[[searchwing-pi-datarate]]

The Raspberry Pi stores the images which are collected during flight. After the plane landed, the images are retrieved from the Pi. The least impact with respect to water sealing would be a wireless transfer via Wifi or Bluetooth. For completeness I also test the transfer rate via USB configured as RNDIS network device. As a seperate topic I look into the transfer rate of the SD Card.

Author: Friedrich Beckmann

I did a headless setup and connected the Pi via USB to startup the pi.

  • Download the Raspbian SD Card image from raspberrypi.org
  • Install Etcher (on MacOS) and download the image to the SD Card
  • Setup the Pi for Headless connection via USB according to this description

In order to be able to login via ssh over usb cable, the following modifications have to be done on the SD Card. All changes are done in the files in the root folder. That folder is formatted FAT32 and can be accessed and modified via MacOS.

config.txt

Append at the bottom of config.txt

dtoverlay=dwc2

cmdline.txt

After rootwait, append

modules-load=dwc2,g_ether

and only add one space between and no newline. The parsing of this line is delicate.

Enable SSH

ssh is disabled by default. In order to be able to login via ssh, the ssh service has to be enabled in the Pi. This can be done by creating an empty file on the SD Card.

Create an empty file “ssh”

That file is deleted every time the pi does a new boot. So if you power off the pi, then you need to create that file again to be able to login via ssh.

Boot the Pi

Attach the Pi via the USB cable to the Mac. There are two USB connectors on the Pi. You must attach the cable to the connector labeled “USB”. Do not attach it to the connector labeled “PWR” - that connector does not provide a data connection. No additional power supply is required. The Pi is powered from the Macbook.

When the Pi boots, the green LED blinks when the SDCARD is accessed. Once boot is finished, the green LED stays on permanently.

Setup on the Mac

Go to System Preferences/Network or Systemeinstellungen/Netzwerk and check if you find RNDIS/Ethernet Gadget. That is the Raspberry Pi which is connected as network device. Then you can login to the Pi. Open a terminal on the Mac and do

ssh pi@raspberrypi.local

The default user on the Pi is pi and the default password is raspberry.

Enable ssh permanently

You need to enable ssh access permanenty with

sudo ssh systemctl enable ssh 

Configure a static IP address for Raspberry Pi

The default behavior of the Pi RNDIS service is to receive an IP address vi dhcp. But the Mac does not provide a DHCP service, so we configure the Pi to have the fixed IP address 192.168.2.2. The Mac is setup to have the IP address 192.168.2.1. To configure the pi append to

/etc/dhcpcd.conf

interface usb0
static ip_address=192.168.2.2/24
static routers=192.168.2.1
static domain_name_servers=192.168.2.1

On the Mac go to Systemeinstellungen/Netzwerk. You need to configure the IP setup for the “RNDIS/Ethernet Gadget” device to “manual”. Set the IP to 192.168.2.1 and the subnetmask to “255.255.255.0”.

Configure the MAC to provide internet access to the Pi

As default the Pi is connected locally to the Mac. To provide internet access to the Pi via the Mac go to Systemeinstellungen/Freigaben and activate Internetfreigabe for RNDIS/Ethernet Gadget.

To check the internet access on the Pi open a terminal on the Mac and login to the pi via ssh. Then check

ping www.hs-augsburg.de

on the Pi. You should get a connection the hs-augsburg web server.

Setup WLAN access as wlan client

To setup the Pi to connect to the WLAN access point as client, I modified the file

/etc/wpa_supplicant/wpa_supplicant.conf

country=DE
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
  ssid="fred"
  scan_ssid=1
  psk="dosenpups"
  key_mgmt=WPA-PSK
}

You can also place that file in the root folder on the Pi from the Mac. That will result in copying the wpa_supplicant.conf file the the /etc/wpa_supplicant directory and start the Wifi service. I am not too sure if this is permant or only for one boot.

