in packet radio we are generally operating on a shared channel. in order for many stations to co-exist peacefully, packet controllers use a scheme known as csma (carrier sense multiple access). in this scheme each controller listens to the channel prior to transmitting, and will not transmit if another carrier is detected.
most of the modems used on packet controllers will accept "white noise", such as an open squelch, and identify it as a carrier. this results in either a continuously flickering or constantly illuminated dcd led, which indicates that the controller has sensed a carrier on the channel. this has resulted in many individuals operating their radios in the squelched mode. while this may reduce one aspect of the problem, it enhances the overall adverse effects of the problem. many radios are very slow to open the squelch, which tends to make csma fall apart, since the modem can't hear the signal while the squelch is opening . if the controller decides to transmit during this time frame, there will be a collision.
摘自
http://www.paccomm.com/dcd.html
