Hidden node

In wireless networking, the hidden node problem or hidden terminal problem occurs when a node is visible from a wireless access point (AP), but not from other nodes communicating with that AP. This leads to difficulties in media access control sublayer.

Hidden nodes in a wireless network are nodes that are out of range of other nodes or a collection of nodes. Take a physical star topology with an access point with many nodes surrounding it in a circular fashion: Each node is within communication range of the AP, but the nodes cannot communicate with each other, as they do not have a physical connection to each other. In a wireless network, it is likely that the node at the far edge of the access point's range, which is known as A, can see the access point, but it is unlikely that the same node can see a node on the opposite end of the access point's range, C. These nodes are known as hidden. The problem is when nodes A and C start to send packets simultaneously to the access point B. Because the nodes A and C are out of range of each other and so cannot detect a collision while transmitting, Carrier sense multiple access with collision avoidance (CSMA/CA) does not work, and collisions occur, which then corrupt the data received by the access point. To overcome the hidden node problem, RTS/CTS handshaking (IEEE 802.11 RTS/CTS) is implemented in conjunction with the Carrier sense multiple access with collision avoidance (CSMA/CA) scheme. The same problem exists in a MANET.

The hidden node problem can be observed easily in widespread (>50m radius) WLAN setups with many nodes that use directional antennas and have high upload. This is why IEEE 802.11 is suited for bridging the last mile for broadband access only to a very limited extent. Newer standards such as WiMAX assign time slots to individual stations, thus preventing multiple nodes from sending simultaneously and ensuring fairness even in over-subscription scenarios.

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