Explicit Call Transfer (ECT)
- Kelli Higdon
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IMG 2020
Explicit Call Transfer (ECT) is a technique for EURO ISDN 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 Stored Program Control Switch (SPCS) on the PBX or TDM switch and tell the PBX/TDM switch to connect these two calls together so they are no longer active on the IMG 2020.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. Explicit Call Transfer supports a portion of specification Q0952_7 (Issue,June 97). See links under Related Topics below for more information on what is supported and how to configure.
Call Flow Diagram
Diagram
Below is diagram explaining how ECT would be function in an ISDN to SIP network.
Two calls Bare 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 the SIP leg is no longer connected. Call A and Call B are now routed through IMG 2020 as displayed below.
With Explicit Channel Transfer Enabled, the IMG 2020 will recognize that the call does not need to be routed through it. The bridged call is using resources that could be used for other functions. The IMG 2020 would then send a request to the SPCS/Switch B (Stored Program Control Switch) 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 the SPCS.
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Configuration
Initial Configuration
The procedure below explains how to configure the IMG 2020 for ECT. 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. 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 Explicit Channel Transfer 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.
Configure B-Channels
In the procedure above, a D-Channel was created. The next few steps describe how to create a set of B-Channels that will be controlled by the D-Channel created earlier.
Right click on the ISDN D-Channel object created earlier and select New ISDN Group. Network specific information is configured in this object. Refer to the ISDN Group topic for more information on configuring the individual fields in this object.
Right click on the ISDN BDN object and select New ISDN Circuit Group. Select the DS-1's/DS-0's that will be configured as B-Channels to the D-Channel configured earlier. Refer to the ISDN Circuit Group topic for more information on configuring the individual fields.
Configure B-Channels into ISDN Channel Group
Right click on the ISDN Channel Group created earlier and select New Circuit Group. Select the DS-1's/DS-0's that will be configured as B-Channels on this channel group. In this Circuit Group object, only the DS-1's/DS-0's that were configured in the section above will be displayed in the drop down menus. Typically the DS-1/DS-0 combination selected in the Circuit Group configured under the D-Channel should also be selected in the Circuit Group under the Channel Group. Refer to the ISDN Circuit Group topic for more information on configuring each of 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
ECT supports EURO ISDN only
The IMG 2020 will take on the role as the Served User.
The IMG 2020 will not support/process an ECT request. The request will be discarded
In order for ECT to be invoked following a SIP Refer, both ISDN B-Channels must be terminated on the same remote switch
ECT will be available only if the initial call was established between ISDN and a SIP device. The direction of the call is irrelevant.
ECT is triggered from the SIP call leg via the REFER method.
Both channel groups must have the ECT feature enabled for the feature to be invoked. Feature will not be available if one of the channel groups does not have ECT 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 ECT request only when both ISDN call legs are answered.
If ECT 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.