Network Throughput Discrepancy

[geffers]

Folks,

I've got a mixed network at home, homeplug and wifi

I've got two Raspberry Pi devices, both connected to Devolo 200mbps homeplugs (One via an extension).

Now, although 200mbps devices the Devolo GUI utility shows around 90mbps between the two plugs connected to the Pis.

Transferring a video file between the two using scp the readout shows 2.1 MB/s

Probably not relevant as command line but receiving Pi runs Raspian and sending Pi runs Ubuntu-Mate

What could be the reason for such a large discrepancy in reported speeds?

Geffers

[Ernst]

Folks,

I've got a mixed network at home, homeplug and wifi

I've got two Raspberry Pi devices, both connected to Devolo 200mbps homeplugs (One via an extension).

Now, although 200mbps devices the Devolo GUI utility shows around 90mbps between the two plugs connected to the Pis.

Transferring a video file between the two using scp the readout shows 2.1 MB/s

Probably not relevant as command line but receiving Pi runs Raspian and sending Pi runs Ubuntu-Mate

What could be the reason for such a large discrepancy in reported speeds?

Geffers

There can not be a discrepancy because these are not the same thing.
90Mbps is line speed in bits per second, 2.1MB/s is transfer speed in Megabyte per second.

The problem is that the transfer speed includes the communication protocol (TCP) overhead, the file transfer (SCP) overhead and re-transmissions when errors are detected. I suspect that the homeplugs work in half-duplex mode, meaning the data cannot be simultaneously received and transmitted. All these little things will influence how fast data can be transferred.
The most effective method to get more information on this "problem" would be to do a packet trace at network level and see what happens during a transfer.
My experience with power line transmission and Wifi is that it can work but not always satisfactory, the best performance is cable.

[fruitoftheloom]

https://www.jeffgeerling.com/blogs/jeff ... networking

[allfox]

The HomePlug speed have a trick, they are labeled on the box with physical layer speed. But at link layer, they are cut to half. 200Mbit/s means a HomePlug AV device, means about 80 Mbit/s at link layer according to Wikipedia: https://en.wikipedia.org/wiki/HomePlug#HomePlug_AV

Transferring data with scp means encrypting would happen. So if you are with a slower Pi, CPU power would be the bottleneck. In fact I'm not sure if Pi 3 have enough power to scp at full speed, I haven't tried it. If it's in a LAN, maybe switch to NFS or Samba would give more performance.

I'm using a pair of Huawei PT500. I could get 8 mega bytes per second or more when forwarding Internet traffic.

[geffers]

Folks,


What could be the reason for such a large discrepancy in reported speeds?

Geffers

There can not be a discrepancy because these are not the same thing.
90Mbps is line speed in bits per second, 2.1MB/s is transfer speed in Megabyte per second.

The problem is that the transfer speed includes the communication protocol (TCP) overhead, the file transfer (SCP) overhead and re-transmissions when errors are detected.

I understand fully overheads and encryption take a toll, also know that mbps is around 8 times MB/s that is why as asked why such a 'large' descrepancy.

Appreciate too that cable is best - ran a long ethernet cable (impractical) to a remote AP and got about same speed as router; substituted the cable for a homeplug and speed dropped to around a quarter.

Geffers