ModemManager provides a unified high level API for communicating with (mobile broadband) modems. While the basic commands are standardized, the more advanced operations (like signal quality monitoring while connected) varies a lot. ModemManager is a system daemon and is not meant to be used directly from the command line. However, a command line client (mmcli) is provided, which may be used to test the different functionality provided during plugin development. ModemManager is a DBus system bus activated service (meaning it's started automatically when a request arrives). It is written in C. The devices are queried from udev and automatically updated based on hardware events. There are DBus-interface specific GInterfaces, which should be implemented by any device specific implementation. There is a generic MMBroadbandModem implementation that provides a generic implementation of the most common operations in both GSM and CDMA modems. Plugins are loaded on startup, and must implement the MMPlugin interface. It consists of a couple of methods which tell the daemon whether the plugin supports a port and to create custom modem implementations. It most likely makes sense to derive custom modem implementations from one of the generic classes and just add (or override) operations which are not standard. Writing new plugins is highly encouraged!

ModemManager requires udev-powered Linux kernels in order to get notified of possible available Modems. udev will report each of the ports found in the device, and ModemManager will probe each of the ports marked with the ID_MM_CANDIDATE tag in udev. This includes both "tty" and "net" ports. Aditionally, users of RS232-based devices may need to request additional manual scans via DBus, in order to detect modems that may have been connected to RS232 to USB adapters. In this case, udev just knows about the USB adapter being connected, not about the RS232 modem connected to the adapter, if any.

Whenever a new device is detected by ModemManager, port probing process will get started, so that we can determine which kind of ports we have, and also which plugin we need to use for the specific device. Devices may expose one or more ports, and all of them will follow the same probing logic. The whole probing process, including pre-probing and post-probing filters, is implemented in the core ModemManager daemon. Plugins will just configure their own special needs in the probing process, so that only the steps required by the given plugin are executed. For example, plugins which do not support RS232 devices will not need AT-based vendor or product filters.

Pre-probing filters are those which control whether the probing, as requested by the specific plugin, takes place. Allowed vendor IDs Plugins can provide a list of udev-reported vendor IDs to be used as pre-probing filters. If the vendor ID reported by the device via udev is found in the list provided by the plugin, port probing will be launched as requested by the given plugin. This filter is specified by the MM_PLUGIN_ALLOWED_VENDOR_IDS property in the MMPlugin object provided by the plugin. Product IDs Plugins can provide a list of udev-reported pairs of vendor and product IDs to be used as pre-probing filters. If the vendor ID and product ID pair reported by the device via udev is found in the list of 'allowed' pairs provided by the plugin, port probing will be launched as requested by the given plugin. This additional filter should be used when the plugin is expected to work only with a given specific product of a given vendor. If the vendor ID and product ID pair reported by the device via udev is found in the list of 'forbidden' pairs provided by the plugin, port probing will not be launched by this plugin. This additional filter should be used when the plugin supports all devices of a given vendor except for some specific ones. These filters are specified by the MM_PLUGIN_ALLOWED_PRODUCT_IDS and MM_PLUGIN_FORBIDDEN_PRODUCT_IDS properties in the MMPlugin object provided by the plugin. Subsystems Plugins can specify which subsystems they expect, so that we filter out any port detected with a subsystem not listed by the plugin. This filter is specified by the MM_PLUGIN_ALLOWED_SUBSYSTEMS property in the MMPlugin object provided by the plugin. Drivers Plugins can specify which drivers they expect, so that we filter out any port detected being managed by a driver not listed by the plugin. Plugins can also specify which drivers they do not expect, so that we filter out any port detected being managed by a driver listed by the plugin. These filters are specified by the MM_PLUGIN_ALLOWED_DRIVERS and MM_PLUGIN_FORBIDDEN_DRIVERS properties in the MMPlugin object provided by the plugin. udev tags Plugins can provide a list of udev tags, so that we filter out any port detected which doesn't expose any of the given tags. This filter is specified by the MM_PLUGIN_ALLOWED_UDEV_TAGS property in the MMPlugin object provided by the plugin.

