- 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 !!!
- A hidden node = a
node
that is outside your range and
interferes with
your transmissions
- 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 !!!)
- Node A is
outside the range
of node C
- 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 !!!
- C
cannot hear
A's transission
- The transmissions of
a node can
collide with
transmissions from
hidden nodes
- 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
- If a node
senses that the
channel is
clear,
this fact
does not guarantee
that there will not be
a collision
- 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 !!!
- The hidden node problem is
not always
present in a
wireless network !!!