Читать книгу Wireless Connectivity - Petar Popovski - Страница 45

We look briefly into the changes required when full-duplex devices operate in a multi-hop setting. Recall from the previous section that the receiver, upon detecting collision, can send a busy tone to the transmitters, such that they can interrupt their transmissions and shorten the time wasted in a collision. Consider the hidden terminals Zoya and Xia from Figure 2.6(c) and assume there is no RTS/CTS mechanism in place. With half-duplex devices, the time that can be consumed by collision can go up to two packet durations; the extreme case is when Xia starts transmission just before Zoya's packet ends and both packets are wasted. With full-duplex devices, the time consumed by a collision is at most a single packet duration, as Zoya and Xia interrupt transmission immediately after a busy tone is sent to them. If the RTS packet is much shorter than the data packet, then it is still useful to limit the maximal duration of the collision to be short, even if the devices have full-duplex capability. Full-duplex operation can also be beneficial to address the exposed terminal problem, Figure 2.6(d). As discussed above, Xia can start to transmit after receiving an RTS from Zoya and no CTS from Yoshi. When Zoya has finished transmitting, she sends a signal to issue a command to Xia to temporarily switch off her transmission, until Zoya receives an ACK from Yoshi. If Xia has a full-duplex device, then while still transmitting to Walt, Xia can detect the command sent by Zoya, suspend her transmission while Zoya receives an ACK, and continue transmitting afterwards.