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Broadband-Hamnet™ Forum :: General |
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Subject :Question about antenna diversity and directional antennas..
2013-06-25- 07:19:24
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| KB9YOJ |
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Joined: 2013-06-23- 06:15:32
Posts: 6
Location: Bloomington, IN |
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Hello- I am new to the message board but am a professional in the wireless networking world. I have been studying the equipment files and have noticed that the most popular "wireless routers" are older b/g routers with 2 antennas. These antennas, when used as an access point(not mesh), were designed to utilize receiver "antenna diversity" and fight the effects of multi-path by selecting the strongest signal at either of the 2 antennas, but not both. Here is my question: Does the HSSM-MESH firmware and OLSR algorithms still use antenna diversity, or is this function disabled? Is it native to the AP and not changed by the firmware update? It makes me wonder if using high gain directional antennas is useful from a design perspective. Maybe high-gain verticals are a better choice? I have many more questions, but I will post those later.
Matt KB9YOJ
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Subject :Re:Question about antenna diversity and directional antennas..
2013-06-25- 07:53:34
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| K5KTF |
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Joined: 2010-01-18- 23:04:04
Posts: 266
Location: 5' from this webserver |
  
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Hi Matt, If you look under the Basic Setup page, (where the IPs and such are set), you can see and set the antennas to either Right, Left, or Diversity (default). Actually, on some models. I have seen where the left and right settings dont really seem to do anything, so I leave all mine on Diversity, and let the router figure out which antenna to use. If you look under Applications - Tower Cam over Mesh, you will see I have 1 dish and 1 omni, and we use that config alot, the dish for a long haul and the omni for close-in local connectivity. Jim K5KTF
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 Jim K5KTF EM10bm Cedar Park, TX  |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-06-25- 09:52:04
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| KB9YOJ |
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Joined: 2013-06-23- 06:15:32
Posts: 6
Location: Bloomington, IN |
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Jim- Thanks for the input. I will keep that in mind when I set up something here. When you talk about local connectivity, you are speaking of the mesh link and not local access via a laptop? I assumed that the PC-to-router link is via the L2 switch connections.
Matt KB9YOJ
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Subject :Re:Question about antenna diversity and directional antennas..
2013-06-25- 11:16:19
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| K5KTF |
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Joined: 2010-01-18- 23:04:04
Posts: 266
Location: 5' from this webserver |
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Bu Local I mean within a block or two. TO access via other Wifi devices, you should setup a Wifi AP and connect it to a LAN port on the node. |
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Jim K5KTF EM10bm Cedar Park, TX |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 15:37:20
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| va3srv |
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Joined: 2012-10-30- 14:19:01
Posts: 9
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A valid question has been asked for the appropriate understanding of what we think the hardware is actually doing. So, in a regular wireless router, the antennas switch back and forth until one antenna shows a good/higher signal. When it's time to transmit, the last antenna used for Rx is then used. Is this still valid? The follow up question....if this switching occurs, what is the merit in setting up "hilltop" stations with one dipole and one yagi when only one antenna seems to do all the work. |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 15:54:59
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| KD5MFW |
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Joined: 2010-01-18- 23:02:11
Posts: 104
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You seem to understand the basics of diversity antennas. In your question, do you intend to say a directional and an omnidirctional antenna? With yaggi as an example of a directional antenna? And "dipole" as an omnidirectional? Dipole makes no sense, to me, in your question. Could you please restate your question? In general, conditions with microwave RF, reflections can change very rapidly. This was the original reason for spending money on a cost reduced consumer device to include more than one antenna. On a hill top node, the directional antenna tends to handle triffic to a distant node and the omni "illuminates" the area near the mesh node with link coverage. -Glenn, KD5MFW |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 16:01:07
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| va3srv |
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Joined: 2012-10-30- 14:19:01
Posts: 9
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Sorry....wrong terms.
I'm thinking of the last QST featuring MESH.
A sends to B sends to C.
Each station has two antennas each.
B is being used as a link between A and C (less like a mesh, more like an extended wi-fi). If B has two directional antennas pointing at each of the other stations (say 180deg apart for sake of argument)...how does the data pass?
