This is the device in the iPhone that manages all the functions which require an antenna. The baseband processor has its own RAM and firmware in NOR flash, separate from the ARM core resources. The baseband is a resource to the OS. The Wi-Fi and Bluetooth are managed by the main CPU, although the baseband stores their MAC addresses in its NVRAM.
The iPhone's baseband processor is the S-Gold 2. The iPhone 3G, the iPhone 3GS and the iPad make use of the X-Gold 608 chip for this purpose. The iPhone 4 (GSM) and iPad 2 uses the XMM 6180, the iPhone 4 (CDMA) uses the MDM6600.
You can check some Baseband Commands.
This is the area in the baseband where the lock state is stored.
0x400--NCK token 0xA00--IMEI signature 0xB00--IMEI 0xC00--Locks table