Asterisk Channels

What is a Channel?

A channel is a connection which brings in a call to the Asterisk PBX. A channel could be a connection to an ordinary telephone handset or an ordinary telephone line, or to a logical call (like an Internet phone call). Asterisk makes no distinction between "FXO" and "FXS" style channels (that is, it doesn't distinguish between telephone lines and telephones). Every call is placed or received on a distinct channel.

Channel Types

Asterisk provides the following channel types in the standard distribution:

• Agent: ACD Agent channel
• Console: Linux console client driver for sound cards (using OSS or ALSA)
• H.323: An older VOIP protocol
• IAX and IAX2: Inter-Asterisk Exchange protocol, Asterisk's own VOIP protocol
• MulticastRTP: Broadcast audio outside the context of a call (new in Asterisk 1.8)
• Local: Loopback into another context
• MGCP: Media Gateway Control Protocol, another VOIP protocol
• mISDN: mISDN channel
• Modem: Confusingly, this is for connecting ISDN lines, not for use with modems. Deprecated.
• Asterisk NBS channels: using Network Broadcast Sound
• phone: Linux Telephony channel
• SIP: Session Initiation Protocol, the most common VOIP protocol
• Skinny: A driver for Cisco Skinny Client Control Protocol (a VOIP protocol)
• Gtalk: Google Talk Channel driver. To be replaced by Asterisk Jingle channels
• Asterisk VOFR channels: voice over frame relay Adtran style
• VPB: For connecting ordinary telephone and telephone lines using Voicetronix cards
• Zap: For connecting ordinary telephones and telephone lines using Digium cards. Also for TDMoE and for zaptelBRI

Channel drivers offering other technologies can be optionally installed:

• Celliax let Asterisk manage GSM and CDMA cellular phones, and Skype calls to/from cellphones
• Bluetooth: Allows the use of bluetooth devices to change routing - see trunk Asterisk mobile channels.
• CAPI: ISDN CAPI channel
• vISDN: vISDN channel (native BRI channel for HFC chipsets)
• SCCP: An alternate Skinny/SCCP channel implementations
• Sirrix: ISDN BRI for Sirrix cards (with optional ISDN encryption)
• UNISTIM: Nortel Unistim channel
• Unicall: Replacement for zaptel, with R2 support
• SS7: SS7 (ISUP on MTP2/3) channel
• NMS: NMS Communications Open Access channel

Configuration

• Digium interface cards: You need to have installed and configured the Zaptel driver to permit Asterisk to use your Digium card. Asterisk accesses Digium cards via Zap channels. Configure these in zapata.conf.
• Voicetronix interface cards: You need to have installed their device driver. Asterisk accesses Voicetronix cards using vpb channels. Configure these in vpb.conf.
• SIP phones are accessed via SIP channels. Configure these in sip.conf.
• IAX phones are accessed via IAX channels. Configure these in iax.conf.
• Some specific VOIP telephones may require special configuration. See Configuration Notes for Specific Phones.

For other connection types, go to the page appropriate for the technology in the list above.

Channel Capabilities: What capabilities are supported by what channels?

Capability	IAX	SIP	Skinny	SCCP	Voice	Zaptel	MGCP	chan_capi	chan_misdn	UNISTIM
Early Voice	?	Y	?	Y	?	?	?	Y	Y	N
Call Transfer	?	Y #/Native	?	Y	?	?	Y	?	Y #/Native	Native
DND	?	Y	?	Y	?	?	?	?	?	?
Receive CallerID	Y	Y	?	Y	?	Y	Y	Y	Y	?
Send CallerID	Y	Y	?	Y	?	Y	Y	Y	Y	?
Group Pickup	?	Y *8#	?	Y *8 /native	?	Y *8	Y *8	Y native	Y *8	*8 (untested)
Directed Call Pickup	?	?	?	Y native	?	?	?	?	?	Native
Call Waiting	?	Y	?	Y	?	?	Y	?	?	Native
Disable CallerID	?	Y	?	Y native	?	?	?	?	Y	?
Call Forward	?	Y	?	Y native	?	?	?	Y call deflection	Y call deflection	?
Three way calling	?	Y	N	Y	?	?	?	Y	?	N
ADSI Screen Phones	N	N	N	?	?	Y	?	?	?	n/a

'Native' indicates that this function is provided by core Asterisk rather than the channel driver or device.