This thread is intended to document as much as is known (or can be figured out through reverse engineering) of the Cellular Services Daemon on the N900.

The Cellular Services Daemon is a daemon that handles most of the communication with the N900 Cellular Modem. It interfaces with other parts of the system over dbus and sends ISI messages to the N900 Cellular Modem via a kernel driver.

The following packages are involved in the working of the Cellular Services Daemon:
csd-base (Cellular Services Daemon)
libisi1 (base library for sending ISI messages to the N900 Cellular Modem)
libisi-glib0 (library for allowing libisi to work with glib)
libtelcommon0 (common utility library used for routing ISI packets)
libcsnet0 (Cellular Services Daemon network service plugin, handles network related things like cell tower change, 2g/3g switch, signal strength, connect/disconnect from cell network, change to a different cell network and date/time info sent by the network)
libsim0 (Cellular Services Daemon SIM plugin, handles talking to the SIM and retrieving info including IMSI, SIM-based operator name, home network and sim status)
libcscall2 (library for handling phone call stuff)
csd-call (Cellular Services Daemon call plugin, handles making and receiving phone calls and related activity)
csd-gprs (Cellular Services Daemon GPRS plugin, handles data transfer via 2G/3G networks)
libphinfo0 (library for retrieving phone information)
csd-info (Cellular Services Daemon info plugin, handles phone information like IMEI, serial number, product code, hardware version and cellular modem software version)
libsms0 (library for handling SMS messages)
libsms-utils0 (library for decoding/encoding SMS messages and doing other SMS related utility tasks)
csd-sms (Cellular Services Daemon SMS plugin, handles SMS messages including Cell Broadcast SMS messages)
libss1 (library for handling supplementary services)
csd-ss (Cellular Services Daemon supplementary services plugin, handles things like call diversion and call barring)
libsimpb0 (Cellular Services Daemon sim phone book plugin, handles talking to the phone book on the SIM)

Next post I will make will talk about the dbus interfaces exposed by the cellular services daemon and its plugins and link to all the known details of those interfaces.