Demand test descriptions and error codes - Avaya Support

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
Note: CO
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
Note:
does not normally provide an On-Hook (Disconnect) signal. Exceptions to this
rule include Netherlands loop start and UK loop-calling guarded-clearing.
282 Maintenance Alarms for Communication Manager, Media Gateways and Servers