ASTERISK “Open Source Communications Platform” Presentation by: Rahul Dev Parashar MTech(CSE) IIT Bombay

References: 1. http://aosabook.org/en/asterisk.html 2. http://www.asterisk.org

Outline: 1. Introduction 2. Critical Architectural Concepts --> Channels --> Channel Bridging --> Frames 3. Asterisk Component Abstractions --> Channel Drivers --> Dialplan Applications --> Dialplan Functions --> Codec Translators 4. Threads 5. Call Scenarios --> Checking Voicemail --> Bridged Call

Introduction: --> Open source telephony applicatios plateform --> Server application for performing custom processing of calls --> Started by Mark Spencer in 1999 --> Name comes from wildcard character(*) of linux --> Customization of phone call features can be done like: Voicemail, Playing back a sound file, Speech recognition --> It supports: VoIP(Voice over IP) Analog and digital connectivity to traditional telephone n/w Connects to PSTN(Public Switched Telephone N/W)

Critical Architectural Concepts: 1. Channels: Ex: caller interacting with voicemail Figure: A Single Call Leg, Represented by a Single Channel

Ex: Connection between two phones 2. Channel Bridging: Ex: Connection between two phones Figure: Two Call Legs Represented by Two Channels

Types of bridging: 1. Generic bridging 2. Native bridging Figure: Example of a Native Bridge

3. Frames: --> Communication within the Asterisk code during a call is done by using frames --> Signalling frames are used to send messages about call signalling events, such as a digit being pressed, a call being put on hold, or a call being hung up. Some available frames: * VOICE --- These frames carry a portion of an audio stream * VIDEO --- These frames carry a portion of a video stream * MODEM --- The encoding used for the data in this frame * CONTROL --- These frames are used to indicate call signalling events. These events include a phone being answered, hung up, put on hold, etc. * DTMF_BEGIN --- Frame is sent when a caller presses a DTMF key * DTMF_END --- Frame is sent when caller stops pressing DTMF key DTMF stands for Dual-Tone Multi-Frequency. This is the tone that is sent in the audio of a phone call when someone presses a key on their telephone.

Asterisk Component Abstractions: 1. Channel Drivers: This component is responsible for translating between the Asterisk channel abstraction and the details of the telephony technology that it implements Figure: Channel Technology and Abstract Channel Layers

--> send_digit_begin --> send_digit_end --> read --> write The most important methods in ast_channel_tech are: --> requester --> type --> call --> answer --> hangup --> indicate --> send_digit_begin --> send_digit_end --> read --> write --> bridge

2. Dialplan Applications: # It defines how asterisk handles inbound and outbound calls. There are 4 main concepts in dialplan: 1. Contexts: Dialplans are divided into parts. Contexts keeps these different parts interect with each other. 2. Extensions: Unique sequence of steps that Asterisk follows. 3. Priorities: Extension has priorities. 4. Applications: Workhorses of dialplans. Ex: answer(), hangup() etc. * This code define the rules for what happens when someone dials 1234. exten => 1234,1,Answer() same => n,Playback(demo-congrats) same => n,Hangup() * This code define the rules for what happens when someone dials 5678. exten => 5678,1,Answer() same => n,Read(DIGITS,beep,4) same => n,SayDigits(${DIGITS})