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# Hidden Station Problem (HSP) in Wireless LAN

Wireless LAN is used to transmit data from one device to another without using connected medium. There are normally several problems in wireless LAN. One of them is Hidden Station Problem (HSP).

Hidden Station Problem (HSP) :
When two stations hidden from each other i.e., not in range of each other send signals to third station at the same considering third station is free. It causes collision at third station and is known as Hidden Station Problem. It reduces capacity of network due to possibility of collision. Following is diagrammatically representation of Hidden Station Problem (HSP) in wireless LAN.

How HSP is created?
In the above shown diagram, station B and C both covers station A in their own range. Each station B and C can send data to station A separately. Both stations B and C are outside of range of each other. Suppose station B is sending data to station A and in middle of transmission station C also has to send data to station A. Since station B and station C are out of each other range therefore station C thinks that station A is free. Station C send data to station A and collision occurs at station A.

How to prevent HSP?
Hidden Station Problem (HSP) can be prevented by using handshake frames.

In the below shown diagram, RTS message from B reaches A but not C. However, both B and C are within range of A. CTS message containing duration of data transmission from B to A, reaches C. Thus C knows some hidden station is using channel and does not transmit until that duration is over.

Note –
CTS frame of handshake prevents collision from hidden station.

RTS : Request To Send
CTS : Clear To Send

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