Android Operating System and Radio Interface Layer

When the mobile phone is switched on, in the first step Linux Kernel is loaded along with the drivers for modem and the first process called "init" is started. 'Init' will start initial dummy process named 'zygote' which helps in starting other services like audio manager, surface flinger, power manager, RIL Daemon (to be explained later), service manager, Telephony service etc 

All the smartphone applications communicate to external ports or environment via Application programming interface (API). For communication towards radio modem, Telephony framework (mentioned above) performs the role of APIs. It helps to access network related services by applications i.e. PDP connection establishment, SMS, MMS etc. Whenever these applications make any query, it is forwarded to Radio interface Layer (RIL) by telephony framework APIs. RIL is interface between telephony framework and radio hardware (modem)

RIL has two sub layers 

• RIL Daemon: It communicates with Vendor RIL for call processing and other functions
• Vendor RIL: It contains Library of functions and drivers to particular modem. It communicates with the radio hardware 

RIL Daemon is responsible for initializing Vendor RIL and forwards all requests from telephony framework to Vendor RIL. Vendor RIL communicates to modem protocol stacks via Hayes AT commands and forwards requests from modem to RIL Daemon or vice versa. Please see the figure above.

So here it is what happens:

1. Android Mobile is switched on
2. Linux Kernel is loaded
3. Initial temporary process named zygote is started on initialization by Linux Kernel
4. Zygote helps in quick start of other services including Android Telephony network and RIL Daemon (RILD)
5. RILD initializes the vendor RIL for communication
6. Vendor RIL initiates the radio modem
7. Network attach procedure is started