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TerryC65
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Network keeps dropping on System Containing Raspberry Pis

Sun Apr 18, 2021 8:41 am

Our re-engineered Minster Music and Bells system at our local Model Town has now been redeployed and has been working in anger (sort-of) since Monday when the site re-opened to the public. There were one or two teething problems which I was able fix by logging in to the system remotely.

However, I've been experiencing a recurrent issue with network access. For the third time since re-opening I found this morning that I was unable to log into one or other of the two Pis in the system and connectivity was only restored after a power cycle. Apart from being inconvenient, this is concerning because I'd like to be able to shut the system down before removing the power. Also, since I am still not visiting the site due to my wife's vulnerability to COVID so I have to phone in and ask the duty manager to do it.

The network architecture is a bit unusual in that we didn't want the expense of an extra Network Switch and USB/Ethernet Adaptor. To solve the former I configured one Pi as a Router and to solve the latter I used USB Networking, which the Pi supports. Here is a quick and dirty diagram:
Music&Bells_Network_Architecture.png
Music&Bells_Network_Architecture.png (22.46 KiB) Viewed 916 times
USB Networking was described to me by thagrol at:

viewtopic.php?f=36&t=295221&p=1785228#p1781380

I describe the system in more detail in the first post in the above link.

Can anyone suggest a reason for these dropouts?

epoch1970
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Re: Network keeps dropping on System Containing Raspberry Pis

Sun Apr 18, 2021 10:04 am

Hah. Where is Thagrol when you need him :)
It would be interesting to know which part of the system needs to be power-cycled in order to restore communication.
If Music Pi stops operating properly for some reason, you’d lose access to both Pis. But if the upstream router loses track of Music Pi you’d get the same effect.
"S'il n'y a pas de solution, c'est qu'il n'y a pas de problème." Les Shadoks, J. Rouxel

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thagrol
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Re: Network keeps dropping on System Containing Raspberry Pis

Sun Apr 18, 2021 12:11 pm

epoch1970 wrote:
Sun Apr 18, 2021 10:04 am
Hah. Where is Thagrol when you need him :)
Right here :D I'm not a mornig person though so don't expect me to respond before 12:00BST/GMT at the earilest.

Not sure what the problem could be, and it's probably not something that's going to be easy to debug remotely or via the forum. Certainly not without a secondary channel into the "bell Pi". (assuming you can still connect to the "music Pi")

This is what I'd probably do:
  • If possible activeate the overlayfs on one or both Pi. That'll make it much, much safer to just pull power but it will complicate updates etc as it means no changes made to them will persist across boots.
  • Enable the gpio-shutdown device tree overlay on both Pi then connect a double pole push to make button to both Pi. One pole per Pi between GPIO 3 and ground.
    The site manager can then push the button to initiate a safe shutdown, wait a couple of minutes then cycle the power.
  • Connect the serial ports of the "music Pi" and the "bell Pi". TX to RX, RX to TX, GND to GND. Serial ports shoudl be enabled by default but on the "music Pi" you'll need to disable login over the serial port (raspi-config) and console output (remove console=serial0,115200 from /boot/cmdline.txt)
  • If you haven't, read the troubleshooting section of my guide (see link in my sig or the sticky in the beginers sub forum)
  • While not a direct solution, you might want to investigate the linux software watchdog. IIRC it can reboot the OS if a it's unable to ping a machine.
  • If you want to be able to remotely peform a hard reboot of the "bell Pi", try this:
    1. Solder on a header for the Run pin.
    2. Disable I2C on the "music Pi"
    3. Connect either GPIO 2 or GPIO 3 on the "music Pi" to the Run pin on "bell Pi".
    4. Pull GPIO 2 or 3 low then back to high on the "music Pi" to force a hard rest on the "bell Pi"
      GPIOs 2 & 3 have external (to the SoC) pull ups to 3.3v so won't go low during reboot. If you want to sue a different GPIO you'll need to add a suitable pull up resistor.
I'm a volunteer. Take me for granted or abuse my support and I will walk away

All advice given is based on my experience. it worked for me, it may not work for you.
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TerryC65
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Re: Network keeps dropping on System Containing Raspberry Pis

Fri May 14, 2021 9:11 am

Both,

My apologies! I forgot that users need to explicitly subscribe to a Topic, even when they have written the original post. :oops:

thagrol,

Many of your suggestions require that I remove the hardware from the Minster model to perform the changes. Depending on how the system performs over the season I may have to do that, but that would be a last resort. At present, when the link is lost, I phone up the Duty Manager and he / she does a power cycle. That is not ideal of course, because there is a risk that the SD Card will become corrupt if I cannot perform a shutdown beforehand. Without access to the Pis over the network, I cannot cannot perform a shutdown...

ephoch,

When this problem occurs, I am usually unable to access the Music Pi, which in turn means that I cannot access the Bells Pi. I have seen the Bells Pi become accessible once or twice, while I was still able to log into the Music Pi.

