Archive for December, 2007

Power management observations with my XV6800

Tuesday, December 11th, 2007

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.

How to not write code for a mobile device

Monday, December 10th, 2007

Earlier this week, I got a shiny, brand-new XV6800 to replace my aging (and rather limited in terms of running user-created programs) BREW-ified phone.

After setting up ActiveSync, IPsec, and all of the other usual required configuration settings, the next step was, of course, to install the baseline minimum set of applications on the device to get by. Close to the top of this list is an SSH client (being able to project arbitrary console programs over SSH and screen to the device is, at the very least, a workable enough solution in the interm for programs that are not feasible to run directly on the device, such as my SILC client). I’ve found PuTTY a fairly acceptable client for “big Windows”, and there just so happened to be a Windows Mobile port of it. Great, or so I thought…

While PocketPuTTY does technically work, I noticed a couple of deficiencies with it pretty quickly:

  1. The page up / page down keys are treated the same as the up arrow / down arrow keys when sent to the remote system. This sucks, because I can’t access my scrollback buffer in SILC easily without page up / page down. Other applications can tell the difference between the two keys (obviously, or there wouldn’t be much of a point to having the keys at all), so this one is clearly a PocketPuTTY bug.
  2. There is no way to restart a session once it is closed, or at least, not that I’ve found. Normally, on “big Windows”, there’s a “Restart Session” menu option in the window menu of the PuTTy session, but (as far as I can tell) there’s no such equivalent to the window menu on PocketPuTTY. There is a “Tools” menu, although it has some rather useless menu items (e.g. About) instead of some actually useful menu items, like “Restart Session”.
  3. Running PocketPuTTY seems to have a significantly negative effect on battery life. This is really unfortunate for me, since the expected use is to leave an SSH session to a terminal running screen for a long period of time. (Note that this was resolved, partially, by locating a slightly more maintained copy of PocketPuTTY.)
  4. SSH port forward support seems to be fairly broken, in that as soon as a socket is cleaned up, all receives in the process break until you restart it. This is annoying, but workable if one can go without SSH port forwards.

Most of these problems are actually not all that difficult to fix, and since the source code is available, I’m actually planning on trying my hand at doing just that, since I expect that this is an app that I’ll make fairly heavy use of.

The latter problem is one I really want to call attention to among these deficiencies, however. My intent here is not to bash the PocketPuTTY folks (and I’m certainly happy that they’ve at least gotten a (semi)-working port of PuTTY with source code out, so that other people can improve on it from there), but rather to point out some things that should really just not be done when you’re writing code that is intended to run on a mobile device (especially if the code is intended to run exclusively on a mobile device).

On a portable device, one of the things that most users really expect is long battery life. Though this particular point certainly holds true for laptops as well, it is arguably even more important that for converged mobile phone devices. After all, most people consider their phone an “always on” contact mechanism, and unexpectedly running out of battery life is extremely annoying in this aspect. Furthermore, if your mobile phone has the capability to run all sorts of useful programs on it, but doing so eats up your battery in a couple of hours, then there is really not that much point in having that capability at all, is there?

Returning to PocketPuTTY, one of the main problems (at least with the version I initially used) was, again, that PocketPuTTY would reduce battery life significantly. Looking around in the source code for possible causes, I noticed the following gem, which turned out to be the core of the network read loop for the program.

Yes, there really is a Sleep(1) spin loop in there, in a software port that is designed to run on battery powered devices. For starters, that blows any sort of processor power management completely out of the water. Mobile devices have a lot of different components vying for power, and the easiest (and most effective) way to save on power (and thus battery life) is to not turn those components on. Of course, it becomes difficult to do that for power hungry components like the 400MHz CPU in my XV6800 if there’s a program that has an always-ready-to-run thread…

Fortunately, there happened to be a newer revision of PocketPuTTY floating about with the issue fixed (although getting ahold of the source code for that version proved to be slightly more difficult, as the original maintainer of the project seems to have disappeared). I did eventually manage to get into contact with someone who had been maintaining their own set of improvements and grab a not-so-crusty-old source tree from them to do my own improvements, primarily for the purposes of fixing some of the annoyances I mentioned previously (thus beginning my initial forey into Windows CE development).

Mini USB charging for devices is a great idea

Thursday, December 6th, 2007

One recent innovation that I recently stumbled across is the use of mini USB ports for power. I actually originally encountered a device doing this with the Morotola HT820 Bluetooth headphones I got some months ago, although at the time, I didn’t realize that the power brick that came with the device was mini USB and not yet another proprietary power cable connection.

For those unaware, mini USB is a small form factor of USB plug (otherwise essentially identical to standard USB). As with regular USB, mini USB devices can be off the bus. Now, standardizing on mini USB for power connectors has some really nice advantages:

  1. Everyone’s power bricks are now compatible. Normally, each gadget you own is going to have its own (incompatible) power transformer brick, which means that if you’re travelling, you’ll quite likely have to lug around several such power bricks (in addition to your laptop AC adapter), even if every device doesn’t really need to be plugged into “wall power” at the same time.
  2. You can charge these devices off of a laptop USB port. Of course, you’ll need a USB to mini USB cable, assuming your laptop doesn’t have any mini USB ports (I haven’t seen any that do). The really cool thing about being able to do this is that for all your mini USB, battery powered devices, you only need to bring one cable for all of them while on the move, since you can just plug the device into your laptop and charge it off of that.

Furthermore, because the connection is still USB, devices can use the same port to charge and transfer data to a computer at the same time.

Now, I didn’t really put all the pieces together about my HT820 being mini USB until I had to go and buy a replacement set of headphones (a different model this time, as Best Buy didn’t carry the HT820 anymore). The volume button on my set had died, which was rather inconvenient, to say the least.

Anyways, I noticed that the new headphones, from a different manufacturer, seemed to have a compatible power cable port from the AC adapter brick to the device’s charging port. This time, the power cable even had the ubiquitous USB logo on it (the Motorola’s charging cable didn’t, for one reason or another). Sure enough, I could use the Motorola charger with the new set of headphones, even though they weren’t of a Motorola make. Cool.

Recently, I finally ditched my old cell phone for a proper smart phone (an XV6800). This, too, is mini USB powered (and it can use the mini USB connection for data as well, at the same time). There’s a lot of other neat things about the XV6800, but the fact that I don’t need to (ever) worry about spare chargers or anything of that sort is really just the icing on the cake.

Thanks to this little advancement, I can now ditch two additional power bricks (one for the cell phone and one for the Bluetooth headset) when traveling, and just charge both of devices off of my laptop. I’m actually kind of surprised that nobody implemented this earlier, given the immediate advantages of standardizing on a uniform power source (especially one that is as easily multiplexed as USB is).

So, the next time you’re looking for a new gadget of some sort or another, look for one chargeable via mini USB and circumvent the gadget charger nightmare (or at least, begin to do so, one gadget at a time).