topic
This question is selected from "Computer Network-Xie Xiren Sixth Edition"
3-25 In the previous question, stations A and B sent data frames at the same time at t=0. When t=255 bit time, A and B have detected a collision at the same time, and the interference signal transmission is completed at t=255+48=273 bit time. A and B choose different r values to back off in the CSMA/CD algorithm. Assume that the random numbers chosen by A and B are rA=0 and rB=1, respectively. When does A and B start to retransmit their data frames? When does the data frame retransmitted by A arrive at B? Will the data retransmitted by A collide with the data retransmitted by B again? Will B stop sending data at the scheduled retransmission time?
t=0, A, B start to send data
t1=225 bit time, A, B detects collision
t2=273 bit time [t1+48], A, B transmission interference signal time
t3=594 bit time [t2+tao +2rAtao+96], where tao is the propagation time of the interference signal (the above is the transmission time), and in 2rtao, because rA = 0, the value is 0, that is, it is retransmitted immediately, 96 is the minimum interval between frames
t4=785 bits Time [t2+2rBtao], at this time B starts to listen to the channel
t5 = 881 bit time [t4+96], after clearing the buffer, if B hears that the channel is free, it can start retransmission.
However,
t6 = 819 bit time [ t3+tao], that is, when A is at 819, the first bit arrives at B, that is, B detects that there is data on the channel, so B will not retransmit at a predetermined time
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Supplementary knowledge:
Enhanced collision
- When a collision occurs, in addition to immediately stopping sending data, in order to let all users know that the collision has occurred, a 32b or 48b interference signal is also sent
- The minimum interval between frames is 9.6 microseconds, 96-bit time, which is the time used by the receiver to clear the receiving buffer