The current situation (without tempting fate) is that If I leave it alone, it seems to carry on working. Every time I have seen the problem to date, it is after I have tried to update the software on the Pis or do something fairly network intensive. My short term solution to this is to not try to do updates and I confine myself to logging in to the Music Pi and checking the Player log. If all is well then the Bells Pi is probably OK, because both Pis rely on the communication between them to trigger events every day.

In the longer term, (ie during the Winter Workshop period), I'll look into including some of thagrol's fixes. This doesn't start until October, so I'll have a better idea of the best way forward. In the meantime I'll investigate the watchdog idea.

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TerryC65
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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 3:16 pm

Another possible solution to this problem has been suggested by a member of our local LUG:

The WiFi Adaptor used for the Minster Music system is the EW-7318USg and according to:

https://help.ubuntu.com/community/Hardw ... ardsEdimax

it uses a Realtek chipset. This person has had problems with Edimax WiFi adaptors that use a Realtek chipset which required a reboot to resolve. Does anyone know if there is any evidence to support this suggestion?

ejolson
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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 3:47 pm

TerryC65 wrote:
Tue May 25, 2021 3:16 pm
Another possible solution to this problem has been suggested by a member of our local LUG:

The WiFi Adaptor used for the Minster Music system is the EW-7318USg and according to:

https://help.ubuntu.com/community/Hardw ... ardsEdimax

it uses a Realtek chipset. This person has had problems with Edimax WiFi adaptors that use a Realtek chipset which required a reboot to resolve. Does anyone know if there is any evidence to support this suggestion?
If you are using WiFi, try keeping an ssh session to the device open with the keep alive flag set. Alternatively set the system that looses connection to periodically ping the other machines in the network.

My suspicion is the loss of connectivity may relate to how ARP packet filtering interacts with certain routers. If so, a work around is to create enough network traffic so the ARP caches don't lose track of the fact that there is a Pi out there.

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TerryC65
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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 4:05 pm

ejolson wrote:
Tue May 25, 2021 3:47 pm
If you are using WiFi, try keeping an ssh session to the device open with the keep alive flag set. Alternatively set the system that losses connection to periodically ping the other machines in the network. The loss of connectivity seems related to how ARP packet filtering interacts with older routers. If so, a work around is to create enough network traffic while issue so the ARP caches don't lose track of the fact that there is a Pi out there.
I can't really keep an SSH connection open because I don't have a machine that can remain on 24/7.

If I've understood your second suggestion properly, you are saying that the network drops the connection to the Pi because there is no traffic on it for several days. I'm not sure about that because although the problem has occurred when I've not tried SSHing into the Pi for several days, it has also occurred when I was trying to perform updates on the Pi. The download started OK and progressed normally and then simply stopped.

I can easily set up a script on the Music Pi to ping the rest of the network, but unless there is something subtle at work here, I can't quite see how that would work. I need someone on site to power cycle the system, so that I can make the modifications and that won't happen until tomorrow morning now.

I'll report back when I'm sure about this as a solution one way or the other.

ejolson
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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 4:08 pm

TerryC65 wrote:
Tue May 25, 2021 4:05 pm
ejolson wrote:
Tue May 25, 2021 3:47 pm
If you are using WiFi, try keeping an ssh session to the device open with the keep alive flag set. Alternatively set the system that losses connection to periodically ping the other machines in the network. The loss of connectivity seems related to how ARP packet filtering interacts with older routers. If so, a work around is to create enough network traffic while issue so the ARP caches don't lose track of the fact that there is a Pi out there.
I can't really keep an SSH connection open because I don't have a machine that can remain on 24/7.

If I've understood your second suggestion properly, you are saying that the network drops the connection to the Pi because there is no traffic on it for several days. I'm not sure about that because although the problem has occurred when I've not tried SSHing into the Pi for several days, it has also occurred when I was trying to perform updates on the Pi. The download started OK and progressed normally and then simply stopped.

I can easily set up a script on the Music Pi to ping the rest of the network, but unless there is something subtle at work here, I can't quite see how that would work. I need someone on site to power cycle the system, so that I can make the modifications and that won't happen until tomorrow morning now.

