- Channel sensing is
useful:
- If there are no
hidden node:
carrier sensing will avoid a collision
- If there are no
hidden node:
- Channel sensing is
not foolproof:
- Due to the
hidden node problem
- Due to the
hidden node problem
- IEEE 802.11
uses channel sensing:
- When a node
senses (detects)
an ongoing transmission,
it will not
start a transmission
Reason:
- The configuration may be
like this:
Sensing will be highly effective in avoiding collision
- The configuration may be
like this:
- When a node
senses (detects)
an ongoing transmission,
it will not
start a transmission
- Reason:
- It is technically not possible to detect collision in wireless transmissions due to the hugh difference in signal strength
- Detailed explanation:
- Signal strength
attentuates
very rapidly
in wireless transmissions
(proportional to
distance3):
- The signal level of
the node's
own transmission
is extremely high:
- because the send antenna of the node is close by to its receive antenna....
- But: the signal level of
another (colliding)
transmission
is extremely weak
- because the send antenna of the other node is far away from the node's receive antenna....
- Result:
- The node's own transmission
will overwhelm
the signal from
the other node(s)
===> Collision detection in 802.11 is not technically feasible
- The node's own transmission
will overwhelm
the signal from
the other node(s)
- Signal strength
attentuates
very rapidly
in wireless transmissions
(proportional to
distance3):