Slaying the Lost Packet Dragon with Pathfinder
By Trevor Stuart on Nov 15, 2017 12:20:00 PM
Using IP radios as your STLs has many advantages. One disadvantage though is that they can be more susceptible to rain and snow fade than other STLs.
Using Axia Pathfinder and syslog, I’ve been able to keep my stations on-air, without hearing a dropped packet. Though I am using syslog, the same result can be achieved using SNMP. Even if the IP radio you have is not capable of sending syslog, you can still make this work for you with a little tweaking...
A situation that this does not help is when there is a sudden loss of radio communications. Perhaps someone trips on the power cord to the IP radio. This means Pathfinder does not get a warning from the radio and IP communication is lost before Pathfinder would be able to instruct the switcher at the transmitter site to switch. Fortunately, our BDI switcher has a 45 second silence sensor in it which covers this scenario.
Here’s a step-by-step guide to implementing this trick:
- I use a BDI composite Switcher; this gives a switchable A/B input and 2 outputs.
- Port A is the composite out of my Omnia.11, which is on Dragonwave.
- Port B is the composite out of my 950 STL, the processor for this one is at the studio end because I also have a 1KW offsite backup TX that this processor feeds.
- BDI has GPIO which is wired to an Axia Multi-Node at the site.
- Dragonwave and Pathfinder are on same network; same physical infrastructure, same VLAN, but different subnet than the rest of Axia…
- Axia is 172.16.2.xxx
- Dragonwave is 192.168.10.xxx
Pathfinder Server’s Axia NIC has 2 IPs; one in Axia subnet, the other in Dragonwave subnet. All static.
In the Dragonwave configuration, under System/Logs, I set the syslog forwarding host to the IP I assigned my Pathfinder. Because I have four radios and two Pathfinders, I configured one radio at each end with Pathfinder 1 and the other radio at each end with Pathfinder 2. This covers me if I’ve lost a radio, I'm on my backup Pathfinder, or basically any combination of those.
Next, in Pathfinder, build a Generic protocol translator. Syslog uses UDP port 514.
I set mine to use all routers.
In stacking events I’ve built a section just for FM Tower Switchers. The qualifiers are the translator itself and a GPO I built that helps avoid having it jump back and forth too frequently.
It took some wireshark sniffing to get the right command I wanted it to switch on. There are several to choose froms, but I've found the one below to give a good lead time before failure, and it's never actually had a failure on-air.
Qualifier 1: The command string is “Major: RSL below threshold rf1 alarm raised.”
Qualifier 2: “Virtual GPO” state.
Action 1: Notify Engineering.
Action 2: Set the GPO of the Multi-Node that is connected to the BDI pin to 1 low, telling BDI to change inputs from A to B.
Action 3: Set the Virtual GPO to low to help avoid loops with A/B switching.
Returning to Dragonwave is done with 2 different events.
Event 1: When Dragonwave states it’s all clear, wait 10 minutes before changing the “virtual GPO” to High. The command is “Major: RSL below threshold rf1 Alarm cleared.”
Event 2: When the “Virtual GPO” has been set to High for another 10 minutes, switch back.
There is roughly a 20-minute window in this scenario; you can adjust the timeframe to your needs.
First, clear any errors on the BDI, in case it had lost audio at some point, as it will not flip between A/B if in an error condition from loss of audio.This is done via Multi-Node GPO, connected to BDI.
Then Multi-Node GPO tells BDI to switch back to input A.
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