Latency over Home plug AV devices

I use a pair of Homeplug AV device (Zyxel AV2000) to extend my network to a media pc and an OpenWrt access point in the front room of my house. The WAN network is very fast (500+ Mbps symmetric reliably) and speed that the Homeplug gives over a dslreports speedtest are between about 30 and 110 Mbps and typically somewhat asymmetric. The speeds are fine, I don't need more than that for streaming soccer games or etc, but the latency under load can go up to 50 or 100 ms (average, much higher peak) on this particular link. It's not too bad, but I sometimes get drops in streaming quality and am concerned that in the front area of the house when my phone attaches to that WiFi AP I could get VOIP degradation due to jitter depending on load. So far it's not a major problem, but probably just because my phone is sticky and tends to stay with the other AP which is on cat5e

At the other end I have a Zyxel managed switch with QoS enabled, and this morning I tried setting a port bandwidth limit of 35Mbps on the link so that at least max bandwidth is consistent. I assume this means Zyxel just drops packets above the given rate. However this didn't really affect latency under load much for the media pc when testing on dslreports. I suspect that the homeplug devices themselves have some buffers and that these tend to fill and drain depending on the burstiness of the traffic. Also I don't think these buffers have any kind of prioritization so that for example DSCP tags matter. At least the buffers never get too huge (like any more than 100ms average is rare)

What strategies can people think of to improve the latency under load on Homeplug links? What experiences do others have on homeplug links? Any general thoughts on latency management in LAN settings?

The particular homeplug devices I have are Zyxel AV2000 if it matters.

also in case it's helpful, here's a speedtest from the media pc with the ~ 35Mbps policing set on that port in the switch. As you can see by investingating the details latency spikes up to 300-400ms occur

EDIT: Not surprisingly it helps to put my custom hfsc shaper on the output of the media pc, that makes upload pretty acceptable. Download is still an issue. It's kinda unusual to have the LAN be the slow link. Any thoughts about controlling latency on this kind of slow LAN link?

Make a separate VLAN for the Ethernet port that goes to the Homeplug. Then software bridge that VLAN back into the main LAN. Now you can apply SQM to that link only.

If you had OpenWrt on both ends you could do this on both sides of the link but control only outflow (going from the router to the Homeplug) on each end.

Yeah, I had an idea like that and just tested it out. I'm keeping the policing on the switch, so that the link itself won't get saturated by the switch. On the far end of the link I have a tp-link "router" that acts as an AP. So what I did was I made "LAN" on that machine a bridge between eth0.1 and eth0.2 then plugged the WAN port into the homeplug and the media PC into the LAN port. Then I put layer cake on the AP on both eth0.1 and eth0.2 egress only.

Now both upload and download on the media PC is fully controlled, still not as good as latency on a cat5e but latency doesn't go crazy under load, stays down in the tens of ms up to maybe 100 or 150 occasionally. I had some speedtests where latency peaked at 3000 ms or more before and took several seconds to fall off back to normal.

So problem solved I guess!

Here's a speedtest after:

I'd check to see that your running the most current firmware on them. My TP-Link AV500 Nano adapters have had a few firmware updates. They also have a configuration utility where you can set the QoS to optimize them. It's a simple QoS and has 4 options like, Web, Gaming, Video, and Voice. I'm not sure if it really impacts anything. You could check and see if yours have a feature like this and set it to Video. You could also try disabling power saving.

Other factors that may cause latency issues could be electronic devices and appliances on the circuit that use fans or AC to DC power converters (laptops, PCs, phone charges...). It could even be a microwave in use, fridge kicking on, or an air conditioner or heater turning on as well. The best practice is to try to put other devices on the circuit(s) the power line adapters use, especially, anything with a fan or AC to DC converter, behind surge protectors.

Also, always make sure the adapters are plugged directly in to the wall outlet. I tend not to plug anything else in to that outlet. Some adapters have pass through, where you can plug in another device. This should be safe to do, as the outlet on the adapter should act in the way a power surge protector would in reducing interference from other devices.

I'm guessing since the speeds are fluctuating so much that there is some underlying interference on the circuit(s). Typically, you should be able to reach ~40% of your maximum download speed on them. On a 200/20Mbps connection I normally reach between 65-85Mbps down and 20Mbps up on my adapters without huge latency spikes.

I'll see if there are any firmware updates, the controller software thing is inconvenient because I have zero windows machines to run it on. Well I guess my laptop dual boots but the last time I booted windows it took literally 24 hours of downloading and installing updates before it would do anything... Sigh.

My speeds don't fluctuate that much during my tests, though they fluctuate from day to day, it's just latency that fluctuates, and yes it could be noisy powerlines. The solution to police the switch port and set up sqm up and down seems to have worked well though.

This topic was automatically closed 10 days after the last reply. New replies are no longer allowed.