I'll report back when I'm sure about this as a solution one way or the other.
There seem to be cases where something subtle was at work and setting up a ping prevented loss of connectivity. If all other machines are off, I'm not sure what you should ping--maybe the router; maybe the system you will turn on later; maybe both.

If the Pi crashed during a file transfer, that sounds more like a problem with a marginal power supply. What is the power setup you're using?

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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 4:27 pm

ejolson wrote:
Tue May 25, 2021 4:08 pm
There seem to be cases where something subtle was at work and setting up a ping prevented loss of connectivity. If all other machines are off, I'm not sure what you should ping--maybe the router; maybe the system you will turn on later; maybe both.
There are plenty of machines on the local network that are on 24/7 (around 15 at the last count), so I suppose I could set up an SSH link to one of those.
ejolson wrote:
Tue May 25, 2021 4:08 pm
If the Pi crashed during a file transfer, that sounds more like a problem with a marginal power supply. What is the power setup you're using?
Each Pi in the system (Music and Bells) has it's own buck converter to generate 5.1 V from a 12 V site supply. The current capability of the buck converter is around 3 A and the actual consumption is less that 0.5 A. Exactly the same setup is used on most of the other Pis on the network without any similar issues. However, they are all networked using Ethernet. Ethernet proved more difficult to implement for this system because of the problem of installing the Cat 5 under paths and model buildings, so the WiFi solution was chosen.

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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 4:31 pm

TerryC65 wrote:
Tue May 25, 2021 4:27 pm
ejolson wrote:
Tue May 25, 2021 4:08 pm
There seem to be cases where something subtle was at work and setting up a ping prevented loss of connectivity. If all other machines are off, I'm not sure what you should ping--maybe the router; maybe the system you will turn on later; maybe both.
There are plenty of machines on the local network that are on 24/7 (around 15 at the last count), so I suppose I could set up an SSH link to one of those.
ejolson wrote:
Tue May 25, 2021 4:08 pm
If the Pi crashed during a file transfer, that sounds more like a problem with a marginal power supply. What is the power setup you're using?
Each Pi in the system (Music and Bells) has it's own buck converter to generate 5.1 V from a 12 V site supply. The current capability of the buck converter is around 3 A and the actual consumption is less that 0.5 A. Exactly the same setup is used on most of the other Pis on the network without any similar issues. However, they are all networked using Ethernet. Ethernet proved more difficult to implement for this system because of the problem of installing the Cat 5 under paths and model buildings, so the WiFi solution was chosen.
Depending on where you sourced the buck converters, some of them may have fake inductors and generate an astonishing amount of switching noise. This could lead to system instability among other things.

Maybe a picture of the converter clearly showing the markings on the components would help determine that. There are, unfortunately, lots of useless electronics sold online which waste people's time.

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TerryC65
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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 4:40 pm

ejolson wrote:
Tue May 25, 2021 4:31 pm
Depending on where you sourced the buck converters, some of them may have fake inductors and generate an astonishing amount of switching noise. This could lead to system instability among other things.
I don't rely on the converters and add my own filtering. We bought around 20 of these originally and have never seen any issues with noise. Most of them have been running for around 3-4 years without any problems.

I didn't see any unusual noise on the 5 V supplies when I integrated this system, but it is possible that a component failure could have occurred since then. However, it would have to be very specific noise to affect only the WiFi and then only after several days or heavy traffic.

Unfortunately I can't measure the noise in situ over SSH :!:

ejolson
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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 5:00 pm

TerryC65 wrote:
Tue May 25, 2021 4:40 pm
ejolson wrote:
Tue May 25, 2021 4:31 pm
Depending on where you sourced the buck converters, some of them may have fake inductors and generate an astonishing amount of switching noise. This could lead to system instability among other things.
I don't rely on the converters and add my own filtering. We bought around 20 of these originally and have never seen any issues with noise. Most of them have been running for around 3-4 years without any problems.

I didn't see any unusual noise on the 5 V supplies when I integrated this system, but it is possible that a component failure could have occurred since then. However, it would have to be very specific noise to affect only the WiFi and then only after several days or heavy traffic.

Unfortunately I can't measure the noise in situ over SSH :!:
I was thinking of noise that would cause the Pi to malfunction and crash.

The fact that the noise which results from using fake parts in a DC to DC converter is difficult to detect is one reason why some people think it's okay to sell such things. It's good if you can rule that out, otherwise all the unknowns could make solving this problem take longer.