Measurement Setup

The Raspberry is connected as WLAN client to the Fritzbox 7390 as access point. The Raspberry Pi Zero only has a 2.4 GHz rf modem and can not connect in the 5 GHz band. The Pi Zero W uses the CYW43438 chipset with 802.11n in single mimo stream setup and a maximum bandwidth of 20 MHz. This setup limits the air rate of the Pi to 65 MBit/s for a standard guard interval of 800ns and 72 MBit/s for a short guard interval. The Macbook is connected via Wifi to a Apple Timecapsule in the 5 GHz band, i.e. the Macbook is connected via seperate band to the FritzBox access point an does not use the channel bandwidth. There are no other clients connected to the Fritzbox AP. The channel is however occupied by other 2.4 GHz stations in the neighbourhood.

  • Raspberry Pi Zero WH
  • SanDisk Edge 8 GB - Class4 U1 SDHC Card
  • Raspbian Buster Lite release from 2019-09-26 with kernel 4.19
  • Macbook Pro (Retina 13 inch, Modell 2015) running MacOS 10.13.6 High Sierra
  • Fritzbox 7390 as WLAN access point for the Raspberry Pi in 2.4 GHz band
  • Apple Timecapsule 3GB version 7.9.1 for the 5 GHZ Wifi connection of the Mac

The Fritzbox reports the Raspberry Pi connection status with 65 MBit/s when I place the Pi in about 1m distance to the AP. There is no possibility to configure the FritzBox for Short Guard Interval, i.e. the system runs with the default 800ns GI.

Install iperf on the Raspberry Pi

To install iperf on Raspberry Pi run

sudo apt install iperf

Install iperf in the Mac

I installed iperf version 2.0.5 via homebrew. So on the Mac

brew install iperf

The iperf datastream goes from the client to the server. For our scenario the relevant direction is Pi to Mac. Therefore I run the iperf server on the Mac and the client on the Pi. iperf opens a tcp connection from the client to the server.

On the Mac

iperf -s

On the Pi

iperf -i 10 -t 300 -c 192.168.2.1

That tests the TCP/IP data transfer speed from the Pi to the Mac via USB connection. Each test interval is 10s and the toal test time is 300s. The result looks like this:

pi@raspberrypi:~ $ iperf -i 10 -t 300 -c 192.168.2.1
------------------------------------------------------------
Client connecting to 192.168.2.1, TCP port 5001
TCP window size: 70.0 KByte (default)
------------------------------------------------------------
[  3] local 192.168.2.2 port 34428 connected with 192.168.2.1 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec   185 MBytes   155 Mbits/sec
[  3] 10.0-20.0 sec   186 MBytes   156 Mbits/sec
[  3] 20.0-30.0 sec   186 MBytes   156 Mbits/sec
[  3] 30.0-40.0 sec   186 MBytes   156 Mbits/sec
[  3] 40.0-50.0 sec   185 MBytes   155 Mbits/sec
[  3] 50.0-60.0 sec   180 MBytes   151 Mbits/sec
[  3] 60.0-70.0 sec   180 MBytes   151 Mbits/sec
[  3] 70.0-80.0 sec   180 MBytes   151 Mbits/sec
[  3] 80.0-90.0 sec   181 MBytes   152 Mbits/sec
[  3] 90.0-100.0 sec   181 MBytes   152 Mbits/sec
[  3] 100.0-110.0 sec   181 MBytes   152 Mbits/sec
[  3] 110.0-120.0 sec   184 MBytes   155 Mbits/sec
[  3] 120.0-130.0 sec   182 MBytes   152 Mbits/sec
[  3] 130.0-140.0 sec   182 MBytes   152 Mbits/sec
[  3] 140.0-150.0 sec   181 MBytes   152 Mbits/sec
[  3] 150.0-160.0 sec   181 MBytes   152 Mbits/sec
[  3] 160.0-170.0 sec   180 MBytes   151 Mbits/sec
[  3] 170.0-180.0 sec   180 MBytes   151 Mbits/sec
[  3] 180.0-190.0 sec   180 MBytes   151 Mbits/sec
[  3] 190.0-200.0 sec   180 MBytes   151 Mbits/sec
[  3] 200.0-210.0 sec   180 MBytes   151 Mbits/sec
[  3] 210.0-220.0 sec   181 MBytes   152 Mbits/sec
[  3] 220.0-230.0 sec   180 MBytes   151 Mbits/sec
[  3] 230.0-240.0 sec   181 MBytes   152 Mbits/sec
[  3] 240.0-250.0 sec   181 MBytes   152 Mbits/sec
[  3] 250.0-260.0 sec   181 MBytes   152 Mbits/sec
[  3] 260.0-270.0 sec   180 MBytes   151 Mbits/sec
[  3] 270.0-280.0 sec   181 MBytes   152 Mbits/sec
[  3] 280.0-290.0 sec   180 MBytes   151 Mbits/sec
[  3] 290.0-300.0 sec   180 MBytes   151 Mbits/sec
[  3]  0.0-300.0 sec  5.32 GBytes   152 Mbits/sec
pi@raspberrypi:~ $ 

