Wireless networking is susceptible to the hidden node problem, which occurs when an STA is visible from the AP, but not from other STAs that are also communicating with the AP. This typically occurs with STAs that are at the far end of the AP’s range. This leads to difficulties in media access control because the STAs cannot sense the carrier and both send packets to the AP at the same time, which results in collisions. This concept is illustrated below in Figure 9-7:
Fig. 9-7. The Hidden Node Problem
Referencing Figure 9-7, the two STAs can see the AP but are unable to see each other because they reside at the very end of the AP’s range. Because the nodes cannot see each other, they may send frames to the AP at the same time. Because the STAs are hidden, CSMA/CA will not work because the STAs are unable to sense the carrier and collisions will occur.
To address this issue, and reduce the probability of STAs colliding because they cannot hear each other, the IEEE defines a Virtual Carrier Sense mechanism. This mechanism uses the Ready To Send (RTS) and Clear To Send (CTS) acknowledgment and handshake packets to partly overcome the hidden node problem.
With the RTS and CTS exchange, STAs must first request access to the medium from the AP with an RTS message. The RTS packet includes the source, destination, and duration information. The STA will refrain from accessing the medium and transmitting its data packets until it receives the CTS from the AP. The CTS is a response control packet from the AP that also includes duration information. The RTS originator then proceeds and sends frames.
All other stations that receive the CTS set their Virtual Carrier Sense indicator, commonly referred to as the Network Allocation Vector (NAV), for the given duration and will use that information along with the physical carrier sense when sensing the medium.