在TIA/EIA-136-362找到了较为全面的答案：
Send state variable V(S)
Each RLC endpoint transmitter shall have an associated send state variable V(S). V(S) denotes the sequence 27
number of the next in-sequence RLC data block to be transmitted. V(S) can take on the value 0 through SNS - 1. 28
V(S) shall be set to the value 0 at the beginning of each TBF in which the RLC endpoint is the transmitter. The 29
value of V(S) shall be incremented by 1 after transmission of the RLC data block with BSN = V(S). In RLC 30
acknowledged mode, V(S) shall not exceed V(A) modulo SNS by more than the maximum allowed number of 31
outstanding RLC data blocks WS.
Acknowledge state variable V(A)
In RLC acknowledged mode, each RLC endpoint transmitter shall have an associated acknowledge state 2
variable V(A). V(A) contains the BSN value of the oldest RLC data block that has not been positively 3
acknowledged by its peer. V(A) can take on the values 0 through SNS - 1. V(A) shall be set to the value 0 at the 4
beginning of each TBF in which the RLC endpoint is the transmitter. The value of V(A) shall be updated from 5
the values received from its peer in the received block bitmap (RBB) of the Packet Ack/Nack message (see 6
Section 9.1.8) 7
Furthermore, [ V(S) - V(A) ] modulo SNS £ WS.