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Topic: Hidden node problem

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	Hidden node problem - Wikipedia, the free encyclopedia
		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.
		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).
		If you have 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, you may have to force that user to move again.
	en.wikipedia.org /wiki/Hidden_node_problem (750 words)

	IEEE 802.11 Layers
		In this protocol, when a node receives a packet to be transmitted, it first listens to ensure no other node is transmitting.
		The reason for this is that when a node is transmitting it cannot hear any other node in the system which may be transmitting, since its own signal will drown out any others arriving at the node.
		This back-and-forth exchange is necessary to avoid the "hidden node" problem, illustrated in Figure 2.
	www.cs.utexas.edu /~ypraveen/surveys/wlan_security/node3.html (465 words)

	802.11 MAC layer
		The hidden node problem occurs in a point to multi-point network and is defined as being one in which three (or more nodes) are present.
		Node A, Node B and Node C. It is possible that in this case Node B can hear Node A (and vice versa) and Node B can hear Node C (and vice versa) BUT Node C cannot hear Node A.
		Hidden Nodes are solved by the use of a RTS (request to send)/CTS (clear to send) protocol prior to packet transmission.
	www.zytrax.com /tech/wireless/802_mac.htm (764 words)

	A Survey of MAC Protocols in Ad Hoc Networks
		When node A is sending a packet to B, C cannot sense the ongoing signal as it is out of the communication range of A and may try to send a packet to B as well.
		This is a typical exposed node problem, in which case a node within the range of the transmitter may be unnecessarily prohibited from accessing the medium and thus decrease the network throughput.
		According to their analysis, the problems are rooted in the fact that the 802.11 MAC protocol does not address the hidden/exposed problems as expected and some inappropriate parameter settings.
	www.utdallas.edu /~mxw013200/MAC_ADHOC.html (9435 words)

	[No title]
		The transmitting nodes do not have any sense of a problem, but their common neighbor is unable to hear either transmission because they interfere with each other.
		It is possible for two nodes that are within transmitting range of each other to transmit at the same time and have both messages arrive intact at the receiving nodes.
		The disadvantage of CSMA is that this protocol suffers from the hidden and exposed node problem.
	filebox.vt.edu /k/kaklilu/paper_bj2.doc (6728 words)

	Antenna Systems and Technology eNewsletter (Site not responding. Last check: 2007-07-08)
		Node 3 will then transmit to node 2, resulting in a devastating collision at node 2 of separate packets from both node 1 and node 3-losing both packets of data, and effectively eliminating the usefulness of the network.
		Node 2 would then send the clear-to-send (CTS) with a directional beam, which would be repeated when the data is sent from node 1 to node 2 and the acknowledgement from node 2 to node 1.
		Although the hidden node problem still exists when using an adaptive array antenna, the adaptive array's increased ability to suppress the interference results in only the loss of the interfering packet - the other packet is still correctly received.
	www.antennasonline.com /ast_newsletter_10-05.htm (5160 words)

	Network Computing \| Workshop \| Infrastructure \| Anatomy of IEEE 802.11b Wireless \| Page 2 \| August 7, 2000
		When node A wants to transmit some data to node B, it first sends an RTS packet.
		Node A must use the standard transmission method to obtain access to send the RTS packet.
		When node B replies with this CTS message, node C (and F and G) hears this response and is made aware of the potential collision, and will hold its data for the appropriate amount of time, preventing a collision.
	www.networkcomputing.com /1115/1115ws22.html (1020 words)

	Troubleshooting Wireless Networks
		For a LAN solutions these include use of RTS/CTS, adjusting the point where the wireless packets are fragmented, increasing the power used by the far nodes and decrease the power used by the nearby nodes, removing the obstacle, moving the node closer, or using a polling mechanism to control access.
		The near/far problem occurs when there are nodes near the access point that have high power settings and other nodes far from the access point with low power settings.
		Possible solutions to the near/far problem include reducing the power of the nearby nodes, increasing the power of the far-off nodes, moving the far-off nodes closer to the access point, and moving the access point to a more central location.
	www.timhogan.net /wireless-troubleshooting.htm (1408 words)

	Comments on: The 802.11 Hidden Node Problem (Site not responding. Last check: 2007-07-08)
		It doesn't see anyone else communicating on the "line", so it goes ahead and begins communications with the access point at A. This is a problem if the P node is communicating with the access point at the same time.
		In this network, N can communicate with A, and P can communicate with P, but N and P are out of range of communications with each other, or in other words, the P node is “hidden” from N, and vice versa.
		The problem arises when, in a CSMA/CA controlled network, N first checks the “line” (checks on the 2.4Ghz spectrum) to see if there is communication currently going on.
	getluky.net /2004/01/08/the-80211-hidden-node-problem/feed (313 words)

	[No title] (Site not responding. Last check: 2007-07-08)
		Since node C is jammed by the DATA sent from D to E, it can not succesfully decode node B's CTS.
		Node C does not know that B is receiving a packet, so he thinks he is free to transmit.
		Thus, if a node is listening in the DATA channel he will be `deaf' to what is going on in the control channel, and may loose state information.
	www.dcs.gla.ac.uk /~stelios/Research/Link_Layer/hidden-exposed-nodes (506 words)

