Question: In a TDD UTRAN scenario, following a "radio link failure", what is the maximum allowed Re-establishment delay (TRE-ESTABLISH) for a UE to camp on a known cell and initiate a CELL UPDATE message, as per 3GPP specifications?
Option 1: 1000 ms
Option 2: 1500 ms
Option 3: 2500 ms
Option 4: 1815 ms
Explanation: The Re-establishment delay (TRE-ESTABLISH) to a known cell is crucial for maintaining seamless connectivity in wireless networks. As per the specifications, this delay encompasses the time from the beginning of the T2 period until the UE camps on Cell 2 and starts sending SYNC-UL for a CELL UPDATE message due to "radio link failure". The maximum allowed value for TRE-ESTABLISH, ensuring efficient network recovery while accommodating various protocol and procedure delays (e.g., TRRC-RE-ESTABLISH, TUE-RE-ESTABLISH-REQ-KNOWN), is set at 1815 ms. This threshold supports the balance between quick re-establishment and the practical limitations of wireless communication protocols.