Thinkpads Forum

Hi.

I have a T61 and discovered that wireless networking at home was very slow. Here's what I discovered and how I worked around the problem.

Hardware/Software:

My machine is a Lenovo T61 7663-AN5, running Windows XP SP2. The wireless adapter is an Intel 4965AG and I have the latest wireless driver 11.1.1.16. The T61's BIOS has also been upgraded to the latest version as at the date of this post.

I have a NetGear MR814 802.11b wireless/DSL router with the latest firmware installed (v4.14RC4).

Symptoms:

Remote web-sites rendered very slowly. Websites on the local network seemed to render ok.

I installed Wireshark and Firebug and noticed that some web requests were taking seconds, if not minutes to complete, if they completed at all.

According to Wireshark, the apparent cause of this is that some (but not all) inbound TCP segments of 1400 bytes in length were being dropped and required TCP retransmission and so this caused a typical HTTP request/response cycle to take much longer than normal.

I then experimented with ping and discovered that the maximum ping size I could send to a remote site was 1464 (framesize 1506).

When I was connected to my router via a wireless connection, pings of this size would be regularly dropped, though the occasional one would get through. I also discovered that if I swapped the wireless connection with an ethernet connection, all pings of size 1464 were reliably replied to.

The fact that pings of size 1464 via the ethernet link worked whereas pings via the wireless link were usually dropped indicates to me that the routing on the Internet side of the ADSL connection is _not_ the factor in my unreliable wireless link.

I also noted that pings via the wireless interface of size 512 were far more reliable.

Per the advice in http://support.microsoft.com/kb/900926 I located the interface registry key which corresponded to the wireless adapter and adjusted the MTU size to be the smallest possible (576). (According to wireshark, the implicit default was 1460).

With this change, the reliability and apparent speed of my wireless routed web connections improved considerably.

Does anyone have a better solution to problems of this kind?

Yeah, you really don't want to set your MTU size to the smallest value possible, but rather tweak it so that it has the maximum value to prevent packet loss. In the linked FAQ, look at 'Max MTU: How do I find mine?'

http://www.dslreports.com/faq/tweaks/4._MTU#695

True. Thanks for the link.

I played around with larger MTU sizes and was able to push it slightly higher without incurring significant packet loss, but not much. I settled eventually on an MTU size of 658.

Any thoughts on why I needed to do this for the T61? I had a T43 previously which worked fine on this wireless network with "default" settings.

Are the T61 defaults (such as receive window size) optimized for a faster wireless network (say, a 54Mbps 802.11g) thereby causing problems when the device is attached to a slower network such as an 11Mbps 802.11b?

I know some routers had problems with mixed b and g environments, but you'd assume that firmware updates would fix that. You really need a new router if it is 802.11b only. The T61 should function in a 'b' or a mixed b/g network.

jon.seymour wrote:I have a NetGear MR814 802.11b wireless/DSL router with the latest firmware installed (v4.14RC4).

Have you tried setting your 4965AGN card to "b only" mode (under the Advanced tab in the Device Manager "Properties" display).

If performance comes good when you do that, you probably really do need to get a new router.

Cheers,

Bill B.

Thanks Bill - yes, I did try that but it didn't seem to have much effect.

My router is certainly getting a bit old, so I may well retire it and get a faster router anyway.

FWIW: the wireless adapter works find with an 802.11g network and default (e.g. blank) MTU size settings.

Regards,

jon.