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Explain hidden station and exposed station problem in WLAN.
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WLAN

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A significant difference between wired and wireless LANs is the fact that, in general a fully connected topology between the WLAN nodes cannot b assumed. This problem gives rise to ‘hidden’ and ‘exposed’ station problems.

Hidden Terminal:

  • As seen in the above problem, the transmission range of A reaches B but not C. Similarly, the range of C reaches B but not A. Also the range of B reaches both A and C.
  • Now, the node A starts to send something to B and C doesn’t receive this transmission.
  • Now C also wants to send data to B and senses the carrier. As it senses it to be free, it also starts sending to B.
  • Hidden terminal problem occurs when two nodes that are outside each other’s range performs simultaneous transmission to a node that is within the range of each of them resulting in a collision.
  • That means the data from both parties A and C will be lost during the collision.
  • Hidden nodes mean increased probability of collision at receiver end.
  • One solution to avoid this is to have the channel sensing range much greater than the receiving range. Another solution is to use the Multiple Access with Collision Avoidance (MACA).

Exposed Terminal:

  • Consider the same above diagram. Here imagine a situation wherein the B node is currently sending some data to node A.
  • Now the other node C which is right now free want to send data to some node D(not in diag) which is outside the range of A and B.
  • Now before starting transmission it senses the carrier and realizes that the carrier is busy (due to interference of B’s signal).
  • Hence, the C node postpones the transmission to D until it detects the medium to be idle.
  • However such a wait was un-necessary as A was outside the interference range of C.
  • Also a collision at B will be a weak enough to be unable to penetrate into C
  • Exposed terminal problem occurs when the node is within the range of a node that is transmitting and it cannot be transmitted to any node.
  • Exposed node means denied channel access unnecessarily which ultimately results in under-utilization of bandwidth resources.
  • It also results in wastage of time-resource.
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