Editing Hidden node problem (section)

==Background==
Hidden nodes in a [[wireless network]] are nodes that are out of range of other nodes or a collection of nodes. Consider a physical [[star network|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.

For example, 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 communicate with a node on the opposite end of the access point's range, '''C'''. These nodes are known as ''hidden''.

Another example would be where A and C are either side of an obstacle that reflects or strongly absorbs radio waves, but nevertheless they can both still see the same AP.

The problem is when nodes '''A''' and '''C''' start to send [[Packet (information technology)|packet]]s simultaneously to the access point '''B'''. As the nodes '''A''' and '''C''' cannot receive each other's signals, so they cannot detect the collision before or while transmitting, [[carrier-sense multiple access with collision detection]] (CSMA/CD) does not work, and collisions occur, which then corrupt the data received by the access point.

To overcome the hidden node problem, request-to-send/clear-to-send (RTS/CTS) handshaking ([[IEEE 802.11 RTS/CTS]]) is implemented at the Access Point in conjunction with the [[Carrier sense multiple access with collision avoidance]] ([[CSMA CA|CSMA/CA]]) scheme. The same problem exists in a [[mobile ad hoc network]] ([[MANET]]).

[[IEEE 802.11]] uses [[802.11 RTS/CTS]] acknowledgment and handshake packets to partly overcome the hidden node problem. RTS/CTS is not a complete solution and may decrease throughput even further, but adaptive acknowledgements from the base station can help too.

The comparison with hidden stations shows that RTS/CTS packages in each traffic class are profitable (even with short audio frames, which cause a high overhead on RTS/CTS frames).<ref>Pommer, Hermann: ''Roaming zwischen Wireless Local Area Networks''. [[VDM Verlag]], Saarbrücken 2008, {{ISBN|978-3-8364-8708-5}}.</ref>

In the experimental environment following traffic classes are included: data (not time critical), data (time critical), video, audio. Examples for notations: (0|0|0|2) means 2 audio stations; (1|1|2|0) means 1 data station (not time critical), 1 data station (time critical), 2 video stations.

[[Image:RTS CTS benchmark.png|250px|thumb|Benchmarks: Net Throughput with/without RTS/CTS (Pommer, p.179)]]

The other methods that can be employed to solve hidden node problem are :       
*Increase transmitting power from the nodes
*Use [[omnidirectional antenna]]s
*Remove obstacles
*Move the node
*Use protocol enhancement software
*Use [[antenna diversity]]