CS485 Sylabus

### The exposed node annoyance (not so much a "problem")

• Exposed Node

• Exposed node:

 Exposed node = a node that is within your range but does not cause a collision

• Example: (exposed node)

• Node C is inside the range of node B

But:

 Node C does not interfere with the transmissions from Node B

• Review: sense to avoid collision

• Recall: collision avoidance

• If a node hears an on-going transmission (from another node):

 the node must delay from transmitting

because:

 The transmission will collide in the on-going transmission

Example:

• Node C is currently transmitting to node A:

• Node B can hear the transmission

 If node B starts transmitting, the transmission will collide with node C's transmission !!!

• The Exposed node "annoyance"

• Exposed node "problem" (annoyance):

• A node can hears an on-going transmission from its exposed node, and:

 This on-going transmission (from the exposed node) will not collide with its own transmission

Example: (exposed node annoyance)

• Suppose Node B is currently transmitting to Node A:

Node C will hear B's transmission

• If node C would transmit to Node D:

There will be no collisions:

 B's signal can not reach node D to cause a collision !!!

Remember:

 A collision is caused by multiple signal reaching the same destination

• Conclusion:

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

• Post script

• The exposed node is not a problem:

• The exposed node is not a problem because:

• Hearing an on-going transmission will prevent a node from transmitting

 It is losing some opportunity to transmit

That's why I call it an annoyance...

• The hidden node is a real problem:

 A node is not aware of its hidden nodes The node's transmissions can be lost due to collisions with those from its hidden nodes !!!