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70cm Backbone

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va3qr
va3qr's picture
70cm Backbone
Ok, here's a weird one for you (or maybe not).

I can not get a stable 2.4GHz or 900MHz link to the MESH in my area - we've tried everything. However, I just came back from a DXPedition with 2 x Micrktik RB411s with Xaygl 70cm WiFi modules in them. I am certain that I can get a link to the remote site via UHF, but I'm unsure how to configure this, and how to setup the AREDN nodes at either end of the bridge to see across the bridge. I've dumped the Mikrotik O/S iin favour of OpenWRT Barrier Breaker. All I want to use the Mikrotiks for is to build a UHF backbone to get me to the MESH.

Has this been done before?

Thanks! VA3QR.
KG6JEI
Not sure I would try using
Not sure I would try using them as backbones, I can't speak to bandwidth limitation where you are at, but here in the states it's restricted to miserably slow on 70cm so it would never make a good backbone.

Perhaps however using them as transparent bridges, plug them into a mesh node somewhere and bring the LAN out via them so you can access from your home PC on 70cm to the LAN port of a remote mesh note which then links into the network.

I've seen this done with a 1.2Ghz DSTAR radio before, but even that was a bit painful at 128kb/s vs mesh direct but proved it could be done.

You wouldn't want to try and bring the DTDLink across as its not tolerant to packet loss and would very quickly eat up any 70cm bandwidth, in addition it would register as a "perfect" link potentially pulling traffic off 100mbps links to a painful 70cm link.
va3qr
va3qr's picture
Bandwidth
These are WiFi modules designed to work in the 70cm band. 5MHz b/w gives me 5-15Mbps over the air. The idea being to put it down at a centre freq. of 427.5MHz and use it to get me past the terrain to the main network.
Ve3xnc
 

 

I have played with these cards using openwrt. If you need help configureing openwrt send me your email address

Brad
 
va3qr
va3qr's picture
E-Mail
My address is va3kpj at gmail dot com. Thanks!
KE2N
KE2N's picture
70 cm

I have done some experimenting with both the Doodle labs cards (no longer made) and Xagyl cards.  My plan was to use them as a tree-penetrating last-mile link. You run these at 5 MHz bandwidth (SISO of course) and probably at less than max encoding, so the throughput is not that great. But it is just enough to stream SD video (in one hop). The main problem I ran into is that the Xagyl card has its maximum sensitivity around 443 MHz right where US repeaters are located, rather than down at 425 MHz where broadband/ATV activity is located. I found that a carrier signal of -70 dBm (at 443 MHz) is enough to cause the reported noise floor to rise (and OFDM signal to drop). Around here, if you poke an antenna up in the air, there will be plenty of signals that strong, in that frequency range.  When the card gets hit with a signal stronger than -70 it tends to drop the link until about 1 minute after the problem signal goes away.  None-the-less, I was able to test a 6 mile hilltop-to ground level link with only occasional drop-outs (using Doodle Labs).  My plan is to use 10 MHz BW helical resonator filters on each end to deal with this but I have been busy with other things recently ...

The filters I am using are from ocicom.com - these are not cheap but about half the cost of the dci.ca filters.  OCI will tune the filter to any center frequency you need. A 10 MHz filter allows a 5 MHz central region with reasonable group phase delay characteristics.

 

AE6XE
AE6XE's picture
What's the environment
What's the environment characteristics that prevent 2.4Ghz or 900Mh from working?    The ground is in the way?   Trees everywhere?   interfere noise?  distance? 
va3qr
va3qr's picture
Terrain
I'm just skipping over the tops of the trees, and the area where the main node is at is a little RF-noisy. During the winter, I can maintain an ok link at 2.4GHz, but when I tried to get connected this summer at 2.4GHz or even 900MHz, I could see nothing. It's 22km as the crow flies from point to point. The cards put out 1W at max, and I'm using small 5-element Cushcraft beams at about 9.5dB gain.
AE6XE
AE6XE's picture
If it were me,  I'd try to
If it were me,  I'd try to strap a couple 5Ghz NanoBridges with 22dBi gain antennas (about 1' diameter) on to a pole at the top of the tree and extend it up another few feet if possible.   If there's sway in the tree, use 16dBi Nanosations with a wide beam width.    These are relatively light weight and inexpensive.   The higher gain and extra height should just do the trick.    

I don't think 9.5dBi gain is sufficient to maintain a good link at that distance regardless of the trees.   We have some local-loco folks attempted to use NanoStation loco 2.4GHz devices.  These have a gain of 8dBi and advertised by Ubiquiti for 5+ miles.  They had completely clear line of sight to a mountain spot and not a usable link. 

Joe AE6XE
va3qr
va3qr's picture
.... the idea is to use the
.... the idea is to use the Mikrotiks/Xagyls to create a backbone between 2 AREDN units - I just want an invisible path. Maybe it's as easy as setting up OLSRD o the Mikrotiks and putting them in the network?
AE6XE
AE6XE's picture
Many hardware WISP-wifi
Many hardware WISP-wifi devices can work, when put in bridge mode, to run AREDN over the top.  It looks like a cat5 cable and if the devices can be configured to carry vlan 2 tags, for DtDlink to work,  then no need for an extra 802.1q switch and connect the devices directly together.   But, whatever the device, I think you'll need at least a 15+ dBi gain antenna to work at that distance.  Most (WISP type devices) do nearly 1W to stay within part 15 limits.    The more ambient noise to deal with, the higher the gain antenna you'll need. 

