Overlap Signaling (Sending)
There are two methods supported by the IMG 2020 to interwork Called Party Number digits between SIP & SS7. One method labeled Enbloc requires that all the Destination address information be included in the IAM or INVITE. The other method is Overlap. The IMG 2020 supports both Overlap Sending and Overlap Receiving. In these methods, the destination address is contained in the IAM message as well as the subsequent SAM sequence of messages. The IMG 2020 creates a series of INVITES. Each invite contains one or more destination address digits. The received digits are buffered until the configured minimum number of digits is received at which point the outgoing call is made with the destination address Enbloc. Refer to the Overlap Signaling (Receiving) topic.
This feature will allow Overlap Sending of destination address digits. In this case each received IAM and subsequent SAM message will be interworked into a series of INVITES each containing all the destination address digits received to that point, similarly when the SIP interface receives a series of INVITES, each containing additional destination address digits to the previous one, these will be mapped into an IAM, SAM sequence with each message containing only the 'new' digits of the latest INVITE.
Call Flow Diagram
The Diagram below assumes that the Minimum Number of Digits field in the Channel Group pane is set to 1
Call Trace Example
Click on the IMG 1010 - Call Trace - SIP to SS7 link for Call Trace Information
Click on the IMG 1010 - Call Trace - SS7 to SIP link for Call Trace Information
Configuration
Overlap sending is configured under the Channel Group Object. The IMG 2020 supports interworking the Overlap signaling messages from SIP to SS7 and from SS7 to SIP, therefore Overlap sending is configured in both the SIP and SS7 Channel Groups. When configuring Overlap, both the incoming and outgoing channel groups must have Overlap set to Enabled. The configuration below describes the configuration needed to receiving overlap signaling on SS7 channel group and transmit onto SIP channel group.
Initial Configuration
The procedure below explains how to configure SIP Privacy on the IMG 2020. Before configuring, the IMG 2020 must have an initial configuration created on it. Follow the Basic Configurations procedure before proceeding.
Create SS7 Channel Group
Right click on Routing Configuration and selecting New Channel Groups. The Channel Groups object is a container object for individual channel groups to be created under. No configuration is needed. For more information, refer to the Channel Groups topic.
Right click on Channel Groups object and select New Channel Group. Enter a Name to identify this channel group and select SS7_ISUP from drop down menu in the Signaling Type field.
In the Overlap Enable field select Enabled from drop down menu. This will enable Overlap and opens the four fields displayed below. Edit the fields described below to configure Overlap signaling. Refer to the Channel Group topic for other information on configuring the channel group object.
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Create and configure a SIP channel group by right clicking on Routing Configuration and select New Channel Group. Either enter a name that identifies this channel group or accept the default entry.
Select SIP from drop down menu in the Signaling Type field.
In the Overlap Enable field select Enabled from drop down menu. This will enable Overlap and opens the following two fields for editing. See screen capture below.
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Refer top the Channel Group topic for information on configuring the individual field within the channel group object.
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.
The Overlap Signaling feature is now configured. Routing and Translations can now be configured.
Additional Information
Overlap Signaling is disabled by default
Overlap sending is used for interworking between SIP and SS7. The IMG 2020 supports receiving Overlap signaling and converting to en-bloc signaling. This feature supports both receiving and sending of Overlap signaling
Each new INVITE message is a new transaction and could potentially be routed to different endpoints/gateways. Ensure that multiple INVITE requests sharing a desired prefix will reach the same external gateway. This could be done through the routing configuration, outbound proxy routing, or load balancing.
If Overlap is enabled and the minimum number of digits is not received, a 484 response will be sent with a reason phrase Overlap Enabled, Insufficient number of digits.
When configuring Overlap, the parameters are configured on a per channel basis.
On inbound SIP side, the SIP side does not buffer or re-order the INVITE messages received as Overlap Signaling.
Receipt of 2nd INVITE message will evoke a 484 response and is handled entirely on the SIP side. No SS7 resources are used.
An initial INVITE message will immediately translate into an IAM if the Minimum number of digits field is set to 1 in the corresponding channel groups. This is only way to guarantee that both SIP and GCL will not buffer the received digits until subsequent INVITE messages are received.
If a subsequent INVITE is received, SIP will propagate only the additional digits received in the R-URI to the SS7 side.
When sending an INVITE message, a timer is initiated to monitor the receipt of another SAM.
If Timer expires before receiving a SAM, an early ACM will be sent.
SIP-T is not supported when overlap signaling is enabled
If an INVITE request is forked the IMG 2020 will decide which media stream to be played. This is decided by the number of digits. A media stream corresponding to the response to an INVITE with a greater number of digits is to be given greater priority.
Timer Values are taken from Q.1912.5, 7.8, table 41