Nortel 1000 Description, Installation And Operation Manual page 557

DCH link failure
The DCH link can change to the RLS (Release) state due to technician action
in LD 96, MSDL or SDI/DCH card failure, or cable failure. This condition is
detected on the DCHIP card by the PC Card signaling that the L2 connection
has failed.
PC Card failure
The PC Card failure can be detected in various ways, such as the following:
• missing heartbeat transmission
• a hardware interrupt
When the software does not send "an activity test message" (heartbeat
message) to the Card Services of the PC Card Device Driver during a period
greater than n seconds, Card Services consider it a breakdown detection. Card
Services tries to reset the PC Card. Card Services are responsible to ensure
the conformance of the reset timing. Card Services also check and wait for the
card to reach the READY state.
Socket Services are responsible for card insertion and removal. There is a
single interruption shared for insertion and removal events and a single
interruption for device-specific interruptions. Socket Services identify which
socket originates the interruption and sends the interruption to the Card
Services interruption handler. Card Services then wait and re-initialize the
PC Card, if the card is plugged in again and is in the READY state.
Do not insert or remove the PC Card when the IP trunk card is plugged in.
DCHIP card failure
This occurs when the DCHIP IP trunk card goes out of service. The DCHIP
IP trunk card failure case is similar to the DCH link failure case. However, all
call reference information is gone. As a result, when the DCHIP comes back
up, it sends a "REStart" message to the other side to re-initialize all the trunks.
All the established calls are terminated.
Maintenance
IP Trunk Description, Installation, and Operation
Page 557 of 644