Explain the concept of hidden node and exposed node with respect to WLAN.

This question appears in Mumbai University > Wireless Networks subject

Marks: 5M , 10 M

Year: May 2012, Dec 2013, May 2014


1.Hidden node problem :

i. Hidden node problem arises when two stations on opposite sides of an AP can hear activity from an AP, but not from each other, usually due to distance or an obstruction.

ii. In such a case, if one station wants to transmit data, there is a fear of collision from the other station i.e. ‘hidden node’.

iii. To solve this problem, 802.11 specify an optional ‘request to send/clear to send (RTS/CTS)’ protocol at the MAC layer.

iv. With this feature, the station which wants to transmits, transmits an RTS and waits for the AP to reply with CTS.

v. Since all stations can hear the AP, the CTS cause the hidden nodes to delay any intended transmissions. Thus, it allows the sending station to transmit and receive a packet acknowledgement without any chance of collision.

vi. Since RTS/CTS adds additional overhead to the network by temporarily reserving the medium, it is typically used only on largest sized packets, for which retransmission would be expensive from bandwidth standpoint.

vii. Since all stations can hear the CTS, this mechanism also reduces the probability of a collision on the receiver area.

viii. During the duration of the RTS, the transmission area is protected from collision during the ACK from the station that are out of range of acknowledgement station.

ix. Also, RTS and CTS are short frames. Thus, if these frames are lost due to collision, there is not much overhead. The RTS and CTS frames are recognized faster.

x. There is a concept of RTS threshold which, if used, must be set on both AP and client side.

xi. The time required to transmit a frame, taking into account the RTS/CTS four way handshake, is given by, (BO+DIFS+4PLCP+(RTS+CTS+D+A)/R+3SIFS)microsec.

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Fig9. Hidden and Exposed node problem

2. Exposed Node Problem

i. In the figure (b), let us assume that node ‘b’ and ‘c’ intend to transmit data only, without receiving data.

ii. Since, node ‘c’ has initiated an RTS and node ‘b’ is in the radio coverage of node ‘c’, node ‘b’ will not initiate the transmission to node ‘a’ because it will detect a busy medium.

iii. The transmission between node ‘b’ and node ‘a’ is therefore blocked even if both of them are idle. This is referred as ‘Exposed Node Problem ’.

iv. To alleviate this problem, a node must wait with a random backoff time between two consecutive new packet transmission times.

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