CAS R1 LOOPSTART NETWORK

Loopstart is a CAS R1 variant.

Loopstart is an asymmetrical variant where a FXS (or "user") and FXO (or "network") sides must be face to face.

Current page describes the Loopstart FXO variant, see the following page for documentation about [[CAS_R1_LOOPSTART_FXS|Loopstart FXS] variant.

Overview

In the Loopstart variant, FXO side is sending CAS bit B, used to make the user's phone ring, while it receives CAS bit A from user side to report user's phone as "on hook" or "off hook".

Initial CAS bits values:

• Bit A (from user) is 1 ("on hook")
• Bit B (from network) is 1 ("ringer off")

Summary of the Loopstart call flow:

• Call establishment:
  • User calling:
    • FXO side detects bit A changes to 1, indicating that user's phone is "off hook"
    • FXO side sends a dial tone
    • FXO side collects the digits entered by the user
    • FXO side stops the dial tone (upon first digit)
    • FXO side proceed with the call
  • Network calling:
    • FXO side changes bit B to 0 ("ringer on") to make user's phone ring
    • FXO side toggles bit B from 0 to 1 (0 for 2 seconds, 1 for 4 seconds)
    • FXO side detects bit A changes to 1, indicating that user's phone is "off hook"
    • FXO side changes bit B back to 1
• Call termination (user side only, network has no way to terminate the call):
  • FXO side detects bit A changes to 0, indicating that user's phone is "on hook"

Note: There is no way, in Loopstart variant, for network side to report when the call is answered, or terminated. The caller considers the call answered when the ring tone ends and when someone starts talking. The caller considers the call terminated when the conversation is over, or when it no more hears the remote party.