Release Link Trunk (RLT)
- Kelli Higdon
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IMG 2020
Release Link Trunk is a technique for the Nortel DMS Variant where if there are two calls active on the IMG 2020 and they are bridged or looped together the resources on the IMG 2020 are being used inefficiently. With this feature the IMG 2020 has the ability to send specific facility messages back to the Nortel DMS and tell the switch to connect these two calls together so they are no longer active on the IMG 2020. This would effectively open up two b-channels that were initially being utilized. Release Link Trunk supports a portion of specification (NISA211_V801_NI-1, chapter 5-12 (Issue, August 98). See links under Related Topics below for more information on what is supported and how to configure.Â
Call Flow Diagram
 Diagram
Below is diagram explaining how RLT would be function in an ISDN to SIP network.
Two calls are in progress. The SIP side sends a REFER message to the IMG 2020, notifying the IMG 2020 to connect the two calls together.
After the call is bridged in the IMG 2020, the SIP leg is no longer connected. Call A and Call B are now routed through IMG 2020 as displayed below.
When Release Link Trunk is Enabled, the IMG 2020 will recognize that the call does not need to be routed through it. The bridged call is using resources on the IMG 2020 that could be used for other functions. The IMG 2020 would then send a request to Switch B (Nortel DMS) that the two independent calls be bridged together as shown below.
At this point, the call is being routed through Switch B and the IMG 2020 has released the call thereby freeing resources on both the IMG 2020 and Switch B.
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Configuration
Initial Configuration
The procedure below explains how to configure the IMG 2020 for RLT. Verify the following entities are configured before proceeding. Refer to the Basic Configurations topic for information on configuring the basic functionality.
 Configure ISDN stack
Right click on the IMG 2020 physical node object and select New Signaling. The Signaling object is a parent or container object and no configuration is needed here. Refer to the Signaling topic for more information on this object.
Right click on the Signaling object created and select New ISDN. The ISDN signaling object is also a parent or container object and no configuration is needed here. Refer to the ISDN - Signaling topic for more information on this object.
Right click on the ISDN signaling object and select New ISDN D Channel. Select which DS-1/DS-0 the D-Channel will get configured on. Also, select the Northern Tel DMS-100 Variant as shown in screen capture below.
Refer to the IMG 1010 - ISDN D Channel topic for more information on configuring the individual fields within this object.
 Configure ISDN Channel Group
Right click on the Dialogic object and select New Routing Configuration. The Routing Configuration object is a parent or container object and no configuration is needed here. Refer to the Routing Configuration topic for more information on this object.
Right click on the Routing Configuration object and select New Channel Groups. The Channel Groups object is a parent or container object and no configuration is needed here either. Refer to the Channel Groups topic for more information on this object.
Right click on the Channel Groups object and select New Channel Group. In the Name field. either enter a Name that will identify this channel group or accept the default entry.
In the Signaling Type field select ISDN from the drop down menu. By selecting ISDN, the Channel Transfer field can now be modified.
To enable the Release Link Trunk feature, select Enabled from the drop down menu in the Channel Transfer field. See screen capture below.
Refer to the Channel Group object for more information on configuring the individual fields.
Click on the Channel Groups object again. The Channel Groups object will have a yellow exclamation point in place of the icon indicating the configuration has not been sent to the IMG 2020. Click on the Download Resource Tables button and the new configuration will be sent to the IMG 2020.
At this point the feature has been configured and enabled. Routing and Translations can now be configured.
Below is screen capture of tree within the Web GUI once the above procedure has been configured.
 Additional Information
RLT supports Northern Telecom DMS-100 and DMS-250 only
The IMG 2020 will take on the role as the User Switched.
The IMG 2020 will not support/process an RLT request. The request will be discarded
In order for RLT to be invoked following a SIP Refer, both ISDN B-Channels must be terminated on the same remote switch
RLT will be available only if the initial call was established between ISDN and a SIP device. The direction of the call is irrelevant.
RLT is triggered from the SIP call leg via the REFER method.
Both channel groups must have the RLT feature enabled for the feature to be invoked. Feature will not be available if one of the channel groups does not have RLT enabled.
Two B-Channels can be controlled by one d-channel or they can be controlled by separate d-channels.
The IMG 2020 will invoke an RLT request only when both ISDN call legs are answered.
If RLT is unsuccessful, the two calls will remain in the same state they were in prior to the request. The IMG 2020 will not initiate clearing of the call.