QSIG Un-Tromboning (TBCT)

Un-Tromboning, also known as TBCT (Two Bearer Channel Call Transfer), or call optimization is now supported on Vega 400s running SIP firmware. Where a call has been established through the Vega then subsequently transferred or forwarded, the situation can exist where a trombone (or hairpin) exists between the Vega and PBX such that two bearer channels are taken up by a single call.
The following scenarios are supported:
• Vega initiated un-tromboning, see diagram below. Un-tromboning initiated by the Vega on QSIG
so that the call is directly connected by the PBX.
• PBX initiated un-tromboning, see diagram below. Un-tromboning initiated by the PBX, resulting
in the transmission of SIP REFER message so that two IP endpoints are directly connected..
Both support for standard and Ericsson proprietary message format are supported.

Open

The Vega’s default behavior relies on detecting that two SIP call legs have the same call ID in order to initiate the QSIG side un-tromboning. Other headers can be checked and verified using the following parameters:
Parameter:
_advanced.sip.loopback_detection.sip_header
Possible Values:
String up to 31 characters – Default “NULL”. This is the header to look for to check for a SIP loopback call
Parameter:
_advanced.sip.loopback_detection.sip_header_regex
Possible Values:
String up to 127 characters – Default “NULL”. Format is in the form of a regular expression - the user must use < and > delimiters to find the unique component within the SIP header.
The flexible approach of specifying a regular expression was chosen as it allows other loopbacks to be detected when interacting with other third party devices. 

Example Using non-Call ID Detection 

In this example the following SIP header is sent to the Vega:
TWID: TW-CALL-SERVER-00000108-48d11387:-T2
It’s this header rather than the Call ID header which needs to be used to detect a SIP loopback. In this case the Call ID is different for the two legs of the call (so cannot be used for detection). 
To detect the “TWID” header the following settings would be used:
set ._advanced.sip.loopback_detection.sip_header=”TWID”
set ._advanced.sip.loopback_detection.sip_header_regex=”<TW\-CALL\-
SERVER\-.*>:.*”
In this case the Vega will look for two calls where the TWID header has the same content. Everything from the start of the TWID header up to (but not including) the “:-T2”. The position of the “<” and “>” indicate the section the vega will use for comparison.

Open

Configuration
The following parameters control Un-Tromboning:
Parameter
e1t1.port.1.isdn.untromboning_enable
Possible values:
0 – Default – Do not allow un-tromboning
1 – Enable un-tromboning
Parameter:
_advanced.isdn.untromboning.type
Possible values:
standard – Default – Use standard message format for un-tromboning
ericsson – Use Ericsson proprietary message format
Parameter:
_advanced.sip.loopback_detection
Possible values:
0 – Default – Disable SIP loop detection
1 – Enable loop detection for SIP calls