[LEDE-DEV] TDW8970 Slow Wireless Performance when using VDSL2

[Sebastian Moeller]

Hi All,


> On Nov 6, 2016, at 11:47, Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> wrote:
> 
> 
> 
> On 06/11/16 10:12, Xander Shelley wrote:
>> Hi there,
>> 
>> I've installed LEDE Nov5 snapshot onto my TP-Link TD-W8970 router to use
>> with a UK fibre ISP (Plus.net)
>> 
>> Out of the box most things work great.
>> 
>> If I connect via Ethernet to the router from my xubuntu T410 laptop, and
>> run a speed test using (speedtest.net) I get close to line speed (31Mb/s
>> Line Speed, measured speed 29MB/s) which is great.
> 
> VDSL2 overheads would account for the drop there.

	Assuming TCP/IPV4 goodput measurements the best you can expect on a VDSL2 line is:
sync bandwidth * coding factor * per packet overhead factor
with coding factot: 64/65
payload factor: variable depending on encapsulation, typically PPPoE with MTU 1500 and VLAN

PPPoE: 2 Byte PPP + 6 Byte PPPoE
VLAN: 4 Byte VLAN
VDSL2 (IEEE 802.3-2012 61.3 relevant fuer VDSL2):  1 Byte Start of Frame (S), 1 Byte End of Frame (Ck), 2 Byte TC-CRC (PTM-FCS), = 4 Byte
COMMON ethernet: 4 Byte Frame Check Sequence (FCS) + 6 (dest MAC) + 6 (src MAC) + 2 (ethertype) = 18 byte
Sum: 8 + 4 + 4 + 18 = 34 Bytes
effective MTU 1500 - PPPoE - PPP = 1500 - 8 = 1492 Bytes
payload factor: (1492 - 20 - 20) / (1492 + 8 + 4 + 4 + 18) = 0.951507208388
PPPoE goodput: 31 * (1492/1526) * (64/65) = 29.8430083678
IPv4 goodput: 31 * ((1492 -20)/1526) * (64/65) = 29.4429680411
TCP/IPv4 goodput: 31 * ((1492 - 20 - 20)/1526) * (64/65) = 29.0429277145
TCP/IPv4 goodput (assuming MTU 1508 baby jumbo frames): 31 * ((1500 - 20 - 20)/1534) * (64/65) = 29.0506468759
So 29 out of 31 is to be expected…

About the wifi problem, I would try to test against a local machine connected to the wired LAN ports of the router to reduce the internet introduced variability…

Best Regards
	Sebastian



> 
>> 
>> However, when I switch over to using wifi, this measured speed drops to
>> 12-16MB/s....
>> 
>> Looking at the LuCI interface it shows that my laptop is connected
>> around 113MB/s which  is as expected.
>> 
>> Am I doing anything incorrectly/missed a step in my setup?
> 
> I can't offer any magic bullet.  Wifi is notoriously variable in terms of bandwidth and I have to say my 2.4Ghz is slower (similar to your measurement) than 5Ghz even though rates would suggest plenty of spare capacity.
> 
> It's worth proving the router isn't running out of CPU though...a top during wired/wireless tests would be helpful.
> 
> Kevin
> 
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