Cannot get Wireless-N speeds, What am I doing wrong?

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givehimagun
DD-WRT Novice


Joined: 09 Oct 2009
Posts: 8

PostPosted: Thu Oct 22, 2009 23:47    Post subject: Reply with quote
My problem isn't getting my connection rate to stay above 130 as much as it is that I don't get actual transfer speeds that match that.

I have a server that is on a wired Gigabit. When I turn off my wireless and wire up the laptop to the router I get transfer rates of 16-17 megabytes/s through samba and sftp.

If I put my laptop on Wireless N and get 130 mbps (let's say I get even 65 mpbs which should be 8 MB/s), my transfer rates on the same samba and sftp shares go down to 2 MB/s or 16 mbps.

16 mbps of actual throughput for something that is displaying as 130-270 mbps seems wildly incorrect.

I've updated ALL of my wireless and wireless related drivers for my Intel Wifilink 5100 AGN. I wonder if that's the case. To me it seems that the problem resides in either DDWRT's wireless settings or my laptop's wireless settings, but I don't know what to do to actually isolate the problem.


RCShadows wrote:
I have a 310 and get solid 216-270mbps.

What I found was, I played with my xmit power till I found what was optimal for my environment. I am sure you will have to do the same.

It took quite a bit of time for me to do this but using inSSIDer helped too. I actually went from 70mw to 250mw watching inSSIDer while doing so. I usually went in increments of 2mw-5mw. This took several hours scattered over weeks until I found what was good for me.

You will find the "sweet spot". It just takes some time to do so.
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phuzi0n
DD-WRT Guru


Joined: 10 Oct 2006
Posts: 10143

PostPosted: Fri Oct 23, 2009 0:57    Post subject: Reply with quote
All wifi specs have overhead and can only achieve roughly 1/2 the reported connection rate as TCP/IP throughput. You should stick to a non-overlapping channel (1, 6, 11) because your current channel 4 with 40MHz width is overlapped by everything 1-11.

Something you need to understand is that the N spec requires for 40MHz width (2x 20MHz channels) to drop back to 1x 20MHz if it detects someone is using the additional 20MHz channel. For N with 40MHz width you should start by checking what channel in the area has the highest interference from other devices in the area, hopefully this won't be channel 6 because this part of the spectrum has to be utilized for 40MHz width in the 2.4GHz spectrum. The channel with high interference is the one you need to avoid when selecting your main channel and upper/lower 2nd channel. So if you stick to non-overlapping channels like you should, there's only 4 total possible configurations:

channel 1 + 6 (channel 1 upper)
channel 6 + 11 (channel 6 upper)
channel 6 + 1 (channel 6 lower)
channel 11 + 6 (channel 11 lower)

I'm not entirely sure about this but I think that in 40MHz width, the channel you select in the UI is the main 20MHz channel number that will always be active regardless of whether anyone else is using it and the upper/lower channel is the 20MHz that will be given up if it's being used. That might be backwards though.

When testing throughput do it ~1 meter from the AP. Doing it far away/behind walls/downstairs/etc will have reduced rates and is a different problem to solve.

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givehimagun
DD-WRT Novice


Joined: 09 Oct 2009
Posts: 8

PostPosted: Fri Oct 23, 2009 1:33    Post subject: Reply with quote
I tried both 1+ upper and 11+lower and couldn't quite adjust it to your recommendations. The 1+upper setting makes the channels 1+3 while the 11+lower makes it 11+9. When I tested them both and I got the same 16 mbps actual rate.

I did some random testing of my own and used just channel 1 with a 20 Mhz width and the actual transfer rates doubled to 5 MB/s, which is the most it has ever been. I tested it within a foot of my router and a room away and the result was the same...double the actual throughput!

Is there a way in DD-WRT to set the second channel when using a 40 MHz width?