Whenever all pre-probing filters of a given plugin pass, ModemManager will run the probing sequence as requested by the specific plugin. The main purpose of the probing sequence step is to determine the type of port being probed, and also prepare the information required in any expected post-probing filter. Custom initialization This property allows plugins to provide an asynchronous method which will get executed as soon as the AT port gets opened. This method may be used for any purpose, like running an early command in the ports as soon as possible, or querying the modem for info about the port layout. This configuration is specified by the MM_PLUGIN_CUSTOM_INIT property in the MMPlugin object provided by the plugin. AT allowed This boolean property allows plugins to specify that they expect and support AT serial ports. This configuration is specified by the MM_PLUGIN_ALLOWED_AT property in the MMPlugin object provided by the plugin. Single AT expected This boolean property allows plugins to specify that they only expect and support one AT serial port. Whenever the first AT port is grabbed, any remaining AT probing in ports of the same device will get cancelled. This configuration is specified by the MM_PLUGIN_ALLOWED_SINGLE_AT property in the MMPlugin object provided by the plugin. Custom AT probing This property allows plugins to specify custom commands to check whether a port is AT or not. By default, the 'AT' command will be used if no custom one specified. This configuration is specified by the MM_PLUGIN_CUSTOM_AT_PROBE property in the MMPlugin object provided by the plugin. QCDM allowed This boolean property allows plugins to specify that they expect and support QCDM serial ports. This configuration is specified by the MM_PLUGIN_ALLOWED_QCDM property in the MMPlugin object provided by the plugin. Check Icera support This boolean property allows plugins to specify that they want to have the Icera support checks included in the probing sequence. They can afterwards get the result of the support check to decide whether they want to create a Icera-based modem object or not. This configuration is specified by the MM_PLUGIN_ICERA_PROBE property in the MMPlugin object provided by the plugin.

Post-probing filters are required to identify whether a plugin can handle a given modem, in special cases where the information retrieved from udev is either not enough or wrong. This covers, for example, RS232 modems connected through a RS232 to USB converter, where udev-reported vendor ID is that of the converter, not the one of the modem. Allowed vendor strings Plugins can provide a list of vendor strings to be used as post-probing filters. If the vendor string reported by the device via AT commands is found in the list provided by the plugin, the plugin will report that it can handle this modem. This filter is specified by the MM_PLUGIN_ALLOWED_VENDOR_STRINGS property in the MMPlugin object provided by the plugin. Product strings Plugins can provide a list of pairs of vendor and product strings to be used as post-probing filters. If the vendor and product string pair reported by the device via AT commands is found in the 'allowed' list provided by the plugin, the plugin will report that it can handle this modem. This additional filter should be used when the plugin is expected to work only with a given specific product of a given vendor. If the vendor and product string pair reported by the device via AT commands is found in the 'forbidden list provided by the plugin, the plugin will report that it cannot handle this modem. This additional filter should be used when the plugin supports all devices of a given vendor, except for some specific ones. These filters are specified by the MM_PLUGIN_ALLOWED_PRODUCT_STRINGS and MM_PLUGIN_FORBIDDEN_PRODUCT_STRINGS properties in the MMPlugin object provided by the plugin. Icera support Plugins can specify that they only support Icera-based modems, or that they do not support any Icera-based modem. When either of this configurations is enabled, the Icera support checks will be included in the probing sequence, and the result of the check will help to determine whether the plugin supports the modem or not. This filter is specified by the MM_PLUGIN_ALLOWED_ICERA and MM_PLUGIN_FORBIDDEN_ICERA properties in the MMPlugin object provided by the plugin. Plugins which require post-probing filters will always be sorted last, and therefore they will be the last ones being checked for pre-probing filters. This is due to the fact that we need to assume that these plugins aren't able to determine port support just with pre-probing filters, as we want to avoid unnecessary probing sequences launched. Also note that the Generic plugin is anyway always the last one in the list.