Does A send to B(ant1) which then buffers and looks for C on ant1 first before switching to ant2 to retransmit? |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 16:21:09
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| KD5MFW |
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Joined: 2010-01-18- 23:02:11
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The easiest way to view the operation is that the radio talks to another radio on the antenna on which it last received the best siganl. TX adnd RX switch very rapidly so the check is done many times a second. If you point antennas at 180 degrees apart, say one aimed east and the other west, and you use highly directional antennas, if one radio consistently comes in on the east antenna to a node, that node will tend to use that same antenna to transmit back to that node. And if there is a node to the west, the node to the west will tend to talk to the west facing antenna. The data is buffered in RAM as it is received and spooled out of RAM during transmit. Diversity was intented to help with a real problem of microwave bouncing around in a house or office like a LASER pointer in a house of mirrors. People walking around drastically change the avaiable paths. But diversity antennas has the effect described above with no hardware or software changes at all. Also the concept of a mesh is abstract. You can have 5 close nodes for example and one node far away and they are still technically in the same mesh if they are linked. The mesh software does not care if the nodes are all close - only if they can link via RF, so you get into a more complex and accurate model of a mesh, that is beyond a simplistic view of all nodes being the same distance apart. Hope this helps. Diverstiy is usually the way to go, as it dynamically figures out the best antenna to use. Even if you have directional antennas and you - a big bag of microwave blocking water, step in front of the directional antenna, the omini might be able to get a better link until you move. -Glenn KD5mFW |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 16:31:46
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| va3srv |
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Joined: 2012-10-30- 14:19:01
Posts: 9
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OK....I figured that the routers has a single transmitter and it couldn't send and receive at the same time, does it transmit out both antennas at once though? but selects the receive antenna with the higher SNR?
If I used a mesh node as a "relay" (only two neighbors, who can't see each other) I wasn't sure what was happening with how the antennas behaved.
You say that the antenna last used for receive will transmit next...I'm unsure what you mean by: "point antennas at 180 degrees apart, say one aimed east and the other west, and you use highly directional antennas, if one radio consistently comes in on the east antenna to a node, that node will tend to use that same antenna to transmit back to that node".
Do you mean by appropriate RF SNR determining or by smarts in the router in ID'ing a node on antx? |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 16:54:35
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| KD5MFW |
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Joined: 2010-01-18- 23:02:11
Posts: 104
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Let's try this example. You have 3 nodes in this example. One is in the east valley (relative to the hill top node). There is a hill top node and there is a node in the west valley, relative to the hill top node. The node on the hill top can see the nodes in each valley. The node in each valley can only see the hill top node. The hill top node has two directional antennas operating in diversity mode. One is aimed at the node in the east valley and the other antenna is aimed at the node in the west valley. The hill top node will both transmit and recieve to the east valley on the its east antenna. The hill top node will both transmit and receive to the west node valley on its west aimed antenna. A packet is sent from the east vally to the hill top node and arrives at the hill top node on the antenna it has aimed to the east. The data goes into RAM in the hill top mesh node. The hill top mesh node then transmits an ACK out its east facing antenna to let the node in the east valley know the packet was received correctly. Then the hill top node will transmit the data, sitting in its RAM to the node in the west valley using its west facing antenna. The node in the west valley will receive the packet relayed via the hill top node, and transmit an ACK that the packet was properly received to the hill top node. The hill top node will receive this ACK on its west facing antenna. And a packet has been relayed from the east valley node to the west vally node. This all assumes that there was a packet that needed to go from the east node to the west node for the sake of discussing diversity. In our current system the nodes do not transmit and receive at the same time. As we move to other hardware, the systems may tranmsit and receive at the same time, using multiple antennas on multiple bands but that is not the story today. I hope this is helpful. If not, you might want to do a Google search for"wi-fi diversity mode antennas" and get some drawings and timing diagrams that are more clear than my text. -Glenn, KD5MFW |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 17:03:49
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| va3srv |
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Joined: 2012-10-30- 14:19:01
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Glenn, thanks for the time on this.
This is how I "presumed" things worked, but I was challenged that it didn't actually work this way and I'm trying to source what is actually happening.
So, from your example....are the node "paths" determined through "beacon" packets in that the Hilltop node knows that antenna1 has the best signal to the Eastern node; and alternatively for the western node.
Now, doing this ignores the real idea for diversity..which is multi-path/scatter resilience....but it seems like a common thing to do. |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 17:29:37
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| KD5MFW |
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Joined: 2010-01-18- 23:02:11
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First it can be irritating to some that the example we just went through does, in fact, work. I have done it many times, and it has even been documented in the past in a QST - The article was something like "HSMM expiriments in the Shenendoh Valley" They have a photo and a description of exactly of the example we just covered - But that is not what it is for! Some will say - well yes it was to deal with nasty reflected signals, but it does, in deed, work as in our example.
