Asterisk

• Server application for everything telephony
• Currently very single server based but future development is to address scalability concerns.

Terms

• Channel - connection between Asterisk and some telephony endpoint
• Channel bridging - connect channels together to pass data between them
  • Generic - handle audio - high level
  • Native - understands underlying comms layer and can possibly use hardware to improve efficiency
• Frames
  • media frames - portion of a stream
  • signalling frames - call events e.g. hang up, digit pressed etc.

Abstractions

• Core acts as a module registry - connects abstract interfaces to make calls work
• Concrete implementations of those interfaces are in loadable modules
• Channel Driver - translates between Asterisk channel abstraction and underlying telephony technology. Examples of methods are:
  • call
  • answer
  • read/write frames
• Dialplan - handles extensions in a cofig file
  • loads applications for specifc number e.g. answer, playback(sound), voicemail
  • series of steps for each extension can allow for chains of applications to be executed
  • can pass parameters to applications e.g. caller id, time call started.
• Codec translators
  • encoding media
  • compress audio, increase volume etc.

Threads

• Heavily threaded application - uses POSIX threads
  • Network Monitor Threads: monitor a network e.g. ip, pstn. Responsible for initial connectin setup and then hand over to a channel thread.
  • Channel Threads: Created for each channel that executes a dialplan. Dialplan applications always execute in the context of a channel thread.

Call Scenarios

Checking Voicemail

1. Channel thread handles dialplan execution. Eventually gets to VoicemailMain application.
2. This handles call interaction but underlying tech to enter button presses etc is abstracted away from it.
3. If the Voicemail application needs to play a sound to the user then the Asterisk channel may need to setu a codec translator to convert the audio to the correct format.

Bridged Call

1. Dialplan executes the Dial application which knows the typed of protocol to use to create the outgoing connection.
2. Once both channels have been setup then the bridging of the channels begins
3. During a bridge audio and signalling events are passed from one channel to the other via a codec translator until an event occurs that causes the bridge to end e.g. one side hangs up.
4. The codec translator communicates with the channel core using framein and frameout calls.