Wifi power saving mode
(added a bit more explanation about the PSM implementation in tablets, as well as reference to longer explanation) |
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Ad-hoc mode cannot use power-saving modes other than reduce transmit power - since there's no master or slave where the slave can spend a lot of time "asleep", it prevents power saving modes. | Ad-hoc mode cannot use power-saving modes other than reduce transmit power - since there's no master or slave where the slave can spend a lot of time "asleep", it prevents power saving modes. | ||
| + | == Routers known to be incompatible with PSM mode == | ||
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| + | * Fon2200 | ||
[[Category:Users]] | [[Category:Users]] | ||
[[Category:Connectivity]] | [[Category:Connectivity]] | ||
[[Category:WiFi]] | [[Category:WiFi]] | ||
Revision as of 13:59, 16 August 2009
The tablets support what is called Wifi Power Saving Mode (PSM). PSM allows the tablet to switch its wifi radio on and off several times a second to save battery. While this does greatly improve battery life (6-8 hours idle on Wifi without PSM, 3-4 days with), there are still many routers on the market which do not support PSM or have a buggy implementation and this can cause problems for the tablets.
Because using PSM introduces extra latency for incoming packets, the tablets try to optimize performance by remaining in Constant Active Mode (CAM) if there is constant traffic. On the other hand, CAM consumes a lot of power, so the timeout should be as short as possible. For more detailed description, see this posting on the subject.
Contents |
Issues
Though PSM does greatly increase idle battery life (meaning leaving the tablet connected to a Wifi network at all times is a reality), poor support in some routers and certain network issues can crop up that cause trouble. In the case of routers with non-existent or buggy support, battery life while connected to Wifi may be greatly reduced or, at worst, connection difficulties may arise.
PSM can cause some network issues even on routers with good support. Particularly with PC-to-tablet stuff like SSH, connections may be dropped while idle or lag severely. There are two ways to reduce the negative impact of aggressive PSM in these situations. Either have an active network process running on the tablet (XChat in an active channel, ping to google.com, etc), or disable PSM completely on connections where you'll be doing a lot of work over SSH.
The default values were tightened in Diablo release, which might be the reason that some people started seing problems then. The problems with PSM are typically random in nature, so with a bit of luck the connection works just fine sometimes, but sometimes problems are seen. The probability depends on the used timeout values.
Configuration
PSM can be configured on a per-connection basis in the Connectivity control panel (Application menu -> Settings -> Control panel -> Connectivity). Under the Other tab of Advanced settings when editing a saved connection. There are 3 options available: On (Maximum), On (Intermediate), and Off (note, the WLAN transmission power setting will not significantly impact battery life—though it will impact Wifi roaming distance—and is simply there for legislative reasons). Maximum and Intermediate change the timeout period (200ms and 1000ms, respectively), and Off turns off PSM entirely.
Ad-hoc prevents PSM
Ad-hoc mode cannot use power-saving modes other than reduce transmit power - since there's no master or slave where the slave can spend a lot of time "asleep", it prevents power saving modes.
Routers known to be incompatible with PSM mode
- Fon2200