I also live in a large apartment complex with about 20 routers within range of my router so I'm sure that is degrading the quality of my actual transfers.
phuzi0n
DD-WRT Guru


Joined: 10 Oct 2006
Posts: 10143

PostPosted: Fri Oct 23, 2009 2:25    Post subject: Reply with quote
The UI displays the channels sort of weird... Channels are specified by their center frequency but instead of displaying (center of main 20 MHz + center of additional 20MHz) the devs opted to display it as (center of main 20MHz + center of 40MHz). Just be aware that the spectrum of channel 6 has to be used to form a 40MHz channel.

I get the impression that you avoided setting channel 6 as the main channel because it has the highest interference? You could try channel 6 upper and channel 6 lower to see if either does any better. With that many AP's around though, you're probably best off sticking to 20MHz.

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Read the forum announcements thoroughly! Be cautious if you're inexperienced.
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givehimagun
DD-WRT Novice


Joined: 09 Oct 2009
Posts: 8

PostPosted: Fri Oct 23, 2009 3:17    Post subject: Reply with quote
Yes, that's exactly why I avoided channel 6. About 1/2 of the routers in my area are on that frequency.

I tried both 6 upper and 6 lower and they were far slower than just the 20 MHz Channel 1.

Looks like I'm stuck with going wired for the larger transfers while I'm at the apartment and gunning at 40 mbps actual throughput the rest of the times.

I am going to add all the great information you posted to the wiki so others don't have to go through the same. Thanks for the help phuzi0n.
RCShadows
DD-WRT User


Joined: 17 Aug 2008
Posts: 435

PostPosted: Fri Oct 23, 2009 4:15    Post subject: Reply with quote
givehimagun wrote:
...I also live in a large apartment complex with about 20 routers within range of my router so I'm sure that is degrading the quality of my actual transfers.


I think you have found your problem my friend...
ddwrtliukuohao
DD-WRT User


Joined: 07 Jul 2008
Posts: 133

PostPosted: Mon Nov 02, 2009 13:15    Post subject: Reply with quote
phuzi0n wrote:
All wifi specs have overhead and can only achieve roughly 1/2 the reported connection rate as TCP/IP throughput. You should stick to a non-overlapping channel (1, 6, 11) because your current channel 4 with 40MHz width is overlapped by everything 1-11.

Something you need to understand is that the N spec requires for 40MHz width (2x 20MHz channels) to drop back to 1x 20MHz if it detects someone is using the additional 20MHz channel. For N with 40MHz width you should start by checking what channel in the area has the highest interference from other devices in the area, hopefully this won't be channel 6 because this part of the spectrum has to be utilized for 40MHz width in the 2.4GHz spectrum. The channel with high interference is the one you need to avoid when selecting your main channel and upper/lower 2nd channel. So if you stick to non-overlapping channels like you should, there's only 4 total possible configurations:

channel 1 + 6 (channel 1 upper)
channel 6 + 11 (channel 6 upper)
channel 6 + 1 (channel 6 lower)
channel 11 + 6 (channel 11 lower)


I'm not entirely sure about this but I think that in 40MHz width, the channel you select in the UI is the main 20MHz channel number that will always be active regardless of whether anyone else is using it and the upper/lower channel is the 20MHz that will be given up if it's being used. That might be backwards though.

When testing throughput do it ~1 meter from the AP. Doing it far away/behind walls/downstairs/etc will have reduced rates and is a different problem to solve.


Dear phuzi0n,

Referring the letters in red colour.

Would you able to write down step by step how to
configure say channel 1 + channel 6 in channel
bonding?


Last edited by ddwrtliukuohao on Mon Nov 02, 2009 13:20; edited 1 time in total
ddwrtliukuohao
DD-WRT User


Joined: 07 Jul 2008
Posts: 133

PostPosted: Mon Nov 02, 2009 13:19    Post subject: Reply with quote
phuzi0n wrote:
The UI displays the channels sort of weird... Channels are specified by their center frequency but instead of displaying (center of main 20 MHz + center of additional 20MHz) the devs opted to display it as (center of main 20MHz + center of 40MHz). Just be aware that the spectrum of channel 6 has to be used to form a 40MHz channel.