If you do this DtDlink mode over an RF bridge, there is a setting to make OLSR more stable to set.  This is something we're looking at putting in the next release as an option.

Joe AE6XE
va3qr
va3qr's picture
Ok, I guess I was just
Ok, I guess I was just overthinking it. I put them in WDS bridge mode and plugged them into a VLAN2 port on each side of a test-bed network in the shack and it acts as a nice invisible bridge in DtD mode. As for antennas, I'mgoing to try the ones I have and see if they'll work. If not, I'll get some higher-gain ones. 

What OLSR settings should I be changing?  Thanks!
AE6XE
AE6XE's picture
DtDlink is meant to be on a
DtDlink is meant to be on a cat5 cable.  Consequently olsr has a setting expecting this and treats it differently.   The threshold to drop a cat5 link from being a routing option occurs with something like 10% loss--can't remember the exact value.  An RF link is not dropped until it is 70% loss.    You don't want the link to bounce up and down with a different thresholds than your other RF links.  

Edit these 2 files and reboot:

/etc/config/olsrd
/etc/config.mesh/olsrd

Comment out "#" this line:

config Interface 
        list interface 'dtdlink'
#       option Mode 'ether'

What throughput and link rate do you get on 70cm, particularly at 20+ km?  (KE2N, did you do any iperf measures with your teseing?)   Given people transmitting a lot of high power signals in UHF lower freq band, I'd be interested in your results.     I see that the devices you are using are also Atheros based and doing 802.11g protocols.    A couple 5Ghz Nanobridges go on ebay for about $50 per unit and you'd get 35+Mbps links at that distance to compare with.

Joe AE6XE

 
va3qr
va3qr's picture
Well part of the reason for
Well part of the reason for using the 70cm gear is that it was lying around from another event. If it doesn't work, I might try to 5GHz stuff.

At 5MHz b/w, it probably maxes out at 18Mbps. I'll let you know what we find when we get it all mounted. 

Out of curiosity, do I make that OLSR config change on the AREDN units on both ends of the bridge?

Thanks for the help!
AE6XE
AE6XE's picture
At that distance the rate
At that distance the rate will be less than half the max link rate in 802.11g of 13.5mbps using 5MHz channel, but that's a guess based on Microwave examples.  The actual throughput of data less than half of that.  70cm band, even less.  other high power repeaters and simplex users in the area, even less and up/down...  

The OLSR setting is on both ends, each node makes independent decisions to route and determine link up/down thresholds.   I should add that one would never want to use this setting on a cat5 or normal DtDLink usage.  So don't forget to undo it if repurposing the node back.    Press the rest button for 15 seconds to move the firmware back to firstboot if any doubt.

Joe AE6XE 
K5DLQ
K5DLQ's picture
to clarify one thing: 
to clarify one thing: 
After a FULL power up... Press the reset button for 15 seconds to move the firmware back to firstboot, if any doubt.
va3qr
va3qr's picture
Well I'm just about to go up
Well I'm just about to go up the tower and mount my end, and I think VE3CWU is coming by to pickup the other end today, so I guess we'll find out soon how well this works, if at all. I'm tuned to 432.5MHz centre at 5MHz bandwidth. That end of 70cm is pretty quiet around here, but we'll see how noisy it is in Brampton.

Will keep you posted!
KE2N
KE2N's picture
coordination

not sure what level of activity you guys have up there, but 432.1 the weak-signal calling frequency on 70 cm.  With a 432.5 center frequency you are going to put a whole lot of OFDM modulated carriers all around, and on top of, the calling frequency. It may take a while for your ham neighbors to find you - but you might expect to see people with torches and pitchforks showing up in your front yard one evening ,,, 

K6AH
K6AH's picture
And leave AREDN out of it...

Being a VHF/UHF contester with a fair bit of experience in that part of the 70 cm band, the weak signal guys are listening in the -120 dB range and below. Your radio will require a signal several hundred times greater to successfully decode OFDM.

I'd ask that, if pursue that segment of the band, you not use the AREDN project name in the context of this experiment... or I'll likely have my contest friends visiting me with torches and pitchforks ;-)

Andre, K6AH
 

va3qr
va3qr's picture
There are very few VHF/UHF
There are very few VHF/UHF guys around and between where the 2 sites are. I've already let certain people know what I'm doing in the event that someone hears some complaining. I might go up the band and do some testing. 

The guys in Niagara (who are linked to the Peel MESH) are all running on 70cm without anyone breaking down doors :)

Ok, configs modified (after I found an Atheros IPK for nano editor - i hate vi). Should have some results later this week. Stay tuned!
w6abj
To make your life a little
To make your life a little easier, you can use WinSCP, and use the built in text editor or setup notepad++ as an external editor.. This setup makes things a whole lot easier for editing configuration files, or transferring files.

73, Rich W6ABJ

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