LTE-RLC中ARQ过程的轮询问题

romiyang

5.2.2.3
Expiry of t-PollRetransmit
Upon expiry of t-PollRetransmit, the transmitting side of an AM RLC entity shall:
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if both the transmission buffer and the retransmission buffer are empty (excluding transmitted RLC data PDU awaiting for acknowledgements); or
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if no new RLC data PDU can be transmitted (e.g. due to window stalling):
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consider the AMD PDU with SN = VT(S) – 1 for retransmission; or
      -
consider any AMD PDU which has not been positively acknowledged for retransmission;
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include a poll in a RLC data PDU as described in section 5.2.2.1.
这段话我看不大明白，什么时候应该传SN = VT(S) – 1 的AMD PDU，什么时候应该传没有被确认已收到的PDU。希望各位大侠不吝赐教