Question: In an AM RLC entity, what does the SOstart field (16 bits) primarily indicate for an RLC SDU with SN = NACK_SN?
Option 1: The total number of bytes received correctly before a detected loss.
Option 2: The position of the first byte of the lost portion within the original RLC SDU, with numbering starting at zero.
Option 3: The sequence number of the next expected RLC SDU after a loss is detected.
Option 4: The total length of the lost portion of the RLC SDU in bytes.
Explanation: The SOstart field specifies the starting point (in bytes) of the lost segment within the original RLC SDU, crucial for identifying and requesting retransmission of the specific lost portion, with numbering starting at zero.