Voice Gateways: Understanding Analog Voice Ports

  • FXS, FXO, E+M, CAMA Ports Defined
  • Analog Voice Port Signalling
  • Analog Voice Port Configuration

Analog = Availability all around the world!

  • FXS – Foreign Exchange Station – Fax Machine/Analog Phone
  • FXO – Foreign Exchange Office – Central Office
  • Ear and Mouth – E+M – Bridge to a PBX via analog. Match E+M card at PBX end and setup a trunk between VG and PBX.
  • Centralised Automated Message Accounting – CAMA – Emergency dialling support solely. Connects to the PSTN. Deliver ANI information (Caller ID)

*DSP – Digital Signal Processor. Horsepower for the router to perform voice functions.


  • Transmits key non-voice data: supervisory (ON/OFF hook) Addressing (DIGITS) and informational (Busy, RING etc…)
  • FXS and FXO: Loop-Start (PSTN) and Ground-Start (PBX) – Makes intentional use of line. I want to ground out this line that Im going to use. Prevents ‘glare’.
  • E+M Operation: (2/4 Wire), Type (1-3,5), Signalling (Immediate-,Wink-,Delay-Start)
  • CAMA: Signalling (CAMA) and ANI Mapping

*FXS-DID – Direct Inward Dial – To dial in directly… T1 line into VG with a block of phone numbers. As phone number comes in, it will deliver what is dialled specifically.


via 2621xm router




  • ‘show voice port summary’ – Ports and Sig Types are shown via this command
  • Phone has a dial tone out of the box.
  • voice-port 1/0/0 ?
  • comfort-noise – Hissing
  • signal ? – ground start or loop start
  • cptone GB – Difference in tones across the world. Call progress tone.
  • ring cadence – Amount of ringing, followed by silence.
  • Phone number is not assigned under the voice-port

Sneak preview:

dial-peer voice 1000 pots
destination pattern 1000
port 1/0/0

csim start 1000 – will actually ring the phone from the VG! Test a dial-peer from the VG 🙂