Slow Internet speed when HomePlug combined with router

Question

I'm using HomePlug AV2 600 devices to connect upstairs with downstairs (router is downstairs). A check with iperf shows bandwidth of ~100Mbps, and data transferred on the local network from downstairs to upstairs is transferred at about that speed. Good times.

But when I try to access the Internet from upstairs, maximum throughput is only 40Mbps. Bad times.

If I try from downstairs, I get the full 80Mbps that my Internet connection allows. This is really puzzling, as I know the HomePlugs are working faster than that for local data.

Here's what my setup looks like (the router is connecting to a fibre modem using PPPoE, everything has gigabit ports, and all testing was using wired connections):

                         +----+                           
                         | PC |                           
                         +-+--+                           
                           |                              
                           |                              
                       +---+----+         +----------+    
                       | Switch +---------+ HomePlug |    
                       +--------+         +-----+----+    
                                                |         
    Upstairs                                    |         
+-------------------------------------------------------+
    Downstairs                                  |         
                                                |         
  +-------+ PPPoE +-----------------+     +-----+----+    
  | Modem +-------+ Wireless Router +-----+ HomePlug |    
  +-------+       +-----------------+     +----------+

If I connect to the router downstairs (wired), then it works great:

                       +--------+         +----------+    
                       | Switch +---------+ HomePlug |    
                       +--------+         +-----+----+    
                                                |         
    Upstairs                                    |         
+-------------------------------------------------------+
    Downstairs                                  |         
                                                |         
  +-------+ PPPoE +-----------------+     +-----+----+    
  | Modem +-------+ Wireless Router +-----+ HomePlug |    
  +-------+       +-----------------+     +----------+    
                           |                              
                           |                              
                        +--+-+                            
                        | PC |                            
                        +----+

If I wire the downstairs HomePlug straight to the modem, and setup a PPPoE connection from the Windows 7 machine upstairs, it also works as expected:

                         +----+                           
                         | PC |                           
                         +-+--+                           
                           |                              
                           |                              
                       +---+----+         +----------+    
                       | Switch +---------+ HomePlug |    
                       +--------+         +-----+----+    
                                                |         
    Upstairs                                    |         
+-------------------------------------------------------+
    Downstairs                                  |         
                                                |         
  +-------+            PPPoE              +-----+----+    
  | Modem +-------------------------------+ HomePlug |    
  +-------+                               +----------+

I've also tried plugging the HomePlugs in right next to each other on the same floor, and get the same poor results:

    Downstairs                                          

  +-------+ PPPoE +-----------------+     +----------+    
  | Modem +-------+ Wireless Router +-----+ HomePlug |    
  +-------+       +-----------------+     +-----+----+    
                                                |
                                                |
                               +----+     +-----+----+    
                               | PC +-----+ HomePlug |    
                               +----+     +-----+----+

So the issue is only there when connecting to the Internet through both HomePlugs and the router, and only affects WAN traffic. I've tried replacing the router with a newer model, but get the same result. I also tried replacing the HomePlugs with a different brand (first tried Solwise, now Devolo).

I've also tried creating a PPTP VPN tunnel between my upstairs computer and the downstairs router - and this does inexplicably gives me the full bandwidth I would expect. But this relies on using DD-WRT on my router - and it's very flakey.

I'm at a loss, so any help would be much appreciated!

score 1 · Answer 1 · answered Jul 24 '14 at 14:09

What happens if you plug both powerline devices into the same plug strip AND turn off every circuit breaker in the house except the one powering this circuit? (i.e. Eliminate all possible sources of powerline noise.) If you find this resolves your speed issue then turn breakers back on one at a time until you find which has the noise source (and it may not just be one, but you get the idea). The difference between protocols I expect is exactly that... other protocols have better error correction than TCP.

score 0 · Answer 2 · answered Aug 16 '16 at 10:31

A bit late to the party. But I suspect that your problem is due to radio interference between your ADSL and your home plug.

Both the home plug and the ADSL work by sending radio signals down wires. The home plug uses your home's electrical mains wiring. The ADSL uses your phone lines.

Where a device is connected to both, there is the possibility of signals leaking on the other network. This causes interference.

If your ADSL modem has poor electrical isolation between the power supply and the phone line, the home plug may interfere with the ADSL connection and cause it to drop.

Normally mains hum on the phone line would not affect the ADSL since ADSL specifically avoids the mains frequency for this reason. The problem comes from the home plug introducing high frequency mains hum.

The solution is to isolate the ADSL equipment from the home plug signal. I personally used a surge protector myself.

score 0 · Answer 3 · answered Jul 25 '14 at 12:16

Well, I still haven't fixed it, but after flashing my router with DD-WRT firmware (TP-LINK Archer C7) I've got 2 reasonable workarounds.

Connect using wireless, just for Internet access, and use the wired connection for the LAN. Using wireless 'n' I can get speeds up to 60Mbps. I've got an 'ac' router and adapter, but for some reason I can only get 30Mbps with that - I guess it's an issue with DD-WRT ('ac' support for my router seems to be new to DD-WRT)
Enable a PPPoE server on the router, and have the upstairs machines connect to that, using their wired connection (i.e. over the HomePlugs). For some inexplicable reason, I get the full 80Mbps download speed doing this

I'd still love to know why on earth option 2 works, when the 'normal', non-PPPoE encapsulated WAN traffic is half the speed?!

Joey · Answer 4 · 2014-07-28T10:17:05.973

I had the exact same issue with Devolo HomePlugs. At times, they even caused the ASDL connection to drop completely. They seemed to be causing issues with the router directly. This actually seems to be a common issue with them.

I since upgraded to TP-LINK's Which are working great with full speed connections and no issues yet.

I'd honestly say give another brand a try. If you still have issues, then return them.

Just to add: Not sure where you're from, but BT Infinity Fibre in the UK are known to have big issues with HomePlugs.

Slow Internet speed when HomePlug combined with router

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