Changing modes and capabilities in ModemManager
I don’t really recall when my first interaction with ModemManager was, I just remember saying “wait, this modem just works?”. But I do remember one day when I spent a couple of hours trying to understand why my modem wouldn’t switch to 2G-only mode even if I explicitly selected it in the network-manager-applet. Truth be told, I didn’t dig much in the issue that day; I barely knew what ModemManager was, or how it would interface with NetworkManager, or how to really debug it. And here I am possibly 4 years after that day, trying to get that same issue fixed.
ModemManager has always allowed to specify which ‘network type‘ to use; or, rather than network type, ‘allowed and preferred‘ modes as we name them. It basically is a way to tell your modem that you want to use one technology preferred over another (e.g. allow 2G and 3G, but prefer 3G), or even tell the modem to use only one technology type (e.g. 3G only). Even modern phones allow you to turn off 3G support and only use 2G, in order to save battery. The main problem with ModemManager’s way of handling this issue was that there was a predefined set of combinations to select (as exposed by the applet), and that not all combinations are supported by all modems. Even worse, ModemManager may not know how to use them, or the modem itself may not support mode switching at all (which was actually what was happening with my modem 4 years ago). Therefore, the UI would just try to show all the options and hope for the best when launching the mobile broadband connection. And there it comes the next issue; allowing to select mode preferences during the connection setup just makes the modem restart the whole radio stack, and the connection attempt may end up timing out, as the whole network registration process needs to be done from scratch before connecting…
Allowed and Preferred modes
In the new ModemManager interfaces, each Modem object will expose a “SupportedModes” property listing all the mode combinations (allowed + preferred) the modem actually supports. Graphical user interfaces will therefore be able to provide mode switching options listing only those combinations that will work. If a modem doesn’t support mode switching, no such list should be provided, and the user will not get confused. At any time, the Modem object will also expose a “CurrentModes” property, showing which is the currently selected combination of allowed and preferred modes. And the “SetCurrentModes()” method will allow to switch current modes, accepting as input only combinations which are given in “SupportedModes” (or the special allowed=ANY and preferred=NONE).
Also, changing current modes directly when calling Simple.Connect() will no longer be possible. This means that NetworkManager will never request allowed mode switching during a connection attempt (and hence no radio stack reloading in the modem causing timeouts). The logical place to put allowed mode switching is therefore a system configuration application like the GNOME Control Center or similar, which should allow mode switching at any time, not only just during a connection attempt. A good side effect of this change is that the NetworkManager connection settings now contain only connection-related configuration, which in the case of 3GPP devices can be linked to the SIM in use, leaving out all modem-specific configuration.
There was a time when modems were either 3GPP (GSM/GPRS/UMTS/HSPA…) or 3GPP2 (CDMA/EV-DO…). Nowadays, modems with multiple capabilities are pretty common, specially since LTE is around (LTE, even if 3GPP, is also 3GPP2’s blessed 4G technology, instead of the superhero named one which is forgotten by everyone already). ModemManager will now allow to change capabilities in addition to allowed and preferred modes; so a user with a modem which can work both in 3GPP and 3GPP2 networks will be able to switch from one to the other directly from the user interface. Of course, if the modem supports this (currently only QMI-based modems).
The new “SupportedCapabilities” property will expose all capability combinations supported by the modem, while “CurrentCapabilities” will expose which are the current ones being used at any given time. For example, a modem with “gsm-umts”, “cdma-evdo” and “lte” capabilities may support configuring only “cdma-evdo”, or “gsm-umts”+”lte”. Changing current capabilities is now possible through the “SetCurrentCapabilities()” method, which has a logic very similar to that of the “SetCurrentModes()” method. If a modem supports multiple capability combinations as exposed by “SupportedCapabilities”, this method will allow changing between them. The main difference with mode changing is that we will force a device power-cycle when this change is done, so the modem will disappear and reappear again with the new capabilities.
Capabilities and allowed/preferred modes have a lot in common, so much that there is a single interface in QMI based modems to change them. Therefore, when a modem allows changing capabilities, the list of allowed/preferred mode combinations may (and very likely will) be different depending on the current capabilities in the modem. For example, LTE-enabled QMI-powered modems will not be able to switch allowed/preferred modes when they have “lte” among the current capabilities, but they will be able if the capabilities are changed to only “gsm-umts”. This is not a big deal, as mode preference (e.g. 3G preferred) is not applicable when the modem does LTE (there is no way of saying allow 2G, 3G and 4G but prefer 3G).
ModemManager also allows to specify which frequency bands to use in the modem, but unlike with modes and capabilities, the “SupportedBands” property is not a list of all possible band combinations supported. Instead, it’s just a bitmask with all supported bands, without specifying whether an actual combination is going to work in “SetCurrentBands()” or not. Listing combinations instead of just the bitmask would be truly too much… But anyway, changing frequency bands is not a feature that a normal user should play with, so just don’t do it. I actually bricked a Pantech UML290 myself playing with this…
All these updates, plus some other ones, are available in the ‘aleksander/api-breaks‘ branch in the ModemManager git repository, which should hit git master very soon, likely this week. These ones should be the last API breaks done before releasing the new ModemManager, and will be kept stable after that.