Now on to the selecting a path for packets - that is seperate from the diversity antenna consideration - one does not know of the other. They both are active at the same time. The Optimum Link State Routing (OLSR) software in the mesh nodes is what makes them opperate very, very differently than the facotry software. So called "Hello" packets are sent out by the OLSR software in the mesh node and it collects answers from all who answer and builds an updated routing table every 5 seconds or so. That provides for auto linking and dynamic auto rerouting of packets, if there is an alternate path in the current mesh, and a node dies. This is where the more mesh nodes the better. Say we have 20 nodes and we somehow have packets that are being transfered from node 7 as a source and say, node 19 as a destination. The quality of the signals from each node to each other node is kept by the OLSR software. So it picks the "best" path currently available. If one of the relay nodes used in the path dies, during the data transfer, that is no longer the "best" path and the next best path, now becomes the "best" path and the packets are automatically rerouted through whatever path seems currently best, between the source and destination mesh nodes. It is instructive to have test gear, start a long transfer and turn on and off mesh nodes and watch the OLSR code reroute the data through various paths until the transfer is complete or there is no path currently available. It is a very slick and dynamic system. If you check out the OLSR link on the home page, you can get deep, deep, deep into what OLSR is doing and how it figures the current quality of the paths to all nodes currently in the mesh. (updated about every 5 seconds) If you have a lot of nodes, you can start a data transfer and have a lot of nodes come and go and the data will get through the little "hero" mesh nodes that fight hard to get your data passed. And, OLSR is not the only such auto link, auto reroute mesh software. It is what we use in our firmware. B.A.T.M.A.N and at least a dozen others do roughly similar things, but all have different ways to do things like how to determine the "optimum" path. But for fast to deploy, robust, cheap, cheap, cheap radios, mesh systems are very good for emergency communications. Check the HSMM documents link on the front page to find a list of presentations that have drawings and photos of a lot of stuff that is hard to describe in words only. Thanks for your interest, the mesh is very different than plain wi-fi. Also down load a free copy of the mesh book at wndw.net It is super. -Glenn, KD5MFW |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 17:34:55
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| va3srv |
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Joined: 2012-10-30- 14:19:01
Posts: 9
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Glenn, if we meet, I owe you a donut!
I hit the OSLR wiki just before reading this and got the answer I was having a hard time figuring the question to....
So, there is pre-mapping done and this is what can allow a router to operate two directional antennas in two different directions though they are no longer optimally set up for multi-scatter correction.
This has worked for us...but I couldn't explain away the comments.
Cheers
Steve
VA3SRV |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-21- 17:54:26
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| KD5MFW |
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Joined: 2010-01-18- 23:02:11
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The unintended effects of the diversity mode antennas is a handy featuer. The diversity mode is independent of the OLSR quality calculation. OLSR does not know what antenna was used. It got a reading not knowing what antenna was used. The diverstity mode was doing its thing, without regard to what the rest of the system does with the data. So the example was of the most simple case. I have been operating and had packets reflected off aircraft and thus find an alternate path for a fraction of as second. One of our test ranges of 10 miles is along a path where large airliners come into Austin to land. I would sometimes notice that the signal strength (usually actually the S/N ratio) improve dramatically for one or two samples - to a level that made it look like a far away node was only a few feet away. Watching long enough, I noted aircraft in the path to that node when the signal got "hot" for a second. Do you think the guys that designed the diversity mode stuff intended to deal with aircraft in the path? ;-) (It is a reflected signal!) Use what works, test it until you can reliably reproduce the results then share the "free" feature with others. -Glenn, KD5MFW |
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Subject :Re:Question about antenna diversity and directional antennas..
2013-08-23- 05:19:23
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| KG1L |
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Joined: 2013-06-28- 12:53:53
Posts: 18
Location: Owings, MD |
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I have considered the possibility of actually building aircraft reflections into the the mesh plan. Here in southern Maryland, terrain and forests conspire to limit line-of-sight. However, there are two major airports and a joint Air Force/Navy airbase and a Naval Air station nearby. There are almost always aircraft visible somewhere.
Two mesh nodes (hidden from each other) point directional antennas, not toward each other, but in the direction of one of the airports. Back scatter from several airplanes, a few seconds at a time from each might be enough to support a connection.
I have not been able to test this, yet. Has anyone else tried this? |
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