	Entry problems
		Pose three problems relevant to the research at the lab.
		Perform an analysis of the masked node problem in a network where each node has enough buffer space for only 1 packet.
		Consider the analysis of the hidden node problem, as described in the paper Performance of Wireless Networks with Hidden Nodes: A Queueing-Theoretic Analysis (available at http://people.bu.edu/staro/ray_hidden.pdf).
	nislab.bu.edu /openings/problems.htm (903 words)

	802.11 Hidden Node Problem
		When such query is sent by base station all other nodes process it and postpone own transmission till the end of activity of base station.
		When a node sends transmission query the other nodes that don’t hear the query itself postpone own transmission as they hear base station acknowledgement.
		During transmission to the nodes, hidden node problem has the least influence as every node can hear base station well and can control own actions.
	aqua.comptek.ru /test/HiddenNode/hidden_node_en.html (1110 words)

	David Cottingham: Notes on Wireless Technologies
		In this protocol, nodes listen to the channel, and if it is idle for a time greater than the Distributed Interframe Space (DIFS) then the node transmits its packet.
		The time at which a node may begin tranmission after a DIFS period expires is slotted, such that the interval between slots is sufficient for a node to detect that another node has begun a transmission in a previous slot.
		The acknowledgements mechanism of the protocol involves the receiving node transmitting an ACK after a short period of time known as the Short InterFrame Space (SIFS), which is shorter than the DIFS.
	www.cl.cam.ac.uk /users/dnc25/wireless.html (1120 words)

	Techniques to Resolve 802.11 and Wireless LAN Technology in Outdoor Environments - Technologies - Security Magazine
		A common problem which occurs when 802.11 compliant equipment is used outdoors is the “hidden node” problem.
		Unfortunately, it still doesn’t address the hidden transmitter problem because it is possible for more than one node to initiate this process at the same time because they cannot “hear” each other directly.
		To avoid problems associated with pure polling schemes, TurboCell also employs a “free for all” period to enable stations that have data available but are low in the polling queue to transmit without much delay.
	www.securitymagazine.com /CDA/Articles/Technologies/57d5cbe0434d8010VgnVCM100000f932a8c0____ (1065 words)

	Andrew E. Scott - Wi-Fi Performance Tweaking
		One node (a Wi-Fi device) 'A' can be hidden from another node 'B', in the sense that it can't hear its transmissions, but a third node 'C' can hear the transmissions from both.
		When multiple nodes wish to transmit on the same channel at the same time, the algorithms in DCF result in the nodes being scheduled to transmit in generally a random order.
		These nodes also listen to transmissions where they are to be the recipient, and these will be sent to them from the access point.
	www.aes.id.au /wifitweaking.html (1112 words)

	PointToPoint - Seattle Wireless (via CobWeb/3.1 planetlab2.cs.columbia.edu) (Site not responding. Last check: 2007-07-08)
		Node A, B, and C are participating in a network.
		When node A can see B, but not node C, and node C can see node B but not node A, a hidden node condition occurs.
		Especially if Node B is routing data between Node A and Node C with an omni directional antenna, as it will require a doubling of the operation above.
	www.seattlewireless.net.cob-web.org:8888 /PointToPoint (3979 words)

	[No title] (Site not responding. Last check: 2007-07-08)
		Before node C starts the RTS it needs to defer (DIFS), backoff and then send and RTS, the backoff timer is dicrete and counted in slots of 6 microseconds each (if I remember well).
		Going back to the example, normally, node B will not reply with a CTS to A in the first place, because when it gets A's RTS, it senses the carrier and although it can't decode the data packet from D to E it can probably detect the transmission in the physical layer and defers...
		Thus, if a node is listening in the DATA channel > he will be `deaf' to what is going on in the control channel, and may loose state information.
	www.dcs.gla.ac.uk /~stelios/Research/Link_Layer/hidden-exposed-continued (937 words)

	Point-to-multipoint basic questions - NetStumbler.org WiFi Forums
		The nodes are hidden from each other, hence the name.
		But yeah, I realise that the problem that the "hidden node" effect creates is that of collision detection being very difficult, as obviously the nodes can't see each other.
		Hidden node is exactly a collision detection problem.
	www.netstumbler.org /showthread.php?p=92773 (3518 words)

	The Hidden/Visible Object Problem
		Hidden object algorithms involve (explicit or implicit) depth sorting (depth priority)
		Another factor in choosing a hidden surface algorithm is its support for other rendering effects such as transparency.
		Ray casting, scanline and the painter's algorithm all handle transparency well since all of the coverage information is known, however, due to the processing loops in the depth buffer algorithm it is harder to incorporate transparency.
	www.cs.fit.edu /~wds/classes/graphics/Hidden/hidden/hidden.html (1667 words)