The transfer rate from the Pi to the Mac over the usb interface is on average 152 MBit/s.

The other direction for a data transfer from the Mac to the Pi shows a data rate of 109 MBit/s. For that test I started the iperf server on the Pi and called the iperf client on the Mac.

Fredo:~ fritz$ iperf -i 10 -t 100 -c 192.168.2.2
------------------------------------------------------------
Client connecting to 192.168.2.2, TCP port 5001
TCP window size:  129 KByte (default)
------------------------------------------------------------
[  4] local 192.168.2.1 port 63206 connected with 192.168.2.2 port 5001
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.0 sec   130 MBytes   109 Mbits/sec
[  4] 10.0-20.0 sec   130 MBytes   109 Mbits/sec
[  4] 20.0-30.0 sec   130 MBytes   109 Mbits/sec
[  4] 30.0-40.0 sec   130 MBytes   109 Mbits/sec
[  4] 40.0-50.0 sec   130 MBytes   109 Mbits/sec
[  4] 50.0-60.0 sec   130 MBytes   109 Mbits/sec
[  4] 60.0-70.0 sec   130 MBytes   109 Mbits/sec
[  4] 70.0-80.0 sec   130 MBytes   109 Mbits/sec
[  4] 80.0-90.0 sec   130 MBytes   109 Mbits/sec
[  4] 90.0-100.0 sec   130 MBytes   109 Mbits/sec
[  4]  0.0-100.0 sec  1.27 GBytes   109 Mbits/sec
Fredo:~ fritz$ 

The transfer rate from the Mac to the Pi is 109 Mbit/s over the USB Interface.

CPU Load during iperf transfer

I measured the cpu load with the top tool. I login to the pi via the wifi interface and run “top” there. The idle sitation is shown here:

top - 13:29:01 up 33 min,  2 users,  load average: 0.03, 0.04, 0.00
Tasks:  77 total,   1 running,  76 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.7 us,  1.0 sy,  0.0 ni, 98.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :    432.7 total,    327.0 free,     36.6 used,     69.1 buff/cache
MiB Swap:    100.0 total,    100.0 free,      0.0 used.    345.0 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND   
  578 pi        20   0   10172   2952   2520 R   1.0   0.7   0:01.11 top       
  556 root      20   0       0      0      0 I   0.3   0.0   0:00.02 kworker/u+
    1 root      20   0   32532   7876   6384 S   0.0   1.8   0:04.33 systemd   
    2 root      20   0       0      0      0 S   0.0   0.0   0:00.00 kthreadd  
    5 root      20   0       0      0      0 I   0.0   0.0   0:00.04 kworker/u+
    6 root       0 -20       0      0      0 I   0.0   0.0   0:00.00 mm_percpu+
    7 root      20   0       0      0      0 S   0.0   0.0   0:00.31 ksoftirqd+
    8 root      20   0       0      0      0 S   0.0   0.0   0:00.01 kdevtmpfs 
    9 root       0 -20       0      0      0 I   0.0   0.0   0:00.00 netns     
   10 root      20   0       0      0      0 I   0.0   0.0   0:00.10 kworker/0+
   11 root      20   0       0      0      0 S   0.0   0.0   0:00.00 khungtaskd

