Power management observations with my XV6800

As I previously mentioned, I recently switched to a full-blown Windows Mobile-based phone. One of the interesting conundrums that I’ve ran into along the way is feeling out what’s actually killer on the battery life for the device and what isn’t.

It’s actually kind of surprising how seemingly innocent little things can have a dramatic impact on battery life in a situation like an always-on handheld phone. For example, one of the main power saving features of modern CDMA cell networks and packet switched data connections is the ability for the device to go into dormant mode, a state in which the device relinquishes it’s active radio link resources and essentially goes into a standby mode that is in some respects akin to how the device operates when it is otherwise idle and waiting for a call. This is beneficial for the device’s battery life (because while in dormant mode, much of the logic related to maintaining the active high-rate radio link can be powered off while the network link is idle). Dormant mode is also beneficial for network operators, in that it allows the resources previously dedicated to a particular device to be used by other users that are actively sending or receiving data.

When data is ready to be sent on either end of the link, the device will “wake up” and exit dormant mode in order to send/receive data, keeping the link fully active until the session has been idle for enough time for the dormant idle timer to expire. (Entering and exiting dormant mode is mostly seamless, but may involve delays of 1-2 seconds, at least in my experience, so doing so on every packet would be highly undesirable.)

What all of this means is that there’s some significant gains to be had in terms of radio power consumption if network traffic can be limited to where it is really necessary. There are a couple of things that I had initially running on my device that didn’t really fit this criteria. For example, I have an always-active SSH connection to a screen session that, among other things, runs my console SILC client, which is based off of irssi (an IRC client). One gotcha that I encountered there is that in the default configuration, this client has a clock on the ncurses console UI. The clock is updated every minute with the current time, which is (normally) handy as you can at-a-glance compare the current timestamp with message timestamps in the backlog.

However, this becomes problematic in my case, because the clock forces the device out of dormant mode every minute in order to repaint that portion of the console UI. (Aside from the clock, the remainder of the SILC/irssi UI is static until there is actual chat activity, which would otherwise make it relatively well suited for this situation.) Fortunately, it turned out to be relatively easy to remove the clock from the client’s UI, which prevents the SSH session from causing network activity every minute while I’ve got the SILC client up.

Another network power management “gotcha” that I ran into was the built-in L2TP/IPsec VPN client. It turns out that while an L2TP/IPsec link is active, the device was, in my case, unable to enter dormant mode for any non-trivial length of time. I suspect that this is caused by the default behavior of L2TP to send keep-alive messages every minute to ensure that the session is still alive (at the very least, packet captures VPN-server-side did not seem to indicate any traffic destined to the device’s projected IP Address).

In any case, this unpleasant side effect of the L2TP/IPsec client ruled out using it for full network connectivity in an always-on fashion. Unfortunately, dialing the VPN link on-demand has it’s own share of pitfalls, as the device seems to make the VPN link the default gateway. When I configured IMAPv4 mail to be fetched every so often via demand-dialing the VPN link (which would alleviate the difficulty with dormant mode not operating as desired with the VPN link always on), my SSH session would often die if either side happened to try and send data while Pocket Outlook was polling IMAP mail.

My solution in this case was to fall back to using SSH port-forwards through PocketPuTTY for IMAP mail (after I fixed SSH port forwards to work reliably, that is, of course). Unlike the L2TP/IPsec link, the SSH link doesn’t inherently force the device out of dormant mode without there being real activity, which allows me to continue to leave the SSH connection always on without sacrificing my battery life.

These two changes seem to have fixed the worst offenders in terms of battery drain, but there’s still plenty of room for improvement. For example, PocketPuTTY will tell the remote end of the session that the window has resized when you switch the device from portrait to landscape mode (or vice versa), such as when pulling out the device’s built in keyboard. While this is a very handy feature if you’re actually using the SSH session (as it allows the remote console to resize itself appropriately, which is how SILC/irssi magically reshapes itself to fit the screen when the device keyboard is engaged), it does present the annoyance that pulling out the keyboard while PocketPuTTY is not active will still cause the packet radio link to be fully established to send the terminal resize notification and receive the resulting redraw data.

This behavior could, for instance, be further optimized to defer sending such notifications until the PocketPuTTY window becomes the foreground window (potentially aborting the resize notification entirely if the screen size is switched back to how it previously was before PocketPuTTY is activated).

One of the nice things about having a (relatively) easily end-user-programmable device instead of a locked-down, DRM’ified BREW-based handset is that there’s actually opportunity to change these sorts of things instead of just blithely accepting what’s offered with the device with no real hope of improving it.

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