NMS ISDN maintenance messages are layer 3 ISDN messages used by ISDN to put B channels and D channels in the in-service or out-of-service state. These messages are supported only on North American variants: 4ESS, 5ESS, NI2, and DMS-100. For 5ESS only, service messages can also be used to place the entire interface in or out of service.
There are two types of IMGT service maintenance messages:
In some ISDN variants, SERVICE messages can be exchanged to put channels in and out of service. Through the IMGT service, the application can send SERVICE messages to the trunk and receive SERVICE or SERVICE ACKNOWLEDGE events. When a SERVICE message is received, the ISDN stack automatically sends a SERVICE ACKNOWLEDGE. No primitives are needed to send a SERVICE ACKNOWLEDGE. The application can also query the stack to learn the status of a particular B channel.
SERVICE message handling is implemented for the following variants:
DMS
AT&T 4ESS
AT&T 5ESS
NI2
Using the IMGT service, the application can receive notification whenever a RESTART message is received. When a RESTART message is received, the ISDN stack automatically sends a RESTART ACKNOWLEDGE.
For the DMS variant, the reception of a RESTART message resets the status of the B channel to the in-service setting.