Читать книгу Wireless Connectivity - Petar Popovski - Страница 44

In order to make the collisions less expensive, we can reuse the concept of a reservation packet from Section 1.4.2. The main idea is to constrain the collisions to occur only for packets that are short. First we look at the simplest setting for hidden terminals from Figure 2.6(b). If Zoya has a data packet for Yoshi, then she sends a short request-to-send (RTS) packet. The RTS packet should contain information about the originator (Zoya) and how long the transmission from Zoya will last. Assume that Xia does not transmit while Zoya transmits an RTS. Then Yoshi receives the RTS from Zoya correctly, and he acknowledges it by sending a short clear-to-send (CTS) packet to Zoya. The CTS should contain the address of Zoya, but it also repeats the information about how long Zoya will need to send her packet to Yoshi. The latter information from the CTS is intended for the terminals that are hidden from Zoya, such as Xia, and the CTS blocks their transmissions while Zoya transmits. If Zoya receives the CTS, then she starts to transmit her data and, at the end of the transmission, she receives an ACK from Yoshi. Note that Xia is in the range of Yoshi, such that she gets inhibited by the carrier sensing mechanism in the case when Yoshi has something to send to Zoya, be it a CTS or an ACK. Recalling from the previous section the use of idle slots for packet prioritization, both control packets CTS and ACK that are sent as responses to other packets can use an idle slot that is shorter compared to an RTS packet. With this, when Yoshi sends a CTS packet, Xia senses a busy medium and is thus prevented from sending an RTS packet.

To see the other effects of the RTS/CTS mechanism, consider the exposed terminal problem in Figure 2.6(d). Zoya sends an RTS, Yoshi sends a CTS; Xia receives the RTS, but not the corresponding CTS. This is an indication for Xia that the intended receiver of Zoya is outside Xia's range and Xia can freely initiate a transmission to Walt. The only problem is that, after Yoshi receives Zoya's packet and sends an ACK to Zoya, Xia may be still transmitting and Zoya will not receive the ACK. Therefore, in this simple form, an RTS/CTS does not completely solve the exposed terminal problem and the reader is encouraged to think what other amendments can be done to the protocol in order to address this problem. We should also note that the utility of an RTS/CTS decreases if the range for carrier sensing is larger than the communication range.