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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 5:07 pm

ejolson wrote:
Tue May 25, 2021 5:00 pm
I was thinking of noise that would cause the Pi to malfunction and crash.
The Pi keeps going as evidenced by the music and bells playing properly. Only the network connectivity fails, hence the suspicion about the Realtek driver.

ejolson
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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 5:14 pm

TerryC65 wrote:
Tue May 25, 2021 5:07 pm
ejolson wrote:
Tue May 25, 2021 5:00 pm
I was thinking of noise that would cause the Pi to malfunction and crash.
The Pi keeps going as evidenced by the music and bells playing properly. Only the network connectivity fails, hence the suspicion about the Realtek driver.
That's a good point. I think the next thing to try is setting up a cron job that pings some other systems on the network every 15 minutes or so. Optionally, that same script could up and down the interface if the pings don't work after some time.

I've had a similar problem here which the above work around solved. Others have also reported success with this idea. I think the problem is, in part, related to the router, otherwise it would affect more people.

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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 5:21 pm

TerryC65 wrote:
Tue May 25, 2021 5:07 pm
ejolson wrote:
Tue May 25, 2021 5:00 pm
I was thinking of noise that would cause the Pi to malfunction and crash.
The Pi keeps going as evidenced by the music and bells playing properly. Only the network connectivity fails, hence the suspicion about the Realtek driver.
I'm not sure whether this applies to realtek chipsets or not, but some folks have reported problems with USB WiFi adapters that have been idel for a while going into a low power sleep mode. In which state they are disconnected from the WiFI and won't receive connections.

That's fine if you're at the machine and can wake it up but no so good if you rely on remote access.

A simple cron job on the affected pi like this one should help:

Code: Select all

* * * * * ping -c 1 -q 10.0.0.1
Replace 10.0.0.1 with the IP of a machine on your network. It shouldn't matter whether the machine is on as the packet gets sent anyway.
I'm a volunteer. Take me for granted or abuse my support and I will walk away

All advice given is based on my experience. it worked for me, it may not work for you.
Need help? https://github.com/thagrol/Guides

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TerryC65
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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 6:08 pm

thagrol wrote:
Tue May 25, 2021 5:21 pm
I'm not sure whether this applies to realtek chipsets or not, but some folks have reported problems with USB WiFi adapters that have been idel for a while going into a low power sleep mode. In which state they are disconnected from the WiFI and won't receive connections.
That's worth looking at because the Manual shows a picture of a Windows Control panel with 'WMM - Power' checkbox which enables Power Saving. I can't find much information because there is no definition of the conditions needed to trigger Power Saving, but I'd rather hope that half-way through a download wouldn't be one of them. Some language that I've seen in the manual would indicate that this might be the case, eg:
Click this button to select the WMM data type which will suppress the function of power saving. When this kind of data is transferring, power saving function will be disabled. Available data types are AC_BK (Background / Low Priority), AC_BE (Best Effort), AC_VI (Video First), and AC_VO (Voice First).
Maybe the trigger is lack of mouse movement. :( Both Pis are headless and running Raspberry Pi OS Lite.

I'll see if the Linux driver has any documentation to show how to disable this.
thagrol wrote:
Tue May 25, 2021 5:21 pm
A simple cron job on the affected pi like this one should help:

Code: Select all

* * * * * ping -c 1 -q 10.0.0.1
Replace 10.0.0.1 with the IP of a machine on your network. It shouldn't matter whether the machine is on as the packet gets sent anyway.
That was the kind of thing that I was going to try. However, if Power Saving kicks in even during data transfer then a 'keep alive' ping is doomed to fail.

This will have to be pursued tomorrow because I have other commitments this evening.

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Re: Network keeps dropping on System Containing Raspberry Pis

Tue May 25, 2021 9:38 pm

TerryC65 wrote:
Tue May 25, 2021 6:08 pm
thagrol wrote:
Tue May 25, 2021 5:21 pm
A simple cron job on the affected pi like this one should help:

Code: Select all

* * * * * ping -c 1 -q 10.0.0.1
Replace 10.0.0.1 with the IP of a machine on your network. It shouldn't matter whether the machine is on as the packet gets sent anyway.
That was the kind of thing that I was going to try. However, if Power Saving kicks in even during data transfer then a 'keep alive' ping is doomed to fail.
Yeah, maybe. I've afeeling ti was a lack of outgoing traffic rather than incoming that would do it. I could easily be wrong though.
This will have to be pursued tomorrow because I have other commitments this evening.
Fair enough.
I'm a volunteer. Take me for granted or abuse my support and I will walk away

All advice given is based on my experience. it worked for me, it may not work for you.
Need help? https://github.com/thagrol/Guides

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TerryC65
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Re: Network keeps dropping on System Containing Raspberry Pis

