CS485 Sylabus

### The hidden node problem

• "Hidden" Node

• Hidden node:

• A hidden node = a node that is outside your range and interferes with your transmissions

Note:

 Because the node is outside your range, you cannot be aware of the hidden node !!!

• Example: (hidden node)

• Node A is outside the range of node C

Node A can interfere with the transmissions from Node C (they have overlapping reach area !!!)

• The "Hidden" node problem

• The hidden node problem:

• The transmissions of a node can collide with transmissions from hidden nodes

 These collisions are undetectable because a node can not hear the transmissions from its hidden nodes !!

Example: (hidden node problem)

• Suppose: Node C wants to send a message to B:

Use channel sensing:

 Node C senses that the channel and find that it is clear (no transmission) ....

• When node C transmits to node B, it can result in a collision:

Observe that:

 C cannot hear A's transission But, when node C does transmit, it will cause an collision at node B !!!

• Conclusion:

• If a node senses that the channel is clear, this fact does not guarantee that there will not be a collision

 due to the hidden node problem

• Comment

• Fact:

• The hidden node problem is not always present in a wireless network !!!

Example:

• Carrier sensing will avoid collision if there are no hidden nodes !!!