Dear Experts, I am actually testing USB tethering throughput over LTE network. I have found that the throughput is quite low - the DownLink speed is about ~ 3MBps. When tested without USB tethering (i.e. Embedded mode of Smart phone), I can reach the throughput more than 13Mbps. But as soon as I turn to the USB Tethering, the throughput significantly drops. I have gathered the PCAP logs and has found lots of Duplicate ACKs and Retransmissions. Many thanks taqi asked 09 Oct '12, 22:57  traza786 edited 09 Oct '12, 23:43 |
One Answer:
a problem with the thethering "driver". Regards answered 09 Oct '12, 23:39  Kurt Knochner ♦ showing 5 of 13 show 8 more comments |
Dear Kurt
Many thanks for your quick reply.
I suspected that too, but under our lab environment (i.e. using LTE test equipment) we can reach desired throughput via tethered mode. But over the live LTE network the problem occurs. That's why I didn't suspect the Driver's issue
how is your lab envirnoment different from the real LTE network? Different PCs? Different LTE equipment? Different mobile devices?
The difference between the lab and field environment is the network simulator and the actual network.
We have tested the Software image of mobile device in the lab environment. Here lab environment means under the network simulator rather than the real network. Then the same mobile device was tested in the live network environment and the issue is observed. We have tested over 3 different PCs but the issue remained the same.
We have repeated the experiment with different mobile devices but in no vain.
Please note, when we test without USB tethering the throughput is Okay. This means the communication between the network and mobile device is Fine (which I further confirmed from Modem's logs)
By the "same device", do you mean the same software image (emulator?) or the same real mobile device?
It means the physical mobile device (smart phone). In other words the device was first tested with network emulator and there was no problem but after getting in the live network field the tethered mode operation has degrade downlink throughput.
Do you think some settings at PC are required? As i can see lots of TCP retransmissons
well, hard to say. It depends, if the lost packets are due to loss on the USB bus (driver) or somewhere in the (LTE) network.
Can you root the mobile device and capture the data flow on the device as well (tcpdump on android or similar on iOS)? Then you could compare the capture files and figure out if packets get lost on the USB bus.
Thanks.. that is a good idea. Can you recommemd some tcpdump program for android?
tcpdump should be installed already on android (depends on version and only accessible on a rooted device). google for "android tcpdump". There are plenty of tutorials for that.
Dear Kurt, thanks. I have got the PCAP logs from the LTE modem part also (i.e. by taking LTE modem logs and then converting into Wireshark logs). And found the problem to be same. I have noticed that the MTU was 1428 when smart phone gave good throughput (i.e. without tethered) and then when we use tethered mode it changed to 1500 (we are not sure why it does so), as a result low throughput is observed. First we suspect MTU size could be the reason. Do you have some previous experience on this that changing MTU could cost so much throughput degradation ?
well, MTU size can have an influence, depending on how the LTE "encapsulation" works. Try to set the MTU to 1428 manually, while you are using thethering.
Thanks Kurt. Yes the issue was with the appropriate MTU size :) Thank your for your prompt responses and feedback.
If you like, you can mark my answer as the correct one (check mark)