| 摘要 |
A method and a system for the retransmission of incorrectly received frames in a transmission system in which information is transmitted in the form of sequentially numbered frames. Each information frame includes a so-called send sequence number, N(S), a bit, R, indicating if this information frame has previously been retransmitted, and an information field containing the information to be transmitted. At the transmitting station, a variable, V(T), whose value indicates the order in which a frame is retransmitted relative to the sequence of frames transmitted for the first time, is associated with each retransmitted frame. At the receiving station, the incoming information frames are checked for errors and the N(S) of frames found to be incorrect is noted. The receiving station then sends to the transmitting station a so-called Checkpoint (CP) message containing a so-called receive sequence number, N(R), which indicates the send sequence number, N(S), of the most recently received, non-retransmitted, error-free information frame, and, if at least one incorrect frame has been received, at least one identifier, N(X), which indicates the N(S) of the incorrect frame whose retransmission is requested. Upon receipt of the Checkpoint message, the transmitting station retransmits requested information frames that have not been previously retransmitted; it also retransmits previously retransmitted frames, only if the V(T) values associated therewith are less than the receive sequence number, N(R), contained in the CP message; and it updates the V(T) value associated with each frame thus retransmitted. In a preferred embodiment of the invention, the Checkpoint messages are transmitted on a periodic basis. Each of these contains an X bit whose value indicates whether the values between N(R) and the highest N(X) are also to be considered as N(X) values. |
