The problem of hidden terminal and exposed terminal in wireless networks
The Hidden Station Problem
Figure 1 illustrates the hidden terminal problem. Suppose that node A wants to transmit to node B located at a distance x from A. By only sensing the medium, node A will not be able to hear transmissions by any node (C) in the dashed area denoted by A(x), and will start transmitting, leading to collisions at node B. This is the well known hidden terminal problem, where the hidden nodes are located in the area A(x).
The Exposed Station Problem
RTS/CTS handshake mechanism was introduced to wireless MAC layers to eliminate the hidden terminal problem. However, this mechanism introduces a new problem termed the exposed terminal problem. We assume here an RTS/CTS exchange so that the issue of hidden terminal is addressed. Let us consider Figure 2 and assume that node A wants to transmit to node B.
Node A sends an RTS and waits for B to send a CTS. Suppose a node D located in area Y (x) wants to transmit data to node C located in area X(x), and D transmits a RTS to C just before A sends the RTS to B. After receiving the RTS from D, C transmits a CTS. This CTS is heard by B upon which B will enter a backo® period preventing B from sending the CTS to A. Therefore, any transmission from a node within the area Y to a node within X(x) will prevent A from transmitting data to B, although simultaneous transmissions from area Y (x) to X(x) would not have interfered with transmission from A to B. We denote the terminals in the region Y (x) as the exposed terminals for the node pair A/B. In this case, the number of transmissions that could occur between nodes from area X(x) and nodes from area Y (x) can be expressed as XY.