RNCs/BSCs?
Option 1: To authenticate User Equipment (UE) across different Radio Access Networks (RANs)
Option 2: To prioritize traffic for Quality of Service (QoS) management in the core network
Option 3: To encrypt user data transmitted over the air interface
Option 4: To identify a tunnel endpoint and associate it with an EPS Bearer, PDP context, or Radio Access Bearer (RAB) within the receiving GTP-C or GTP-U protocol entity
Explanation: The correct answer is Option 4 because the TEID serves as a unique identifier for tunnel endpoints in GPRS tunnelling protocol, enabling the association of specific bearers or contexts (like EPS Bearer, PDP context, or RAB) with the correct tunnel endpoint. This ensures proper routing and management of user data across network elements. The other options are incorrect because they describe functions unrelated to the primary purpose of TEIDs as outlined in the context of GPRS tunnelling protocol.