Hidden node problem
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In wireless networking, the hidden node problem occurs when a node is visible from a wireless access point (AP), but not from other nodes communicating with said AP. This leads to difficulties in media access control.
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[edit] Background
Hidden nodes in a wireless network refer to nodes that are out of range of other nodes or a collection of nodes. Take a physical star topology with an access point with many nodes surrounding it in a circular fashion; each node is within communication range of the AP, however, not each node can communicate with, or has line of sight with each other. For example, it is likely that the node at the far edge of the circle can see the access point, which is known as r, but it is unlikely that the same node can see a node on the opposite end of the circle, 2r (or simply the diameter). These nodes are known as hidden. The problem is when node r and r2 start to send packets simultaneously to the access point. Since node r and r2 can not sense the carrier, Carrier sense multiple access with collision avoidance (CSMA/CA) does not work. To overcome this problem, handshaking is implemented in conjunction with the CSMA/CA scheme. The same problem exists in a MANET.
The hidden node problem can be observed easily in widespread (>50m radius) WLAN setups with many nodes that use directional antennas and have high upload. This is why IEEE 802.11 is suited for bridging the last mile for broadband access only to a very limited extent. Newer standards such as WiMAX assign time slots to individual stations, thus preventing multiple nodes from sending simultaneously and ensuring fairness even in over-subscription scenarios.
IEEE 802.11 uses 802.11 RTS/CTS acknowledgment and handshake packets to partly overcome the hidden node problem. RTS/CTS is not a complete solution and may decrease throughput even further, but adaptive acknowledgments from the base station can help too.
The other methods that can be employed to solve hidden node problem are :
- Increase power to the nodes
- Use omni directional antennas
- Remove obstacles
- Move the node
- Use protocol enhancement software
- Using Space Diversity
[edit] Increase Power to the Nodes
Increasing the power (measured in milliwatts) of the nodes can solve the hidden node problem by allowing the cell around each node to increase in size, encompassing all of the other nodes. This configuration enables the non-hidden nodes to detect, or hear, the hidden node. If the non-hidden nodes can hear the hidden node, the hidden node is no longer hidden. Because wireless LANs use the CSMA/CA protocol, nodes will wait their turn before communicating with the access point.
[edit] Use omni directional antennas
Since nodes using directional antennas are nearly invisible to nodes that are not positioned in the direction the antenna is aimed at, directional antennas should be used only for very small networks (e.g., dedicated point-to-point connections). Use omni directional antennas for widespread networks consisting of more than two nodes.
[edit] Remove Obstacles
Increasing the power on mobile nodes may not work if, for example, the reason one node is hidden is that there is a cement or steel wall preventing communication with other nodes. It is doubtful that one would be able to remove such an obstacle, but removal of the obstacle is another method of remedy for the hidden node problem. Keep these types of obstacles in mind when performing a site survey.
[edit] Move the Node
Another method of solving the hidden node problem is moving the nodes so that they can all hear each other. If it is found that the hidden node problem is the result of a user moving his computer to an area that is hidden from the other wireless nodes, it may be necessary to have that user move again. The alternative to forcing users to move is extending the wireless LAN to add proper coverage to the hidden area, perhaps using additional access points.
[edit] Equalizing Technology
Equalizing technology, which is completely compatible with 802.11, works by taking advantage of the natural inclination of Internet connections to back off when artificially restrained.
Equalizing constantly (every second) measures the total aggregate bandwidth throughput traversing the AP. If it senses the upper limit is being reached, Equalizing will then isolate the dominating flows and encourage them to back off by artificially restraining them. Thus freeing up the frequency for lesser powered remote nodes.
By keeping track of every flow going through the AP, Equalizing technology can make a determination of which ones are getting an unequal share of bandwidth and thus crowding out flows from weaker nodes.
Equalizing determines detrimental flows from normal ones by taking the following questions into consideration:
- How persistent is the flow?
- How many active flows are there?
- How long has the flow been active?
- How much total congestion is currently on the trunk?
- How much bandwidth is the flow using relative to the link size?
The key to making this happen over 802.11 relies on the fact that if you slow a stream down, the application at the root cause will back off and also slow down. This can be done by the deploying equalzing technology after the access point without any changes to the 802.11 protocol since the throttling is actually done independent of the radio. The throttling of heavy streams happens between the AP and the connection to the Internet (or other external source).
Traffic Equalizing technologies are not universally applicable solutions to the hidden node problem. Rather, they are primarily a pragmatic fix to reduce symptoms without fixing the underlying problem.
[edit] Use protocol enhancement software
There are several software implementations of additional protocols that essentially implement a polling or token passing strategy. Then, a master (typically the access point) dynamically polls clients for data. Clients are not allowed to send data without the master's invitation. This eliminates the hidden node problem at the cost of increased latency and less maximum throughput.
