Mumbai University > Computer Engineering > Sem6 > Mobile Communication and Computing

Marks: 5M

Year: May 2015

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).

• 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.