Interactions Between Switch and CO
The following sequences show the interactions between the switch and the CO during call setup for both loop-start and ground-start trunks.
Loop Start Operation
Idle State:
Tip = ground, Ring = CO Battery
Outgoing Call:
- PBX Off-Hook (Seize Message): Closes the Tip-Ring Loop.
- CO Response: DC loop current + Dial tone.
- PBX On-Hook (Drop Message): Open Tip-Ring loop, no loop current.
- CO Response: CO goes to idle state (see Note).
Incoming Call:
- CO Applies Ringing Voltage.
- PBX Response: Detect ringing current.
- PBX Off-Hook (Answer Message): Close Loop.
- CO Response: Trip ringing, provide loop current.
- PBX On-Hook (Drop Message): Open Tip-Ring Loop, no Loop Current.
- CO Response: CO goes to idle state (see Note).
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Note:
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CO does not normally provide an On-Hook (Disconnect) signal. Exceptions to this rule include Netherlands loop start and UK loop-calling guarded-clearing. |
Ground Start Operation
Idle state:
Tip = open, Ring = CO Battery
Outgoing Call:
- PBX Off-Hook (Seize Message): Places ground on Ring.
- CO Response: Places ground on Tip.
- PBX Response: Close the loop.
- CO Response: Provide loop current.
- PBX response: Dial out digits.
- PBX On-Hook first (Drop Message): Open the Tip-Ring Loop, no loop current.
- CO Response: Open circuit on Tip.
- CO On-Hook first (Disconnect): Open circuit on Tip, no loop current.
- PBX Response: Open Tip-Ring loop.
Incoming Call:
- CO Off-Hook (Seizure): CO applies ground on Tip and applies ringing voltage.
- PBX Response: Make trunk busy for outgoing calls.
- CO Ringing: CO applies ringing voltage.
- PBX Response: Detect ringing, ring destination.
- PBX Off-Hook (Answer Message): Close loop.
- CO Response: Trip ringing, provide loop current.
- PBX On-Hook first (Drop Message): Open the Tip-Ring Loop, no loop current.
- CO Response: Open circuit on Tip.
- CO On-Hook first (Disconnect): Open circuit on Tip, no loop current.
