The problems described here were all caused by a faulty switch, see my answer for the fix.

What I learnt from this experience is:

  • Don't trust the hardware. I thought I'd configured something incorrectly to cause the slow speeds when it turned out the switch was faulty.
  • If you have spare kit, swap out one thing at a time to see if you still have a problem. unfortunately I did not have a spare switch in this instance.
  • iPerf is great. Without it I would not have been able to properly test the network speed.

Original Question

I have a LAN setup as shown. The mac is my main computer and the linux box is a NAS running OpenMediaVault, connected by a gigabit switch. The router is downstairs connected by powerline adapters; the router is also the DHCP server.

enter image description here

I'm having problems with file transfer speeds between the mac and linux boxes when the switch is connected to the router via the powerline adapters. If I disconnect the switch from the powerline adapter and manally assign IP addresses to the mac and linux boxes I get fast transfers.

enter image description here

But that breaks the internet connection. When I reconnect the rest of the network file transfers slow to a crawl and there's a lot of activity (flashing lights) on the powerline adapter. It's as if the data is travelling down the powerline to the router and back again before getting to the linux box.

enter image description here

I can't turn off the DHCP from the router as a number of other devices use it for wireless connection, and I don't want to have to manage static IPs for the entire network.

I've considered running a cable from the router to the upstairs but it's not practical; not impossible but it would be real pain and that's what I thought powerline adapters were for, extending networks easily :)

I also want the linux NAS to be accessible from the wireless network and still be able to transfer files at a reasonable speed.

I'm looking for suggestions on how to find the problem, because at the moment I have to disconnect plugs and reconfigure NICs to talk to different parts of the network/internet.


Router: Technicolor TG582n

Powerline: BT Broadband Extender Flex 500

Mac: Macbook Pro (2011)

Linux: OpenMediaVault v2.1 (with all updates applied)

Switch: Netgear GS608 v2 (8 port)


I've seen this question: Slow Internet speed when HomePlug combined with router but that is asking about internet connection speed rather than local file copy speed.


I have a hunch I may have left the wifi connected when I saw the strange behaviour of the powerline flashing away while transferring files to the NAS. I think the wifi was being prioritised by the network, so data was flowing down the wireless to the router and back up the powerline.

I've also been testing with iperf performing 2 or 3 connections per test setup, results below, which has shown interesting results but I'm no nearer to understanding why this is happening. All cables used are CAT6.

mac <-> linux direct connection

[  4] local port 5001 connected with port 39347
[  4]  0.0-10.0 sec   882 MBytes   740 Mbits/sec
[  4] local port 5001 connected with port 39348
[  4]  0.0-10.0 sec   882 MBytes   739 Mbits/sec

mac <-> switch <-> linux

[  4] local port 5001 connected with port 39349
[  4]  0.0-10.1 sec  90.6 MBytes  75.5 Mbits/sec
[  4] local port 5001 connected with port 39350
[  4]  0.0-10.0 sec   114 MBytes  95.3 Mbits/sec
[  4] local port 5001 connected with port 39351
[  4]  0.0-10.0 sec   115 MBytes  96.2 Mbits/sec

I then connected the powerline adapter to the switch as well. The mac and linux connections were not changed.

mac <-> switch <-> linux

[  4] local port 5001 connected with port 39352
[  4]  0.0-10.2 sec  33.4 MBytes  27.5 Mbits/sec
[  4] local port 5001 connected with port 39353
[  4]  0.0-10.4 sec  37.4 MBytes  30.2 Mbits/sec
[  4] local port 5001 connected with port 39354
[  4]  0.0-10.1 sec  37.4 MBytes  31.1 Mbits/sec

To see if there's something up with the switch I moved all the connections to different ports

mac <-> switch <-> linux

[  4] local port 5001 connected with port 39355
[  4]  0.0-10.0 sec  85.6 MBytes  71.7 Mbits/sec
[  4] local port 5001 connected with port 39356
[  4]  0.0-10.0 sec  83.0 MBytes  69.5 Mbits/sec
[  4] local port 5001 connected with port 39357
[  4]  0.0-10.0 sec  76.4 MBytes  63.9 Mbits/sec

Whis increased the speed but I did not move them back again. For the next test I disconnected the powerline adapter again.

mac <-> switch <-> linux

[  4] local port 5001 connected with port 39358
[  4]  0.0-10.0 sec   184 MBytes   154 Mbits/sec
[  4] local port 5001 connected with port 39359
[  4]  0.0-10.0 sec   211 MBytes   177 Mbits/sec
[  4] local port 5001 connected with port 39360
[  4]  0.0-10.0 sec   205 MBytes   171 Mbits/sec

There's definitely something strange going on with the switch, unfortunately I don't have another to test with. I appreciate the netgear switch is a cheap bit of kit and it's probably worth me investing in something better, thoughts anyone?

  • What are the IP addresses assigned to the Mac and to the Linux box? What is the make and model of the switch? How are you attempting file transfers between the two machines? By name? By IP address? – David Schwartz Mar 27 '16 at 22:41
  • macbook:, linux:, switch: netgear gs608 v2 (8 port). file transfer was to a share on the NAS using Apple Filing Protocol (AFP) performed in GoodSync, which shows the transfer rate. slow transfers were happening at 3-4MB per second, fast transfers were 10 times that. – Tony Mar 27 '16 at 23:00
  • I should add the macbook currently has a manually assigned IP and the linux box has managed to get one from the DHCP server - I've configure the DHCP server to always give that IP to make sure I know where to find the server on the network. However, I tend to have to disconnect the powerline adapter after the IP has been assigned to be able to talk to the NAS reliably. – Tony Mar 27 '16 at 23:05

It sounds rather much as problems with the switch. That switch model is known by those sorts of issues: http://forums.whirlpool.net.au/archive/2249158

  • You may have a point. I bought the switch in June '09 so it's nearly seven years old. It has been worked the hardest in the last 7 or so months where it has been on 24/7, so that might have pushed it over the edge. – Tony Mar 28 '16 at 19:54

After reading the link posted by Tamadite, and further investigation, I found a blog post which described HowTo: Fix a Broken Netgear GS108

I have a version 2 switch but the problems they have are similar. After taking the switch apart (easily done, the torx screws are accessed by flipping open the rubber feet covers) I could see the capacitors for smoothing the power supply voltage were bulging, not by much, but the tops are rounded. Compare with the black capacitor behind.

bulging capacitors

I ordered some new capacitors and replaced them. Running the iPerf test again I got much better speeds, although they still varied depending on which ports the two computers were connected to.

Client connecting to, TCP port 5001
TCP window size:  129 KByte (default)
[  4] local port 51621 connected with port 5001
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.0 sec   492 MBytes   413 Mbits/sec
[  4]  0.0-10.0 sec   683 MBytes   572 Mbits/sec
[  4]  0.0-10.0 sec   552 MBytes   462 Mbits/sec
[  4]  0.0-10.0 sec   890 MBytes   746 Mbits/sec
[  4]  0.0-10.0 sec   843 MBytes   707 Mbits/sec  

I've not tested all combinations of ports but even at 400 Mbits/sec that's a great improvement and means the switch has been saved from the scrap heap... for now.

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