Q & A on Delay for CSMA Carrier Sense Q: I had a question about CSMA. How long a computer will "sense" before deciding that NO DATA is being sent? As far as I know, it is equal to the time required for one bit to propagate from one end of the segment to the other end. Am I right? A: If a station is ready to transmit and there is no carrier present (i.e., no transmission is in progress), the station can begin to transmit. Q: Any reasoning behind the waiting period? Perhaps you are thinking about the collision detection problem that occurs when two stations both transmit. In that case, it can take *twice* as long as the maximum propagation time to detect the problem. To see why, consider two stations at the opposite ends of a cable. Station A begins to transmit, and the signal propagates along the cable. Unfortunately, if station B begins to transmit jsut before the signal from A arrives, the signal from B will not reach A until it propagates back along the cable. Hence, the worst case requires almost twice the maximum propagation time. Q: What I am asking about is the ``sense'' time. Is it a function of the propagation delay, or it is a function of the bandwidth? A: Because carrier sensing is performed by analog hardware, a sufficient amount of signal must arrive at the correct rate for the hardware to distinguish it from noise. To make synchronization possible, each transmission starts with a string of alternating 1's and 0's which allows the receiver to determine its presence unambiguously. Thus, there is a short time between the arrival of the signal and the receiver's recognition. The lapse depends on the bit rate being used. Similarly, there is a minmum time after a transmission (related to the propagation delay) during which no station is permitted to transmit.