G_MODULE_EXPORT MMPlugin * mm_plugin_create (void) { static const gchar *subsystems[] = { "tty", NULL }; static const guint16 vendor_ids[] = { 0xabcd, 0 }; static const mm_uint16_pair product_ids[] = { { 0x1234, 0xffff } }; static const gchar *vendor_strings[] = { "vendor", NULL }; return MM_PLUGIN ( g_object_new (MM_TYPE_PLUGIN_IRIDIUM, MM_PLUGIN_NAME, "Example", /* Next items are pre-probing filters */ MM_PLUGIN_ALLOWED_SUBSYSTEMS, subsystems, MM_PLUGIN_ALLOWED_VENDOR_IDS, vendor_ids, MM_PLUGIN_ALLOWED_PRODUCT_IDS, product_ids, /* Next items are probing sequence setup */ MM_PLUGIN_ALLOWED_AT, TRUE, /* No post-probing filters */ NULL)); } G_MODULE_EXPORT MMPlugin * mm_plugin_create (void) { static const gchar *subsystems[] = { "tty", NULL }; static const gchar *vendor_strings[] = { "vendor", NULL }; static const mm_str_pair product_strings[] = { "another-vendor", "product xyz" }; return MM_PLUGIN ( g_object_new (MM_TYPE_PLUGIN_IRIDIUM, MM_PLUGIN_NAME, "Example", /* Next items are pre-probing filters */ MM_PLUGIN_ALLOWED_SUBSYSTEMS, subsystems, /* Next items are probing sequence setup */ MM_PLUGIN_ALLOWED_AT, TRUE, /* Next items are post-probing filters */ MM_PLUGIN_VENDOR_STRINGS, vendor_strings, MM_PLUGIN_PRODUCT_STRINGS, product_strings, NULL)); } static gboolean parse_custom_at (const gchar *response, const GError *error, GValue *result, GError **result_error) { if (error) { *result_error = g_error_copy (error); return FALSE; } /* Otherwise, done. And also report that it's an AT port. */ g_value_init (result, G_TYPE_BOOLEAN); g_value_set_boolean (result, TRUE); return TRUE; } static const MMPortProbeAtCommand custom_at_probe[] = { { "AT+SOMETHING", parse_custom_at }, { NULL } }; G_MODULE_EXPORT MMPlugin * mm_plugin_create (void) { static const gchar *subsystems[] = { "tty", NULL }; static const guint16 vendor_ids[] = { 0xabcd, 0 }; return MM_PLUGIN ( g_object_new (MM_TYPE_PLUGIN_EXAMPLE, MM_PLUGIN_NAME, "Example", /* Next items are pre-probing filters */ MM_PLUGIN_ALLOWED_SUBSYSTEMS, subsystems, MM_PLUGIN_ALLOWED_VENDOR_IDS, vendor_ids, /* Next items are probing sequence setup */ MM_PLUGIN_CUSTOM_AT_PROBE, custom_at_probe, MM_PLUGIN_ALLOWED_AT, TRUE, /* No post-probing filters */ NULL)); }

Once a port passes all probing filters of a given plugin, the plugin will grab the port. When the first port of a given device is grabbed, the plugin will create the required Modem object. While preparing to get the Modem object grab the specific port probed, udev-based port type hints can be used to specify AT port flags (e.g. if a port is to be considered primary, secondary or for PPP).

Once all ports of a given modem have been probed and grabbed by a newly created Modem object, ModemManager will start the global state machine for the modem, as defined in the picture below.

The state machine of a modem can be summarized in 5 main sequences: initialization, enabling, connection, disconnection and disabling.

The modem initialization sequence starts only when all ports have been probed and grabbed by a given plugin. This is done so that the proper AT port (that suggested to be Primary) is used as control port. The global initialization sequence is itself splitted into N per-interface initialization steps (being N the number of interfaces implemented by the modem object). The following list provides the steps required in the initialization sequence of a Broadband modem object. Modem interface initialization The Modem interface provides common actions and information available in the majority of the modems (including Broadband-specific items which won't be implemented by POTS modems). One of the key things done during the initialization of this interface is the checking of supported capabilities. Broadband modem objects are able to handle 3GPP-only, CDMA-only and mixed 3GPP+CDMA modems, but in order to properly handle the distinctions required in these, ModemManager first needs to know exactly which is the current set of capabilities. The other key step in this sequence involves checking the lock status of the modem and/or SIM . If the modem/SIM is found to be locked, the whole initialization sequence is halted and the modem is left in a locked state until unlocked by the user. Note, therefore, that modems that are locked will not expose additional feature-specific DBus interfaces until they get unlocked. It may be the case that some of the steps in the initialization of the Modem interface require the modem itself to be unlocked. If the modem is found locked during the first initialization attempt, as soon as it gets unlocked the initialization sequence will be re-executed. 3GPP interface initialization The 3GPP interface provides common actions and setup for modems which provide 3GPP capabilities. Therefore, this interface initialization sequence will only be run in 3GPP-enabled modems. CDMA interface initialization The CDMA interface provides common actions and setup for modems which provide CDMA capabilities. Therefore, this interface initialization sequence will only be run in CDMA-enabled modems. Additional feature-specific interface initializations Modems with additional features will export feature-specific interfaces, such as the Location or the Messaging ones. These interfaces also have their own initialization sequences, where the first step in the sequence is always the check of whether the given modem supports the given feature. In other words, modems will only end up exporting the interfaces for the features they support.

