- 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
- Node C is
inside the range
of node B
- 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 !!!
- If a
node
hears an on-going transmission
(from another node):
- 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
- A node
can hears an
on-going transmission
from its
exposed node, and:
- Conclusion:
- If a node senses that the channel is busy, this fact does not guarantee that there will be collision
- 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
- Hearing an
on-going transmission will
prevent a
node from
transmitting
That's why I call it an annoyance...
- The exposed node is
not a
problem because:
- 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 !!!
- A node is
not aware of
its hidden nodes