Maintenance Busy / T309 Expired

Symptom:
In Dialogic Gate Control Element Management System (GCEMS) ClientView (for Dialogic IMG 10xx Integrated Media Gateway) or in the WebUI (for Dialogic IMG 2020 Integrated Media Gateway) highlighting an ISDN D-Channel may display a status indicating the D-channel is down with a status of 'T309 Expired'.   This message may also be preceded by 'Maintenance Busy'.  

Below is a screenshot showing an ISDN D-Channel Status in this 'As-Configured' state. {All screenshots in this article were taken using ClientView and an IMG 1010, but apply equally to the IMG 2020 when viewing the corresponding panes in the WebUI.}
 

Open

Reason for the issue:
In most cases, if the ISDN D-Channel in question has been in service and operational prior to the above status, then the messages can be attributed to an underlying issue with the circuit on which the D-Channel resides.  If there is an issue with the circuit facilities, wiring, or ISDN carrier, that causes errors on the T1/E1 or ISDN data channel, first 'Maintenance Busy' will be displayed, followed by 'D Channel Down T309 Expired'.  

While some ISDN timers are documented and, optionally, configurable in the ISDN Timers object under each D-Channel, Timer 309 is not configurable. The 309 timer is defined in ISDN Specification ITU-T Q.931, and is used to release active calls upon expiry.  When the underlying Data Link Layer goes down, the Network Layer is notified and T309 is started for any call in the Active State. 

Fix / Solution:
'D Channel Down T309 Expired' is an informational message and a side effect of D-Channel or span instability.  

In order to troubleshoot this issue, first determine the scope of the issue.  Determine if only one span is affected, or only spans to a particular carrier or piece of equipment.  

The GCEMS server alarm.log (IMG1010 and IMG 1004) or the Event Log (IMG 2020) contains detailed messages that indicate when and where the D-channel issues originated.  These messages are also aggregated in EventView for all Spans and D-Channels :
 
Here we see a D-Channel out of service on Signaling Interface 3:

Open

A closer look at the events prior to the T309 message indicates an issue with the underlying T1, in this case Signaling Span 3, which should be investigated with the carrrier or whoever is responsible for the customer premise equipment:

Workaround:
Route ISDN calls on the affected channel group to alternate channel groups.

Product List
Dialogic IMG 1010 Integrated Media Gateway
Dialogic IMG 2020 Integrated Media Gateway (IMG 2020), formerly referred to as Dialogic BorderNet™ 2020 Session Border Controller