Internals of Telephony subsystem

Internals of Telephony Subsystem

1.1. Part I, the key items

Assumption: We are in the top level of Android source code repository

• Files
  frameworks\base\telephony  the core of the telephony subsystem implementation, GSM phone and so on
  packages\apps\Phone        Phone application implementation, UI related stuffs reside
  hardware\ril               Radio reference implementation
  

• The big picture
  

• The key interfaces and classes
  

  1. Phone interfaces
     Phone.java:Base abstraction of the phone
     

     frameworks\base\telephony\java\com\android\internal\telephony\Phone.java

     PhoneBase.java:An abstract class which extends Phone interface and implements the Event registration and notification
     

     frameworks\base\telephony\java\com\android\internal\telephony\PhoneBase.java

     GSMPhone.java:A concrete phone implementation
     

     frameworks\base\telephony\java\com\android\internal\telephony\gsm\GSMPhone.java

     CommandsInterface.java:Radio operation abstraction
     

     frameworks\base\telephony\java\com\android\internal\telephony\gsm\CommandsInterface.java

     BaseCommands.java:Radio event registration and notification
     

     frameworks\base\telephony\java\com\android\internal\telephony\gsm\BaseCommands.java

     RIL.java: Radio interface library implementation, responsible for sending request to Radio layer and
     distribute the response from Radio layer to request sender
      

     frameworks\base\telephony\java\com\android\internal\telephony\RIL.java

     Native stub for RIL.java: communicates with RIL.java through local socket
     

     hardware\ril\libril\*

     Reference implementation of Radio library:communicates with Radio hardware through AT command
     

     hardware\ril\reference-ril\*

     Radio interface lib daemon runs as a stand alone process
     

     hardware\ril\rild\*

  2. RIL request and response
     1, Solicited and Unsolicited response
     Solicited response response to the upper layer request, this is a kind of passive response,
     for instance, response the request of Singal strength request
     Unsolicited response a response triggered by the underlying hardware, for instance, the state change
     Radio will trigger an RIL_UNSOL_RESPONSE_RADIO_STATE_CHANGED response to notify the upper layer
     

  3. Request sending and handling
     RIL_Sender thread
     gets the reqeust from mRequestsList and send it to Radio reference library through local socket
     RIL Receiver thread
     receives the response from Radio reference library and process it with processUnsolicited()/processSolicited()
     according to response type RESPONSE_UNSOLICITED/RESPONSE_SOLICITED
     processUnsolicited() is responsible for processing active event from Radio layer
     processSolicited() is responsible for processing passive response to upper layer request
     

  4. Message identity localization and Parcel
     The mechanism used here is localize the identity of message and put it into a Parcel, send it to the target,
     the target receives the Parcel, put the wanted stuffs into Parcel and return back it to the sender, it is
     the sender's responsibility to classify and handle the response corresponds to the specific request.
     
     

     We explain this trick by an example, User dialed a number *A user's dialing activity runs a long trip to get to the point dial() in RIL.java * RILRequest rr = RILRequest.obtain(RIL_REQUEST_DIAL, result);      send(rr);      put the RIL_REQUEST_DIAL ID into Parcel embedded in rr and send it to the target, the target here is      reference-ril.c * refernece-ril.c receives the request and handle it with requestDial() * requestDial() completes it work and send response back by RIL_onRequestComplete() * The implementation of RIL_onRequestComplete() resides in ril.cpp      p.writeInt32 (RESPONSE_SOLICITED);      p.writeInt32 (pRI->token);      put the response type to RESPONSE_SOLICITED, because the request is initiated by upper level, and it      is a passive response, put the RIL_REQUEST_DIAL into the Parcel (pRI->token == RIL_REQUEST_DIAL) for      upper layer to handle the response to the RIL_REQUEST_DIAL request. * thanks to the local socket, the control flow arrived at RIL.java again, this time it hits at      RIL_Receiver(),for the reason of RIL_Receiver's listing on SOCKET_NAME_RIL socket      the RIL_Receiver delegates the response to the processSolicited(), it was responsibility of      processSolicited() to notify the registrarant according the content of the response.