Mumbai University > Computer Engineering > Sem 6 > Mobile Communication & Computing

Marks: 4 Marks

Year: May 2016

Hidden nodes in a wireless network are nodes that are out of range of other nodes or a collection of nodes.

The hidden terminal analogy is described as follows:

• Terminal A sends the data to B,terminal C cannot hear A
• Terminal C wants to send data to B,terminal C senses a "free" medium(CS fails) and start transmitting.
• Collision at B occurs, A cannot detect this collision(CD fails) and continues with its transmission to B.
• Terminal A is "hidden" from C and vice versa.

The methods that can be employed to solve hidden node problem are :

1. Increasing Transmitting Power From the Nodes: Increasing the transmission power of the nodes can solve the hidden node problem by allowing the cell around each node to increase in size, encompassing all of the other nodes.
2. Omnidirectional antennas: Since nodes using directional antennas are nearly invisible to nodes that are not positioned in the direction the antenna is aimed at, directional antennas should be used only for very small networks (e.g., dedicated point-to-point connections).
3. Removing obstacles: Increasing the power on mobile nodes may not work if, for example, the reason one node is hidden is that there is a concrete or steel wall preventing communication with other nodes. It is doubtful that one would be able to remove such an obstacle, but removal of the obstacle is another method of remedy for the hidden node problem. Keep these types of obstacles in mind when performing a site survey.
4. Moving the node: Another method of solving the hidden node problem is moving the nodes so that they can all hear each other. If it is found that the hidden node problem is the result of a user moving his computer to an area that is hidden from the other wireless nodes, it may be necessary to have that user move again.