Hidden node problem

In wireless networking, the hidden node problem occurs when a Node (networking)
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.

Background

Hidden nodes in a wireless network refer to nodes that are outof range of other nodes or a collection of nodes. Take a physical
star topology with an access point with many nodes surrounding itin 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 accesspoint, which is known as r, but it is unlikely that the samenode 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.

RTS/CTS benchmarks [Pommer, Hermann: "Roaming zwischen Wireless Local Area Networks". VDM Verlag, Saarbrücken 2008, ISBN 978-3-8364-8708-5.] : The comparison with hidden stations shows that RTS/CTS packages in each traffic class are profitable (even with short audio frames, which cause a high overhead on RTS/CTS frames).

In the experimental environment following traffic classes are included: data (not time critical), data (time critical), video, audio. Examples for notations: (0|0|0|2) means 2 audio stations; (1|1|2|0) means 1 data station (not time critical), 1 data station (time critical), 2 video stations.

The other methods that can be employed to solve hidden node problem are :
*Increase power to the nodes"'
*Use omnidirectional antennas"'
*Remove obstacles"'
*Move the node"'
*Use protocol enhancement software"'
*Using Space Diversity"'

Increase power to the nodes

Increasing the power (measured in milliwatts) of the nodes can solve the hidden nodeproblem by allowing the cell around each node to increase in size, encompassing all ofthe other nodes. This configuration enables the non-hidden nodes to detect, or hear, thehidden node. If the non-hidden nodes can hear the hidden node, the hidden node is nolonger hidden. Because wireless LANs use the CSMA/CA protocol, nodes will wait theirturn before communicating with the access point.

Use omnidirectional 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 omnidirectional antennas for widespread networks consisting of more than two nodes.

Remove obstacles

Increasing the power on mobile nodes may not work if, for example, the reason onenode is hidden is that there is a cement or steel wall preventing communication with othernodes. It is doubtful that one would be able to remove such an obstacle, but removal ofthe obstacle is another method of remedy for the hidden node problem. Keep these typesof obstacles in mind when performing a site survey.

Move the node

Another method of solving the hidden node problem is moving the nodes so that they canall hear each other. If it is found that the hidden node problem is the result of a usermoving his computer to an area that is hidden from the other wireless nodes, it maybe necessary to have that user move again. The alternative to forcing users to move isextending the wireless LAN to add proper coverage to the hidden area, perhaps usingadditional access points.

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.

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.

See also

* Wireless LAN
* Exposed terminal problem
* Point Coordination Function
* Hybrid Coordination Function

References

External links

* [http://www.netequalizer.com/Hidden_Node_White_Paper.php AirEqualizer, an equalizing hidden node solution]
* [http://patraswireless.net/software.html Wireless Central Coordinated Protocol (WiCCP), a software solution of the hidden node problem]
* [http://www.extremetech.com/article2/0,1697,2170789,00.asp Equalizing Hidden node solution compatible with 802.11]
* [http://frottle.sourceforge.net/ Frottle, a client/server software solution]
* [http://aqua.comptek.ru/test/HiddenNode/hidden_node_en.html Benchmarks comparing pure CSMA/CA with RTS/CTS and Polling]