	[No title]
		The problems are rated according to the application level, but feel free to answer any of the questions.
		Make sure to explain why these problems are (i) interesting, (ii) important, (iii) relevant to the lab’s research. Provide credible plans of attack to two of the problems in the PhD category.
		Consider the analysis of the hidden node problem, as described in the paper Performance of Wireless Networks with Hidden Nodes: A Queueing-Theoretic Analysis (available at HYPERLINK "http://people.bu.edu/staro/ray_hidden.pdf" http://people.bu.edu/staro/ray_hidden.pdf).
	nislab.bu.edu /openings/problems.doc (677 words)

	IEEE 802.11 RTS/CTS - Wikipedia, the free encyclopedia
		Any other node receiving the CTS frame should refrain from sending data for a given time (solving the hidden node problem).
		The amount of time the node should wait before trying to get access to the medium is included in both the RTS and the CTS frame.
		Any other node receiving the RTS frame but not the CTS frame is permitted to transmit to other neighboring nodes (solving the exposed node problem).
	en.wikipedia.org /wiki/IEEE_802.11_RTS/CTS (341 words)

	Page 4 - Introduction to Wireless
		As shown, node A can hear node B, but it cannot hear node C. Node C can also hear node B, but it cannot hear node A. Because 802.11 is a shared-access physical medium, only one device can transmit at any given time.
		The Hidden Node problem is that node A and node C cannot hear each other, and neither node will detect a collision.
		Hidden Node issues reduce throughput in this example by at least 50%.
	www.devshed.com /c/a/Practices/Introduction-to-Wireless/3 (1403 words)

	RE: [manet] Hidden- & exposed terminal problem in MANET (via CobWeb/3.1 planetlab2.cs.umd.edu) (Site not responding. Last check: 2007-07-08)
		If there are several nodes along a line, as shown in the original e-mail question and answer, then the hidden node problem still exists.
		If you expand the problem to a 2D plane, then the probability, even with a physical beam width of a degree or two, is greatly reduced and reduces still further when you expand to a 3D volume.
		The hidden node problem was demonstrated with a simple chain.
	www1.ietf.org.cob-web.org:8888 /mail-archive/web/manet/current/msg01022.html (680 words)

	ProNet Systems Inc - TECH TALK (via CobWeb/3.1 planetlab2.cs.umd.edu) (Site not responding. Last check: 2007-07-08)
		Unfortunately, there are many problems associated with using the 802.11 technology outdoors and well intended WLAN resellers are quick to recommend its deployment outdoors to clients and this despite a lack of understanding of its limitations in such environments.
		When this happens, multiple nodes will attempt to transmit their data over the shared medium simultaneoulsy, causing signal interference with one another.
		TurboCell resolves the ‘Hidden Node’ problem by centralizing control of the wireless network at the TurboCell access point or base station.
	www.pronetsystemsonline.com.cob-web.org:8888 /tech_talk.php (1513 words)

	Free Press : Open-source mesh group releases software, discusses social goals
		(In a hub-and-spoke network, hidden nodes can see the hub not other spokes and can disrupt other network traffic by improperly sending at times when other nodes are transmitting resulting in interference and back-off behavior that reduces network performance.
		The software release by CUWiN of a CD-ROM image containing bootable node software along with the developer’s resource (distributed under a BSD license with plans to move to a GPL license) is part of their effort to bring in more programming aid on the project.
		“One of the problems that we see a lot is that people develop really cool hardware that’s far beyond the means of people on the ground where this equipment is supposed to be used.” The group hopes to have the initial documentation complete in six months.
	www.freepress.net /news/3315 (1069 words)

	Patras Wireless Network
		You have to configure all the nodes to work in AD-HOC mode, using the same channel and essid.
		Say that one of the nodes is going to be the AP (possibly the one that everyone can have line-of-site with).
		This scenario allows communications with hidden nodes, since AP performs lan emulation.
	patraswireless.net /software.html (531 words)

	NodeFremont - Seattle Wireless
		I have a few thoughts about my experiment with wireless hardware that I thought I would share: I believe the point to multipoint (hidden node) problem was the show stopper which prevented my node from having the utility we needed from the network to make it something that would scale.
		At one time there were more people that wanted to peer with my node than channel space would allow.
		The open-source community has not come with a workable solution to the hidden node problem - until it does, SeattleWireless will not be able to grow in the way it was first envisioned.
	www.seattlewireless.net /index.cgi/NodeFremont (503 words)

	Patras Wireless Network - Wikipedia, the free encyclopedia
		Claim to fame for PWN was that members on the network solved the hidden node problem, by designing and implementing the Wireless Central Coordinated Protocol (WiCCP), a protocol booster for 802.11b DCF based wireless networks, that provides cyclic token-passing medium access, and scheduled allocation of the available network resources, eliminating the hidden node problem.
		This topology is severely flawed, mainly due to the hidden node problem, so the network moved to a bus-ring topology, providing more reliability to the users.
		One of the first and most promising projects is the design and implementation of the Wireless Central Coordinated Protocol (WiCCP), a protocol booster for 802.11b DCF based wireless networks, that provides cyclic token-passing medium access, and scheduled allocation of the available network resources, eliminating the hidden node problem.
	en.wikipedia.org /wiki/Patras_Wireless_Network (858 words)

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