Wed May 26, 2021 8:39 am

thagrol wrote:
Tue May 25, 2021 9:38 pm
Yeah, maybe. I've afeeling ti was a lack of outgoing traffic rather than incoming that would do it. I could easily be wrong though.
Well. I've made some progress although the information that I've found is somewhat contradictory. At https://routerguide.net/wmm-apsd-on-or-off/ it says:
WMM APSD stands for Wi-Fi Multimedia (WMM) Automatic Power Save Delivery. It is basically a feature mode that allows your mobile devices to save more battery while connected to your wifi network. By allowing your mobile devices to enter standby or sleep mode, it conserves battery. The APSD allows smooth transition in and out of sleep mode by allowing the mobile devices to signal the router of its status.
So will my Music Pi be signalling to the WiFi AP? In this case it's a TP-Link TL-WA7210N 2.4GHz 150Mbps Outdoor Wireless Access Point, which does support WMM. Even if it doesn't the WMM function appears to work by buffering packets and only sending them when the device is out of sleep mode. I would have thought that would happen over milliseconds not days.

However, that site also says:
When your phone or mobile devices do not play well with the specific router that you have. There are reported cases of mobile devices dropping connections when the WMM APSD options are turned on.

The reason for this is that the mobile devices cannot retrieve the data from router after being awaken from the sleep state. Usually in this case, you can either turn the WMM APSD power saving option off from your router or on the mobile devices. Alternatively, you should see if you can check to see if the router company has released a new firmware that solves the problem. The best way is to actually experiment and troubleshoot for the problem.
That implies that devices might have this problem if they do not trigger the WMM active state for a long time, but they are talking about phones, etc that go into sleep mode when not being used and the Music Pi certainly does not do that.

So it looks unlikely that this is the cause of the problem for the reasons stated above. However, I'd like to know how to disable WMM - Power, since I don't have to worry battery state an that at least removes that from the possible causes. Any pointers to this? Apparently the driver is the rt73usb.

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Re: Network keeps dropping on System Containing Raspberry Pis

Thu May 27, 2021 4:01 pm

I've just regained control of the system by way of a power reset administered by a volunteer on site and added the following small script to run at boot-up:

Code: Select all

#!/bin/bash

# ping the WiFi Antenna above the Shelter near the hedge to stop WMM Power causing dropouts.
ping -i 900 -q 192.168.0.253
The pinged device is the nearest of the two outdoor WiFi APs, which presumably is the device that is turning off the tap if no traffic is detected from the Music Pi.

It'll be a few days before I know if it has worked.

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Re: Network keeps dropping on System Containing Raspberry Pis

Sat May 29, 2021 9:34 am

Sitrep:

It's too early yet to tell if the pinging is helping with the dropped connections, but this morning I tried to run apt update and upgrade on the Music Pi. The upgrade failed with multiple download errors. This lead me back to an earlier theory that the signal reception inside the Model Minster Nave is too poor. I had raised this as a possibility early on in the saga, but my on-site co-volunteer said that his phone showed the signal strength when inside the Nave was pretty identical to the signal strength when outside so I wrote that off as a possible cause.

This time, having a nasty suspicion that we were being lead up the garden path, I decided to check the signal strength seen by the Pi rather than a phone. Using iwconfig, I got:

Code: Select all

wlan0     IEEE 802.11  ESSID:"WMT-Guest"  
          Mode:Managed  Frequency:2.412 GHz  Access Point: C4:E9:84:26:E7:00   
          Bit Rate=54 Mb/s   Tx-Power=20 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:on
          Link Quality=32/70  Signal level=-78 dBm  
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:18331   Missed beacon:0
Now I was definitely smelling a rat, so I installed wavemon and got:
Wavemon.png
Wavemon.png (80.48 KiB) Viewed 234 times
My theory now is that because my co-volunteer was holding his phone while stood outside the Nave, with the phone inside the Nave, his body was acting as an aerial for the phone and fooling us. (He has confirmed that this is what he did.)

This may not be the sole cause of the dropped connections, but it can't help. I'm pretty sure it is the cause of the download errors though, so it needs to be fixed. When the system was installed, the Antenna was plugged directly into the Equipment Box inside the Nave instead of being installed in the Bell Tower (which has windows with vents in). I did provide a 6 m SMA to SMA RF cable to do this, but since the system seemed to work at the time, it was never extended. The penalty of making assumptions, instead of actually checking the thing. :cry:

To rectify this, we will have to remove one of the Tower Windows to gain access to the Bell Tower, which is not easy or a good thing while the site is open to the public. :(

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