Then I start the iperf client and transfer the data to the iperf server on the Mac. The cpu load on the Pi looks like this:

top - 13:30:54 up 35 min,  2 users,  load average: 0.61, 0.17, 0.05
Tasks:  77 total,   2 running,  75 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.7 us, 46.7 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi, 52.6 si,  0.0 st
MiB Mem :    432.7 total,    326.0 free,     37.5 used,     69.2 buff/cache
MiB Swap:    100.0 total,    100.0 free,      0.0 used.    344.0 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND   
  580 pi        20   0   26708   2944   2252 S  80.7   0.7   0:20.34 iperf     
    7 root      20   0       0      0      0 R  14.1   0.0   0:03.64 ksoftirqd+
  299 root      20   0   27640   1368   1240 S   3.6   0.3   0:01.11 rngd      
  578 pi        20   0   10172   2952   2520 R   0.7   0.7   0:01.97 top       
   91 root      20   0       0      0      0 I   0.3   0.0   0:00.51 kworker/u+
  562 pi        20   0   12172   4056   3248 S   0.3   0.9   0:00.37 sshd      
  579 root      20   0       0      0      0 I   0.3   0.0   0:00.18 kworker/0+
    1 root      20   0   32532   7876   6384 S   0.0   1.8   0:04.33 systemd   
    2 root      20   0       0      0      0 S   0.0   0.0   0:00.00 kthreadd  
    5 root      20   0       0      0      0 I   0.0   0.0   0:00.04 kworker/u+
    6 root       0 -20       0      0      0 I   0.0   0.0   0:00.00 mm_percpu+

The CPU is fully loaded with the iperf process (80%) and the interrupt service (14%). The transfer rate is again 150 MBit/s.

I created 3 files with different size

  • tf1mb - 1 MB
  • tf3mb - 3 MB
  • tf100mb - 100Mb

The following example creates the file “tf3mb” with random content and a size of 3000000 bytes (3 MB).

pi@raspberrypi:~ $ dd if=/dev/urandom of=tf3mb bs=1000 count=3000
3000+0 records in
3000+0 records out
3000000 bytes (3.0 MB, 2.9 MiB) copied, 0.272827 s, 11.0 MB/s
pi@raspberrypi:

The I copied the file from the Pi to the Mac via scp.

Fredo:~ fritz$ scp pi@192.168.2.2:~/tf1mb .
pi@192.168.2.2's password: 
tf1mb                                         100%  977KB   4.8MB/s   00:00    
Fredo:~ fritz$ scp pi@192.168.2.2:~/tf3mb .
pi@192.168.2.2's password: 
tf3mb                                         100% 2930KB   5.1MB/s   00:00    
Fredo:~ fritz$ scp pi@192.168.2.2:~/tf100mb .
pi@192.168.2.2's password: 
tf100mb                                       100%   95MB   5.2MB/s   00:18    
Fredo:~ fritz$ 

The reported rate is 5.2 MB/s. The transfer time as reported by scp -v is shown in the following table together with the effective rate.

File Size Time Rate
tf1mb 1000000 Bytes 0.6 s 13 MBit/s
tf3mb 3000000 Bytes 0.9 s 27 MBit/s
tf100mb 100000000 Bytes 18.8 s 42 MBit/s

The transfer rate depends on the file size. Compared to the rate of 150 MBit/s as reported by iperf, there is substantial mismatch. The CPU load during the transfer as shown by top is

top - 14:55:32 up  2:00,  3 users,  load average: 0.52, 0.15, 0.13
Tasks:  82 total,   3 running,  79 sleeping,   0 stopped,   0 zombie
%Cpu(s): 53.3 us, 35.9 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi, 10.8 si,  0.0 st
MiB Mem :    432.7 total,    217.3 free,     41.1 used,    174.3 buff/cache
MiB Swap:    100.0 total,    100.0 free,      0.0 used.    339.5 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND             
  948 pi        20   0   12320   4244   3320 R  89.2   1.0   0:13.13 sshd                
  950 pi        20   0    6648   1828   1724 S   7.5   0.4   0:01.12 scp                 
  299 root      20   0   27640   1380   1240 S   1.3   0.3   0:19.77 rngd                
    7 root      20   0       0      0      0 R   1.0   0.0   2:32.59 ksoftirqd/0         
  918 pi        20   0   10168   2848   2428 R   1.0   0.6   0:00.71 top                 
   43 root     -51   0       0      0      0 S   0.3   0.0   1:25.49 irq/86-mmc1         
    1 root      20   0   32532   7884   6384 S   0.0   1.8   0:05.07 systemd  

The CPU is fully loaded. 90% of the time is spent in the ssh daemon. This is probably due to the encryption of ssh. The rate is limited by CPU load. For short files the initial transactions probably limit the rate.

