4G/LTE download speed limited on WRT-based routers like the RT-AC56U?

[Ryan Hardy]

Hey guys!

I'm new to this forum, however I have several years of experience as IT pro. I do have some trouble regarding a phenomenon I'm seeing on two routers now:
First I tried my Huawei E3272-153 4G/LTE USB stick (non-HiLink) that is known to achieve around 75/30 Mbit/s on various PCs (not limited by the provider) on a Linksys WRT160NL with DD-WRT with the funny result of constant 20 Mbit/s download while upload remains on full speed.
My guess was that maybe this old hardware isn't capable of higher speeds (still puzzled by the faster upload though), that's why I bought a faster recent model in the form of an Asus RT-AC56U. I have flashed the latest Merlin firmware onto it, but guess what: I'm still seeing the same problem with download speeds limited to nearly exactly 20 Mbit/s while upload is on normal speed.
The fast upload and the led on the stick guarantees that the stick really is in 4G/LTE mode, else I could have explained it by using 3G/UMTS.

Anyone have an idea how to solve this problem?

Best regards
Ryan

 

[dlugi]

Hello,
This not a hardware limitation. Your problem is related to buggy/not optimized cdc_ncm driver. I have the same problem on my rt-n66u. I've found somewhere a "hack" that can improve performance of cdc_ncm but it is far from optimal solution.
If you can to compile firmware on your own, here is the change:

file:
~/include/linux/usb/cdc_ncm.h

lines:
/* Maximum NTB length */
#define CDC_NCM_NTB_MAX_SIZE_TX 16384 /* bytes */
#define CDC_NCM_NTB_MAX_SIZE_RX 16384 /* bytes */

change size of the buffers to 4096 or 8192

As said this in not a final solution but you can gain some speed.

I've heard that performance problems related to cdc_ncm are solved in newer Linux kernels (3.15 and up)
If you are on Linux 2.6.36 or newer you can try to port driver yourself. Older kernels require lot of changes.

If you not care about double NAT you can convert of your modem to Hi-link. cdc_ether driver is a bit better - still not ideal unfortunately.

=Dlugi

 

[vaszago]

You must install the hilink firmware on stick then you get the full speed

 

[Ryan Hardy]

Actually I got it working using the firmware with double Ds - there the modem is used by the cdc-ncm driver, so full speed. But then again: no hardware acceleration any more. :/
HiLink seems to be something most people want to get away from (double NAT), but for speed issues it may be the way to go... Well, sad we can't have both.
But thank you for your answers!