[Cialug] Slightly OT, .. WiFi Latency

[Zachary Kotlarek]

On Aug 22, 2012, at 7:25 PM, L. V. Lammert wrote:

> It 'feels' to me like it takes longer to establish a connection over
> WiFi, .. some of the notes I find online indicate that is fact, but I
> don't see any explanation.


Without seeing the WiFi frames it's just a guess, but it could be power-saving mode on the client WiFi. It's not uncommon for them to enter a low-power mode, where they do *not* listen continuously for packets; typical sleep periods are on the order of 300ms (3 x 100 ms beacon interval), but that number can vary a lot depending on the client and AP config.

If that's the problem you should be able to either disable sleep entirely on the client, or increase the sleep-idle interval so it doesn't impact your normal operations, or even just set another host to continuously ping/etc. the client so is never able to sleep.

Technical details of WiFi power-saving:
Clients can choose to enter power-saving polling-mode (PSPM) for any or no reason, and can theoretically always operate in this mode (though most Ethernet-replacement systems do not). When entering sleep the client announces to the AP that it's going to stop listening for the next X beacon intervals with a PSPM frame. The AP acknowledges, then the client powers down and the AP caches any packets bound for the client. At the expiration of the period the client wakes up listens while the AP sends a beacon frame that includes a Traffic Indicator Map (TIM) which specifies whether there are pending packets for sleeping clients. If there's no pending traffic the client goes back to sleep and the cycle repeats. If there are cached packets the client can either request the release of a specific amount of data, after which the sleep cycle resumes, or can re-enter normal full-power mode where it listens continuously.

	Zach

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