I installed the lighttpd web server on the Raspberry Pi and tested the file transfer via wget.

Fredo:~ fritz$ wget 192.168.2.2/tf100mb
--2019-11-27 16:52:57--  http://192.168.2.2/tf100mb
Verbindungsaufbau zu 192.168.2.2:80 … verbunden.
HTTP-Anforderung gesendet, auf Antwort wird gewartet … 200 OK
Länge: 100000000 (95M) [application/octet-stream]
Wird in »tf100mb.2« gespeichert.

tf100mb.2                        100%[========================================================>]  95,37M  18,6MB/s    in 5,2s    

2019-11-27 16:53:02 (18,5 MB/s) - »tf100mb.2« gespeichert [100000000/100000000]

Fredo:~ fritz$ 

It takes 5.2s to transfer the 100MB file. That corresponds to a speed of 153 Mbit/s. The full usb speed according to iperf is used. The transfer times for the 1MB file are 0.06s and for the 3MB file 0.2s.

In my setup the Pi and Macbook both connect to a Fritzbox and both receive their IP address via DHCP. The IP address of the Pi is 192.168.178.84 and the IP Address of the Macbook is 192.168.178.34. The iperf server is started on the Macbook and the client on the Pi. The FritzBox access point is operates on 2.4 GHz channel 1. The fritzbox reports 8 others stations on that band. The distance between the Fritzbox and the Pi is approximately 2m. The Fritzbox reports that the wifi rate of the Pi is 65 Mbit/s for upstream and downstream.

pi@raspberrypi:~ $ iperf -i 10 -t 300 -c 192.168.178.34
------------------------------------------------------------
Client connecting to 192.168.178.34, TCP port 5001
TCP window size: 74.4 KByte (default)
------------------------------------------------------------
[  3] local 192.168.178.84 port 53842 connected with 192.168.178.34 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  46.9 MBytes  39.3 Mbits/sec
[  3] 10.0-20.0 sec  48.9 MBytes  41.0 Mbits/sec
[  3] 20.0-30.0 sec  49.2 MBytes  41.3 Mbits/sec
[  3] 30.0-40.0 sec  47.1 MBytes  39.5 Mbits/sec
[  3] 40.0-50.0 sec  45.6 MBytes  38.3 Mbits/sec
[  3] 50.0-60.0 sec  48.5 MBytes  40.7 Mbits/sec
[  3] 60.0-70.0 sec  48.9 MBytes  41.0 Mbits/sec
[  3] 70.0-80.0 sec  48.8 MBytes  40.9 Mbits/sec
[  3] 80.0-90.0 sec  48.5 MBytes  40.7 Mbits/sec
[  3] 90.0-100.0 sec  48.2 MBytes  40.5 Mbits/sec
[  3] 100.0-110.0 sec  48.6 MBytes  40.8 Mbits/sec
[  3] 110.0-120.0 sec  47.2 MBytes  39.6 Mbits/sec
[  3] 120.0-130.0 sec  48.5 MBytes  40.7 Mbits/sec
[  3] 130.0-140.0 sec  48.2 MBytes  40.5 Mbits/sec
[  3] 140.0-150.0 sec  47.9 MBytes  40.2 Mbits/sec
[  3] 150.0-160.0 sec  47.9 MBytes  40.2 Mbits/sec
[  3] 160.0-170.0 sec  49.6 MBytes  41.6 Mbits/sec
[  3] 170.0-180.0 sec  47.0 MBytes  39.4 Mbits/sec
[  3] 180.0-190.0 sec  45.4 MBytes  38.1 Mbits/sec
[  3] 190.0-200.0 sec  44.5 MBytes  37.3 Mbits/sec
[  3] 200.0-210.0 sec  47.1 MBytes  39.5 Mbits/sec
[  3] 210.0-220.0 sec  44.0 MBytes  36.9 Mbits/sec
[  3] 220.0-230.0 sec  45.5 MBytes  38.2 Mbits/sec
[  3] 230.0-240.0 sec  48.4 MBytes  40.6 Mbits/sec
[  3] 240.0-250.0 sec  48.5 MBytes  40.7 Mbits/sec
[  3] 250.0-260.0 sec  48.6 MBytes  40.8 Mbits/sec
[  3] 260.0-270.0 sec  48.4 MBytes  40.6 Mbits/sec
[  3] 270.0-280.0 sec  48.0 MBytes  40.3 Mbits/sec
[  3] 280.0-290.0 sec  46.5 MBytes  39.0 Mbits/sec
[  3] 290.0-300.0 sec  47.5 MBytes  39.8 Mbits/sec
[  3]  0.0-300.0 sec  1.39 GBytes  39.9 Mbits/sec
pi@raspberrypi:~ $ 

