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Almost nothing happens in Asterisk without a channel being involved. A channel is an entity inside Asterisk that acts as a channel of communication between Asterisk and a thinganother device. That is, a phone, a devicePBX, another Asterisk system, or even Asterisk itself (in the case of a local channel).

Our documentation and many Asterisk users speak in terms of "calls". A call can be one or more channels creating a path of communication or activity through the Asterisk system.

To give you an idea about the versatility of what channels do, here are a few facts about them:

  • Channel Drivers provide channels in Asterisk.
  • Channels can interface with each other through bridges.
  • Applications and functions can affect the state or attributes of a channel or its media stream.
  • Channels are commonly passing audio between communication endpoints, but can pass other data, such as video or text messages.
  • Channels execute instructions with dialplan, but can be controlled by other APIs (AGI,AMI,ARI) or interfaces (CLI).
  • Channels are unique and can be tracked through logs.
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Common Asterisk Channels

One of the many benefits of Asterisk is the capability to interface with as many different technologies as you have channel drivers! However, most administrators will only make use of a few types at a time.

Here are a few commonly used channel types:

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Most channel drivers have an associated configuration file. Some channels may require the configuration of dependent resources for optimal operation. For example, SIP channels, configured in sip.conf or pjsip.conf use RTP resources which can be configured in rtp.conf.

The Channel Drivers configuration section contains information on configuring documented channel drivers. In other cases the configuration file itself contains configuration documentation.

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Flat text configuration isn't the only option. A few channel drivers provide support for the ARA (Asterisk Realtime Architecture) and can therefore pull configuration from a local or remote database. Use of the ARA requires configuration of additional resources and dependencies outside the channel drivers themselves.

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  • A device configured in the channel driver communicates to Asterisk (e.g. over a network) that it wants to make a call.
  • A user executes a command (such as Originate) to create a new channel.
  • An existing channel executes dialplan that calls an application (such as Dial) to create a new channel.
  • Asterisk receives API calls that create a new channel or channels.

Once a channel is established, the events that occur are channel technology-dependent. That is, whether audio, video or other data communication begins over the channel will depend on signaling that occurs over SIP, ISDN, H.323 or other protocols implemented via the channel driver.

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