I get the impression that you avoided setting channel 6 as the main channel because it has the highest interference? You could try channel 6 upper and channel 6 lower to see if either does any better. With that many AP's around though, you're probably best off sticking to 20MHz.


Dear phuzi0n,

Referring the letter in green colour.

So, if I am using channel 1 and channel 3 for channel
bonding. Does this means that channel 6 is used as
well.
phuzi0n
DD-WRT Guru


Joined: 10 Oct 2006
Posts: 10143

PostPosted: Tue Nov 03, 2009 1:03    Post subject: Reply with quote
ddwrtliukuohao wrote:
phuzi0n wrote:
The UI displays the channels sort of weird... Channels are specified by their center frequency but instead of displaying (center of main 20 MHz + center of additional 20MHz) the devs opted to display it as (center of main 20MHz + center of 40MHz). Just be aware that the spectrum of channel 6 has to be used to form a 40MHz channel.

I get the impression that you avoided setting channel 6 as the main channel because it has the highest interference? You could try channel 6 upper and channel 6 lower to see if either does any better. With that many AP's around though, you're probably best off sticking to 20MHz.


Dear phuzi0n,

Referring the letter in green colour.

So, if I am using channel 1 and channel 3 for channel
bonding. Does this means that channel 6 is used as
well.

DD-WRT displays it somewhat deceptively. As I explained the first channel it displays is the center of the main 20MHz channel but the second number is the center of the 40MHz channel (using 20MHz channel numbers though) rather than the center of the second 20MHz channel. The four combinations that I stated and which you highlighted are pure 20MHz equivalents for easier understanding. ie. DD-WRT will displays channel 1+3 when you select "channel 1 upper" but the 20MHz channels that the 40MHz channel incorporates are channel 1 and 6.

_________________
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ddwrtliukuohao
DD-WRT User


Joined: 07 Jul 2008
Posts: 133

PostPosted: Tue Nov 03, 2009 8:07    Post subject: Reply with quote
Dear phuzi0n,

Thanks for your reply.
Very Happy

I have attached a graph that I drew of what I
understand about using:

channel bonding in

channel 1 + 3 or channel 1 upper

The centre frequency of 20MHz is Channel 1 =
2412 MHz

The centre frequency of the 40MHz is Channel =
2422 MHz


Once you see the graph, is that why channel 6 is
being used if channel bonding is enabled- due to
overlapping bleeding frequencies from the 40MHz band?

Note:
The solid purple/blue line on the graph is the 40MHz
channel bonding.

The dotted blue line is channel 6 in 20MHz bandwidth

Sorry for the huge pic attached. Sad

Thanks again.



channelbonding.jpg
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channelbonding.jpg




Last edited by ddwrtliukuohao on Tue Nov 03, 2009 10:39; edited 3 times in total
phuzi0n
DD-WRT Guru


Joined: 10 Oct 2006
Posts: 10143

PostPosted: Tue Nov 03, 2009 8:30    Post subject: Reply with quote
Yeah that looks correct as far as I understand it at least. The "20 MHz wide" channels are actually 22MHz wide and spaced 5MHz apart. I haven't studied the true width of "40MHz wide" bonded channels so I'm not exactly sure where they end. It may actually be something more akin to "1+5" though it won't change the fact that channel 6 will always overlap "40MHz wide" channels.

Wikipedia has a good illustration of the 2.4GHz 802.11 channels. As soon as I figure out exactly what frequency range bonded channels use I'm going to create a picture along with a small guide.

http://en.wikipedia.org/wiki/File:2.4_GHz_Wi-Fi_channels_%28802.11b,g_WLAN%29.png

_________________
Read the forum announcements thoroughly! Be cautious if you're inexperienced.
Available for paid consulting. (Don't PM about complicated setups otherwise)
Looking for bricks and spare routers to expand my collection. (not interested in G spec models)
ddwrtliukuohao
DD-WRT User


Joined: 07 Jul 2008
Posts: 133

PostPosted: Tue Nov 03, 2009 10:45    Post subject: Reply with quote
Dear phuzi0n,

Here is the link about 20MHz or 22MHz bandwidth:

http://wifijedi.com/2009/01/25/how-stuff-works-channel-bonding/

See the last pic at the bottom attached.
:D

BTW, do you have a link where can I read about to
prove your facts that channel bonding using channels:
1+3 really uses the frequencies of channel 6?


