An Introduction to Asterisk

Warren Turkal and Sean Reifschneider

What a PBX is

• (Phone) channel switch
• Call routing
• Call/Channel bridging
• Applications (voice mail, call transfer, call waiting, conference calling, etc)
• Source? Other switches, phones, POTS lines, etc...

Traditional Hardware PBXs

• Specialized hardware
• Key systems: Panasonic
• Lucent/AT&T/Avaya, Nortel, Polycom
• Modules: Voicemail, POTS, T1/T3, IVR
• Mini-phone switch
• Proprietary
• Thousands of dollars
• Limited

Software PBXs

• Primarily Asterisk, but there are other alternatives
• http://www.asterisk.org/
• Using commodity PCs
• Interface to other systems:
  • Hardware line interfaces (POTS, T1/T3, ISDN), Bluetooth
  • Software/VoIP: SIP, IAX2, H.323, Skinny, Skype
  • http://www.chanskype.com/
• Caller-ID, Voicemail, Automated attendant, Meet Me/Bridge, Music while waiting, ENUM, Complex dial plans, External programable interfaces (AGI), Time-based calling plans
• http://www.asterisk.org/support/features

Soft phones

• Turns your laptop into a cell phone
• Varying quality
• Many protocols (SIP, IAX, Skype)
• kiax, kphone, gnophone, twinkle, shtoom, linphone, cornfed...

Asterisk Limitations

• Conferencing requires "zaptel" timer hardware
  (OpenPBX.org does not have this requirement)
• No T.38 support, important for faxing (OpenPBX.org)
• No STUN support (OpenPBX.org)
• Multi-architecture is not a priority
• Patent issues: G.729 voice codec

Programming

• External program
• Can provide any functionality an external program can do
• Reads headers from stdin

  agi_language: en
  agi_type: IAX2
  agi_uniqueid: 1168166161.26
  agi_callerid: 9705552917
  agi_context: incoming_call
  agi_extension: s
  agi_priority: 2
  [...]

• Writes commands to stdout

  SET VERIABLE app_username jafo
  SET CONTEXT card_valid
  SET EXTENSION s
  SET PRIORITY 1

Examples

• Emergency escalation tree
• Regex-based blacklist
• Automated attendant
• IVR example: Time and temp
• Voicemail (Standard Asterisk)

Other Alternatives

• OpenPBX.org http://www.openpbx.org/
  • Forked from Asterisk 1.2
• YATE http://yate.null.ro/pmwiki/
• FreeSwitch http://www.freeswitch.org/
• Bayonne (IVR-only): http://www.gnu.org/software/bayonne/
• SipX http://www.sipfoundry.org/sipX

Getting Asterisk

• Debian Stable: 1.0.7
• Debian Testing: 1.2.13
• Ubuntu Dapper/LTS (universe): 1.2.7
• Ubuntu Edgy (universe): 1.2.12
• Sean uses CentOS and installs from source (1.4.x)
• http://www.asterisk.org/