Modem Manager (MM) specification: http://projects.gnome.org/NetworkManager/developers/mm-spec-04.html

OBS: ModemManager-0.3 does not implement all methods/signal from that specification. 

Requirements
============
Backend tested using git versions of NetworkManager and ModemManager as of May 16, 2010. To verify which interfaces ModemManager and your modem supports do:

dbus-send --system --print-reply --dest=org.freedesktop.ModemManager /org/freedesktop/ModemManager org.freedesktop.DBus.Introspectable.Introspect

dbus-send --system --print-reply --dest=org.freedesktop.ModemManager /org/freedesktop/ModemManager/Modems/<modem number> org.freedesktop.DBus.Introspectable.Introspect

where:
<modem number> is returned by dbus-send --system --print-reply --dest=org.freedesktop.ModemManager /org/freedesktop/ModemManager org.freedesktop.ModemManager.EnumerateDevices

I had to add the udi() method to NetworkManager backend because most MM methods requires a path that looks like /org/freedesktop/ModemManager/Modems/<modem number>. udi() return such path, so it is the easier way to find a MM object from a NM object.

Implementation
==============
The backend organizes MM interfaces as follow:

class ModemManager
org.freedesktop.ModemManager

class ModemInterface
org.freedesktop.ModemManager.Modem
org.freedesktop.ModemManager.Modem.Simple
org.freedesktop.ModemManager.Modem.Gsm (this interface includes only enums and flags)

class ModemLocationInterface inherits from ModemInterface
org.freedesktop.ModemManager.Modem.Location

class ModemCdmaInterface inherits from ModemInterface
org.freedesktop.ModemManager.Modem.Cdma

class ModemGsmCardInterface inherits from ModemInterface
org.freedesktop.ModemManager.Modem.Gsm.Card

class ModemGsmContactsInterface inherits from ModemInterface
org.freedesktop.ModemManager.Modem.Gsm.Contacts

class ModemGsmNetworkInterface inherits from ModemInterface
org.freedesktop.ModemManager.Modem.Gsm.Network

class ModemGsmSmsInterface inherits from ModemInterface
org.freedesktop.ModemManager.Modem.Gsm.SMS

class ModemGsmHsoInterface inherits from ModemInterface
org.freedesktop.ModemManager.Modem.Gsm.Hso

class ModemGsmUssdInterface inherits from ModemInterface
org.freedesktop.ModemManager.Modem.Gsm.Ussd

Current Implemenation Status
============================
org.freedesktop.ModemManager
        - fully implemented
        - still needs testing: signals DeviceAdded and DeviceRemoved

org.freedesktop.DBus.Properties
        - fully implemented and tested

org.freedesktop.ModemManager.Modem
        - fully implemented, needs more testing
        - modem MD300 does not supports: method GetIP4Config

org.freedesktop.ModemManager.Modem.Simple
        - fully implemented, needs more testing

org.freedesktop.ModemManager.Modem.Location
        - fully implemented, not tested yet
        - modem MD300 does not support this interface (it is not listed when calling org.freedesktop.DBus.Introspectable.Introspect)

org.freedesktop.ModemManager.Modem.Cdma
        - fully implemented, not tested yet
        - modem MD300 does not support this interface

org.freedesktop.ModemManager.Modem.Gsm
        - fully implemented and tested

org.freedesktop.ModemManager.Modem.Gsm.Card
        - fully implemented, needs more testing

org.freedesktop.ModemManager.Modem.Gsm.Contacts
        - fully implemented, not tested yet
        - modem MD300 does not support this interface

org.freedesktop.ModemManager.Modem.Gsm.Network
        - fully implemented
        - to be tested: methods Register and SetApn
        - still not working (returns error): method scan
        - modem MD300 does not support: method GetBand and probably method SetBand too. SignalQuality method only works in 2G mode for MD300 modem, maybe for other modems it works in 3G mode.

org.freedesktop.ModemManager.Modem.Gsm.SMS
        - fully implemented, not tested yet
        - modem MD300 does not support method List and probably all other methods in this interface.                                                                                                                                   
org.freedesktop.ModemManager.Modem.Gsm.Hso                                                                                                                           - fully implemented, not tested yet
        - modem MD300 does not support this interface

org.freedesktop.ModemManager.Modem.Gsm.Ussd
        - fully implemented, not tested yet
        - modem MD300 does not support this interface

Pending Problems
================
1. Needs more testing with other modems/cell phones.

2. When I hook up a second 3G modem (my cell phone actually) in my notebook
sometimes ModemManager disconnects the first modem from Internet. This is not a
KDE problem, just to warn everybody.

3. I still need to implement some caching to prevent stress in the DBus bus.

4. Missing comments about methods, enum and signal in some classes. Now that everything is implemented I will add the missing comments.

Command line
============

The dbus methods can be tested in command line with:

dbus-send --system --print-reply --dest=org.freedesktop.ModemManager /org/freedesktop/ModemManager org.freedesktop.ModemManager.EnumerateDevices

dbus-send --system --print-reply --dest=org.freedesktop.ModemManager /org/freedesktop/ModemManager/Modems/<modem number> org.freedesktop.ModemManager.Modem.Gsm.Network.GetSignalQuality

dbus-send --system --print-reply --dest=org.freedesktop.ModemManager /org/freedesktop/ModemManager/Modems/<modem number> org.freedesktop.ModemManager.Modem.Gsm.Network.GetRegistrationInfo

dbus-send --system --print-reply --dest=org.freedesktop.ModemManager /org/freedesktop/ModemManager/Modems/<modem number> org.freedesktop.DBus.Properties.GetAll string:<interface name>

<modem number> is returned by the first command (EnumerateDevices).
<interface name> is either of

org.freedesktop.ModemManager (do not use /Modems/<modem number> with this one)
org.freedesktop.DBus.Properties (do not use /Modems/<modem number> with this one)
org.freedesktop.ModemManager.Modem
org.freedesktop.ModemManager.Modem.Simple
org.freedesktop.ModemManager.Modem.Location
org.freedesktop.ModemManager.Modem.Cdma
org.freedesktop.ModemManager.Modem.Gsm
org.freedesktop.ModemManager.Modem.Gsm.Card
org.freedesktop.ModemManager.Modem.Gsm.Contacts
org.freedesktop.ModemManager.Modem.Gsm.Network
org.freedesktop.ModemManager.Modem.Gsm.SMS
org.freedesktop.ModemManager.Modem.Gsm.Hso
org.freedesktop.ModemManager.Modem.Gsm.Ussd

Devices Known to Work
=====================

. Sony Ericsson MD300 modem
    - connects to the Internet.
    - Signal Quality indication only in Edge mode. When in 3G mode the modem only reports the number of bars (zero to five), but Modem Manager does not report that either. We can change Modem Manager to report an approximation of the signal quality based on the number of bars.

. Samsung Omnia HD i8910
    - connects to the Internet.
    - Select "Any" in Plasma NM -> Manage Connections -> Mobile Broadband -> <connection name> -> Edit -> Type or it will not connect
    - Access technology not reported by Modem Manager.

. Nokia N95
    - connects to the Internet.
    - Access technology not reported by Modem Manager.

. Huwaei e226
    - connects to the Internet.