:o

Here is another link from Intel to back up your
theory about channel bonding:

http://www.intel.com/support/wireless/sb/CS-025343.htm

See the first pic attached

Thanks!
Very Happy



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phuzi0n
DD-WRT Guru


Joined: 10 Oct 2006
Posts: 10143

PostPosted: Tue Nov 03, 2009 11:59    Post subject: Reply with quote
ddwrtliukuohao wrote:
BTW, do you have a link where can I read about to
prove your facts that channel bonding using channels:
1+3 really uses the frequencies of channel 6?

I don't know what more proof you're looking for? The wikipedia pic that illustrates the wifi channels shows their middle frequency and spacing. From that you can see adding 20MHz to the upper side of channel 1 will overlap at least until channel 9. Your links also confirm the fact that only one wide channel will fit in the 2.4 GHz spectrum.

As I said before, I'm not entirely sure that channel 1+6 is really an accurate way to describe it either and that it may be more like 1+5.

_________________
Read the forum announcements thoroughly! Be cautious if you're inexperienced.
Available for paid consulting. (Don't PM about complicated setups otherwise)
Looking for bricks and spare routers to expand my collection. (not interested in G spec models)
ddwrtliukuohao
DD-WRT User


Joined: 07 Jul 2008
Posts: 133

PostPosted: Wed Nov 04, 2009 5:40    Post subject: Reply with quote
phuzi0n wrote:
ddwrtliukuohao wrote:
BTW, do you have a link where can I read about to
prove your facts that channel bonding using channels:
1+3 really uses the frequencies of channel 6?

I don't know what more proof you're looking for? The wikipedia pic that illustrates the wifi channels shows their middle frequency and spacing. From that you can see adding 20MHz to the upper side of channel 1 will overlap at least until channel 9. Your links also confirm the fact that only one wide channel will fit in the 2.4 GHz spectrum.

As I said before, I'm not entirely sure that channel 1+6 is really an accurate way to describe it either and that it may be more like 1+5.


Dear phuzi0n,

Sorry being pesky.

I am just asking only.

Thanks.
Very Happy
KelleyCook
DD-WRT Novice


Joined: 24 Nov 2009
Posts: 17

PostPosted: Tue Nov 24, 2009 22:15    Post subject: Reply with quote
phuzi0n wrote:
http://en.wikipedia.org/wiki/File:2.4_GHz_Wi-Fi_channels_%28802.11b,g_WLAN%29.png


Except for the fact that illustration that was lifted out of the eBook made for our OLPC was not to scale.

Someone converted it to SVG and then I actually fixed it a few weeks ago:

http://commons.wikimedia.org/wiki/File:2.4_GHz_Wi-Fi_channels_%28802.11b,g_WLAN%29.svg

And the answer everyone has kind of figured out is mostly correct. The channel you choose is what 20Mhz clients (G, B or N clients with 40Mhz not enabled ) will communicate at. The choice of upper or lower will also effectively take over the channel at either +4 (upper) or -4 (lower) away. But the center of the 40Mhz channel frequency is actually +2 or -2 away.

So "1 upper", it will use channel 1 as a primary and also reserving channel 5 of the 20Mhz bands. When communicating at 40Mhz, that will actually be at a center of 3.

Now "5 lower" is very similar, but it will use channels 5 as a primary and also use 1 of the 20Mhz bands. When communicating at 40Mhz, it will still be at a center of 3.

I'll code up the DD-WRT wiki table tonight or tomorrow.
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