A threshold standard, which judges whether a frame need to retry to send or discard, gives an utmost times to retry to
send a short frame packet from the MAC layer. A short frame means the lenth of the frame doesn't preponderate over the RTS/CTS threshold.
Once the times to retry to send a short frame from MAC layer preponderates over the frame retry times, the frame would be discarded.
The range of the short frame retry times is from 1 to 255 times, and the default of the short frame retry times is 7 times.