The average data rate from the Pi to the Mac via 2.4 GHz Wifi measured with iperf is 39.9 MBit/s.

CPU load during iperf wlan transfer

top results during iperf data transfer

top - 13:59:35 up  1:04,  3 users,  load average: 1.29, 0.92, 0.53
Tasks:  80 total,   1 running,  79 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.7 us, 22.2 sy,  0.0 ni, 44.7 id,  0.0 wa,  0.0 hi, 32.4 si,  0.0 st
MiB Mem :    432.7 total,    321.6 free,     41.0 used,     70.1 buff/cache
MiB Swap:    100.0 total,    100.0 free,      0.0 used.    340.5 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND                                           
  556 root      20   0       0      0      0 I  24.9   0.0   0:27.56 kworker/u2:1-brcmf_wq/mmc1:0001:1                 
    7 root      20   0       0      0      0 S  14.2   0.0   1:33.28 ksoftirqd/0                                       
   43 root     -51   0       0      0      0 S  11.7   0.0   0:34.95 irq/86-mmc1                                       
  662 pi        20   0   26708   3116   2052 S   2.3   0.7   0:04.38 iperf                                             
  665 pi        20   0   10168   3004   2584 R   1.3   0.7   0:00.57 top                                               
  657 root      20   0       0      0      0 I   0.6   0.0   0:48.06 kworker/u2:0-brcmf_wq/mmc1:0001:1                 
  299 root      20   0   27640   1380   1240 S   0.3   0.3   0:17.01 rngd                                              
  533 pi        20   0   12172   4016   3208 S   0.3   0.9   0:00.22 sshd                                              
    1 root      20   0   32532   7876   6384 S   0.0   1.8   0:04.41 systemd         

The iperf client transfer results in a CPU load of 50%. The iperf process consumes only 3%, while most of the cpu power goes to the interrupt handling of the wlan driver.

The ssh results for the wifi transfer are

Fredo:~ fritz$ scp pi@192.168.178.84:~/tf1mb .
pi@192.168.178.84's password: 
tf1mb                                              100%  977KB   2.1MB/s   00:00    
Fredo:~ fritz$ scp pi@192.168.178.84:~/tf3mb .
pi@192.168.178.84's password: 
tf3mb                                              100% 2930KB   2.3MB/s   00:01    
Fredo:~ fritz$ scp pi@192.168.178.84:~/tf100mb .
pi@192.168.178.84's password: 
tf100mb                                            100%   95MB   1.6MB/s   00:58    
Fredo:~ fritz$ scp pi@192.168.178.84:~/tf100mb .
pi@192.168.178.84's password: 
tf100mb                                            100%   95MB   1.8MB/s   00:54    
Fredo:~ fritz$ scp pi@192.168.178.84:~/tf100mb .
pi@192.168.178.84's password: 
tf100mb                                            100%   95MB   1.8MB/s   00:53    
Fredo:~ fritz$ scp pi@192.168.178.84:~/tf100mb .
pi@192.168.178.84's password: 
tf100mb                                            100%   95MB   1.7MB/s   00:55    
Fredo:~ fritz$ scp pi@192.168.178.84:~/tf100mb .
pi@192.168.178.84's password: 
tf100mb                                            100%   95MB   1.9MB/s   00:51    
Fredo:~ fritz$ scp pi@192.168.178.84:~/tf1mb .
pi@192.168.178.84's password: 
tf1mb                                              100%  977KB   1.8MB/s   00:00    
Fredo:~ fritz$ scp pi@192.168.178.84:~/tf3mb .
pi@192.168.178.84's password: 
Permission denied, please try again.
pi@192.168.178.84's password: 
tf3mb                                              100% 2930KB   2.0MB/s   00:01    
Fredo:~ fritz$ 

