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Ping of AP from Laptop slow on battery

Hi,

I have an HP Laptop (Compaq nc6120) which has integral Intel Centrino wireless and I also have a Cisco 857W Router.

OS is Windows XP SP2. I am using WPA TKIP.

On one occasion I noticed that the wireless performance was poor and a set the dot11 interface as follows:- speed basic-1.0 basic-2.0 basic-5.5 6.0 9.0 basic-11.0 basic-12.0 basic-18.0 basic-24.0

The PC is 10 feet from the AP, line of sight is unobstructed.

The issue:-

I have now noticed that the ping time to the router over the wireless link was sometimes concerningly slow.

I have now noticed that the different behaviours occur depending on whether the Laptop is connected to the mains or running off battery.

Here is what I see:- The #### indicate that the power lead was removed or re-connected. I flagged a couple of interesting reports with #.

Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 #### Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=12ms TTL=255 Reply from 172.18.46.1: bytes=32 time=16ms TTL=255 Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 # Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=17ms TTL=255 Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 # Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=18ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 #### Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255 Reply from 172.18.46.1: bytes=32 time=1ms TTL=255

I have tested pinging to 127.0.0.1 and the address of the PC's wireless card 172.18.46.201.

In these cases there is no change to the reported RTT which is

Reply to
anybody43
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Check the WiFi card in the Device Manager for a power management tab, or something in details for power use...

Reply to
William P.N. Smith

William P. N. Smith wrote:-

Thanks. There were settings there.

Still don't understand it though. Will check out Intel.

Interestingly the phenomenon seems to have gone away while I am doing a 100M download from the internet.

Reply to
anybody43

802.11's power-saving mode works by having the mobile station's receiver stay in a shutdown / "sleep" state during idle periods, waking up only at set intervals (typically every 10-20ms) to check for an incoming packet. Its wake interval is synchronized with the access point, which holds any packets destined for a sleeping client until the next wake-up cycle.

Once a steady data transfer is underway, the mobile receiver will remain powered on full-time until the link becomes idle idle again (for a configurable period of time, e.g. 100ms). So, bulk data tranfers aren't affected much, but interactive/sporadic communication sees higher latency, like you noticed with the ping test.

I tried this for a while with my laptops, but thought the battery life savings, at less than 10%, wasn't worth the extra lag in ssh & telnet session. PDAs, with their lower power budget proably see more of a benefit...

It sounds like your laptop enables Wifi power-saving automatically, whenever it's on battery, but this should be overridable in the Control Panel settings.

Reply to
Jordan Hazen

Jordan said:-

Thanks, I guess that I had just about enough experimental data to work that out for myself however I had never heard of a power saving mode.

Any recommended books on 802.11 that actually tell you how it works?

Reply to
anybody43

snipped-for-privacy@aug.com (Jordan Hazen) hath wroth:

A bit more detail on how the power save mode works: |

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infrastructure mode, power saving is coordinated with the beacon interval. I've found that it sometimes fails to function if one turns off SSID broadcast in the wireless access point. Enabling flow control also does weird things. Some access points have a minimal packet buffer, which promptly overflows when a clients power save bit is set. The AP has to buffer packets while the client is dozing. When I set my BEFW11S4 wireless router to beacon once per second instead of the usual 10 times per second, large file downloads began slowing down or hanging as the card (WG511) would go to doze mode between beacons. Lots of ways to do it wrong or screw it up.

From: |

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Decrease in throughput. Keep in mind that to achieve significant battery savings using sleep mode, you have to be willing to live with extremely low throughput. Some applications that require frequent communications with the clients will not operate well with power saving turned on.

Methinks the above is generally correct. One would logically expect the power save mode to be disabled temporarily for large file transfers. I don't think all cards do this.

I know that Intel 2200BG with Proset 10.1 does it right. If you sniff the wireless traffic, the power save bit in the management frames is reset to 0 (off) when traffic gets heavy, and returns back to 1 (power save on) when things slow down. I vaguely recall the response time is about 4 seconds.

Reply to
Jeff Liebermann

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