There are two methods supported by the IMG 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 currently supports Overlap Receiving where the destination address is contained in an IAM and SAM sequence of messages or a series of INVITES, each containing 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 IMG 1010 - Overlap Signaling - Receiving and IMG 1010 - Overlap Signaling - Sending - SIP to SS7 topics.
This feature will allow Overlap Sending of destination address digits. In this case each received IAM and subsequent SAM message will be interworked to 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 Examples
Call Trace - SS7 to SIP - Overlap
Call Trace - SIP to SS7 - Overlap
Configuration
Overlap sending is configured under the IMG 1010 - Channel Group Object. The IMG 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 Enabled. The configuration below explains receiving overlap signaling on an SS7 channel group and transmitting out a SIP channel group.
Initial Configuration
The procedure below explains how to configure SIP Privacy on the IMG. Before configuring, the IMG must have an initial configuration created on it. Below are links to the initial configuration needed.
Configure these initial objects before proceeding to the procedure below
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. Refer to the IMG 1010 - Channel Groups topic for more information on this object.
Right click on Channel Groups object and select New Channel Group. Enter a name to identify the channel group being created.
Select SS7 from the drop down menu of the Signaling Type field.
In the Overlap Enable field select Enabled from drop down menu. This will enable Overlap and opens the next four fields for editing. See screen capture below.
Termination Digit- Select from drop down menu the characters #, *, and 0-9
Minimum # of Digits- Minimum number of digits received to be able to route call.
Inter SAM Timeout- Maximum amount of time between each Subsequent Address Method (SAM). (Default = 1500 milliseconds or 1.5 seconds)
Total Overlap Timeout- Max time allowed for a call to be setup. (Default = 18000 milliseconds or 18 seconds)
Refer to the IMG 1010 - Channel Group topic for more information on configuring the channel group object.
Once the SS7 Channel group is created, the SIP channel group needs to be created. Right click on Routing Configuration and select New Channel Group.
Termination Digit - Not Used. SIP does not use termination digits.
Minimum # of Digits - Minimum number of digits received to be able to route call.
Inter SAM Timeout - Maximum amount of time between each Subsequent Address Method (SAM ). (Default = 4000 milliseconds or 4 seconds)
Total Overlap Timeout- Not Used
Overlap is enabled on both SS7 Channel Group and SIP Channel Group.
Additional Information:
Overlap Signaling is disabled by default
Overlap sending in Feature 1524 is used for interworking between SIP and SS7. The IMG already 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 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 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