During the transfer of the 100 MByte file, the cpu load according to top is

top - 15:36:21 up 18 min,  1 user,  load average: 0.96, 0.86, 0.58
Tasks:  76 total,   2 running,  74 sleeping,   0 stopped,   0 zombie
%Cpu(s): 23.3 us, 39.5 sy,  0.0 ni,  3.9 id,  0.0 wa,  0.0 hi, 33.3 si,  0.0 st
MiB Mem :    432.7 total,    222.8 free,     40.1 used,    169.7 buff/cache
MiB Swap:    100.0 total,    100.0 free,      0.0 used.    341.3 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND       
  906 pi        20   0   15852   7740   3208 R  29.5   1.7   0:08.32 sshd          
  314 root      20   0       0      0      0 D  29.2   0.0   0:50.23 kworker/u2:3++
   43 root     -51   0       0      0      0 D  28.8   0.0   1:20.66 irq/86-mmc1   
  908 pi        20   0    6620   1760   1656 S   3.2   0.4   0:00.70 scp           
    7 root      20   0       0      0      0 S   2.6   0.0   0:12.60 ksoftirqd/0   
  530 pi        20   0   10172   2856   2432 R   1.0   0.6   0:08.43 top           
  292 root      20   0   27640   1420   1292 S   0.6   0.3   0:01.62 rngd          

The CPU is fully loaded and 30% are used for the ssh daemon and 60% are required for the wifi driver. It takes about 55s to transfer the 100 MB file. The rate is therefore around 14 MBit/s. The 1MB files take 0.8s (=10 MBit/s) and the 3MB file takes 2.5s (=10 MBit/s).

The results for the wget transfer time are 0.8s for the 1MB file, 1.1s for the 3MB file and 30s for teh 100MB file. However, these measurements were made at 19.00, i.e. there is quite some traffic on channel 1 now…

The transfer times for the 100MB were 30s, 24s, 25s, 26s. This corresponds to a transfer rate of 30 MBit/s. Some times for the 1MB file were: 0.2 0.2 0.2 0.9 0.3 0.7 0.7 0.2 → There is heavy congestion on channel 1.

Based on the 0.2s for the 1 MB File, this translates to 5 MB/s = 40 MBit/s rate for an undisturbed channel for a transfer via http. This is again exactly the achieved iperf rate as shown above.

  • WIFI Only datatransfer
  • Router: FritzBox 7430 AW
  • Placement: Pi Zero and Laptop placed 10cm above router.
  • data ammount: 540 MByte
  • Laptop and Raspberry Pi share the same Wifi channel ⇒ Each packet goes from Pi to AP and from AP to Laptop. ⇒ Congestion on the Wifi Channel.

wget

  • time=213sek
  • rate=2.53 Mbyte/sek
  • cpuusage=70%

ftp

  • time=361sek
  • rate=1.495 Mbyte/sek
  • cpuusage=50-60%
  • The Laptop and the Pi should not be connected via a single 2.4 GHz access point as laptop and Pi share the same Wifi channel. This results in 50% throughput reduction (see Julians 2.5 MB/s for wget).
  • The transfer rate via wget on an undisturbed 2.4 GHz wifi channel is 5 MB/s. (Only Pi connected to AP, No other stations use this channel). This 40 MBit/s throughput on http level compares well to the maximum 65 MBit/s wifi data rate of the Pi Zero W.
  • ssh transfer is limited by CPU load on the Pi for encryption and results in 1.8 MB/s for Wifi.
  • Short Guard Interval setting could possibly increase the the raw wifi rate from 65 MBit/s to 72 MBit/s.
  • searchwing-pi-datarate.txt
  • Last modified: 2020/01/08 22:00
  • by beckmanf