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.
- 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).
- 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.