How to debug an unstable connection in a P2P configuration

Question

have a setup where I have two computers connected to each other (C1 and C2). Both have a static IP and they know each others IP. C2 sends UDP packets to C1 and C1 receives and logs them.

Seems simple but here is the catch: There is about 1 kilometer of coax cable connecting those two devices. The connection between C1 and C2 is through a ethernet over coax converter which also transfers PoE. C2 is at a remote location and the only connection is a COAX wire.

The setup seems to work at times, I have had it running for several hours. But then at another time, without changing the configuration, the connection seems to become unstable. Since it looked like a connection issue I used wireshark to see what is being transmitted over the network.

When the system is running ok I can see all the packages from C2 reaching C1. When there are connection issues I can see that C2 is broadcast the command

Who has ip ip addr C1 - tell ip addr C2

This feels like it has lost it's connection. To verify that it is not a power issue I have checked the uptime of C2 and this is OK. It has been running continuously over the connection problems.

I am trying to figure out what causes this connection problems. The questions I have at this point are:

1. When does an device emit the 'Who has IP' message?
2. What tests can I do to check to establish if it is a software or hardware issue?

Final comment: I have placed a router between the devices on the C1 side. The connecting outages till occur but it seems to be less frequent. Is a router able to stabilize connections like these?

Update: I managed to find the specsheet of the cable we are using.

Answer 1

Initially I was thinking the same thing as Carlos. but ARP aging would account for brief ping loss, and not minutes or hours of no connectivity.

Coax cable systems compared to others are considered high maintenance. The reason for this is that connectors are usually exposed to the weather. Proper grounding is also paramount to keeping signal to noise ratios within manufacturers specifications.

1. Inspect all connectors indoor and outdoor looking for any corrosion, or oxidization on intermediary connectors. A good terminated connector should have 0.3db to 1.0db of loss (this includes any "barrel" or "double female" connectors.) As connectors age and are exposed to high humidity etc, they become oxidized, oxidization where metal meets for a connection causes higher ohmic resistance (DC) and higher impedance (AC resistance). Both apply here as your adapters also send 55vdc over the coax, and high frequency AC for communication. with oxidization, connectors can easily reach 1.5db loss per connector. (2 connectors @ -1.5 = -3db = 50% signal loss) this can completely break a link of this type.

Bad Cable that has oxidization on the center pin, and should be cut off & re-terminated.

Good bright center-pin.

In my experience the best thing you can do on either end is use high quality compression fittings to terminate these cables. (see below) they will last a considerable amount more time then their cheap counter-parts, and when terminated properly have little loss in comparison.

2. Inspect each ends grounding. grounding is paramount in center coax connections, and ideally the block itself should be oxidization free. and so should the copper terminating to the grounding block.

3. Make sure you match connector impedance with cable impedance. If the cable is 50ohm, get 50ohm connectors.

Without knowing what cable you are actually using, its hard to tell you if its in spec at this distance. but here is what i found in the data-sheet.

Low-q CATV wiring = 800m or 2624.67ft

RG/59u = 2500m or 8202.1ft

RG-6 = 3500m or 11482.94ft

RG-11 = 5000m or 16404.2ft

I did notice in the datasheet that these adapters have a limited lifetime warranty. if you clean up the connectors/grounding and it still isn't stable. I would have the manufacture replace them.

If you are terminating yourself. Give this link a read. https://www.belden.com/blog/digital-building/13-common-bnc-connector-installation-mistakes

Answer 2

Connection between the two systems using IP(L3) goes over ethernet(L2). To be able to send something to C1 (IP address), C2 has to find the ethernet address (MAC) of C1. That's done using ARP (a dedicated protocol) which in turn uses broadcast to reach C1, thus obviating the need to know C1's MAC.

The system keeps a table of IP - MAC mappings, and those entries last some time (5 minutes?). When they "age" out, ARP is called again. (if needed to keep talking to some device, C1 in this case).

That answers your Q1.

Q2 is more complicated.... what kind of coax converter are you using ? Is it able to drive 1km ? I guess that going to the remote site and seeing there with wireshark if you are receiving the packets should be conclusive, but my hunch is that you are pushing the limits of the equipmnent.

After looking at the specs of the device you are using (NV-EC1701) this is the range depending on the cable type and power delivered:

If the cable is "like" an RG/59U (note the device says it supports 25-100ohms cables) then for a 6W remote you are on its limits.