Modem enabling is the user-requested sequence with the sole purpose of bringing the modem to a state where it can get connected. As with the initialization sequence, the global enabling sequence is itself splitted into N per-interface enabling steps (being N the number of interfaces exported by the modem). Those interfaces implemented by the object but not supported by the modem will not be enabled. Modem interface enabling The sequence to enable the Modem interface takes care of different important steps, such as powering up the radio interface or configuring the best charset to use. 3GPP interface enabling Modems with 3GPP capabilities will enable the 3GPP interface as part of the global enabling sequence. This sequence involves setting up the automatic registration of the device in the network, as well as configuring 3GPP specific indicators and unsolicited message handlers. CDMA interface enabling Modems with CDMA capabilities will enable the CDMA interface as part of the global enabling sequence. This sequence involves setting up the periodic checks of registration in the CDMA network. Additional feature-specific interface enablings Each feature-specific interface will have its own enabling sequence, with operations which are directly related to the purpose of the interface. For example, enabling the Location interface will involve setting up the initial location information; and enabling the Messaging interface will involve loading the initial list of SMS available in the SIM or Modem.

Connecting the Modem is done through the Bearer objects. Once such an object is created, the user can request to get the given bearer connected. Broadband Modems will usually create Broadband Bearers. This kind of bearers can run either the CDMA connection sequence (if the modem has CDMA capabilities) or the 3GPP connection sequence (if the modem has 3GPP capabilities). For the special case of mixed 3GPP+CDMA modems, it is assumed that the plugin implementation needs to decide how the connection gets done. By default, anyway, the 3GPP sequence is used in this case. Modems which are both LTE (3GPP) and CDMA can hand over from LTE to CDMA transparently and automatically when no LTE network is available, even keeping the same IP address. When this happens, the modem will get notified about the access technology change, and ModemManager will update that information.

Users can disable the modems, which will bring them to a state where they are in low power mode (e.g. RF switched off) and not registered in any network. As with the initialization or enabling sequences, the global disabling sequence is itself splitted into N per-interface disabling steps (being N the number of interfaces exported by the modem). Those interfaces implemented by the object but not supported by the modem will not be disabled. The global disabling sequence will go on disabling the interfaces one by one, but starting with the interface which was last enabled during the enabling sequence, and backwards. This ensures that the Modem interface gets disabled last. Additional feature-specific interface disablings Each feature-specific interface will have its own disabling sequence, with operations which are directly related to the purpose of the interface. For example, disabling the Location interface will involve shutting down the location gathering; and disabling the Messaging interface will involve unexporting all SMS objects from DBus. CDMA interface disabling Modems with CDMA capabilities will disable the CDMA interface as part of the global disabling sequence. This sequence involves cancelling the periodic checks of registration in the CDMA network. 3GPP interface disabling Modems with 3GPP capabilities will disable the 3GPP interface as part of the global disabling sequence. This sequence involves, among other things, cleaning up 3GPP specific indicators and unsolicited message handlers. Modem interface disabling The sequence to disable the Modem interface takes care of different important steps, such as powering down the radio interface.

ModemManager plugins exist in order to handle all non-standard vendor-specific behaviour that needs to get supported. Plugins will provide their own Modem object implementations, usually subclassing the generic MMBroadbandModem object. As previously explained, this object implements every interface that may be exported by the Modem object in DBus; and then, depending on the per-interface support checks, the interface will end up being really exported or not. Each interface defines every step to be run during the initialization, enabling or disabling sequences. Then, the object implementing the interface may or may not provide the implementation of such step. By default, the generic MMBroadbandModem object implements already most of the steps in the interfaces providing common features:

Vendor-specific subclasses of MMBroadbandModem are then able to either provide their own implementation of a given step (in the image below, a custom implementation for capabilities checking); or even completely disable the step if they know that there is no way to run it (in the image below, revision string loading is removed).

These subclass-able steps are all implemented as standard GIO asynchronous functions, so subclassing a step involves implementing both the async method which receives the input arguments to the action and the corresponding _finish() method which provides the results of the action once the operation is ready. It is worth noting that these steps and the asynchronous methods implementing them don't assume that an AT port will be used to implement the real action. This means that any other kind of port may be really used (e.g. QCDM or QMI) in the implementation, or even that a static reply can be returned (e.g. Iridium modems will always report "Iridium" as current OperatorName).