Nortel 1000 Description, Installation And Operation Manual page 157

In a VoIP network, there are three primary benefits of TAT.
1 As in a TDM network, TAT eliminates tromboning of trunks and frees
up valuable trunking resources.
2 TAT provides a method of reducing bandwidth requirements, which can
be crucial over a slow WAN link. If TAT is not used, a tromboned call
using a G.729 codec can theoretically use 60-70Kbps on a WAN link. By
using TAT, bandwidth can be reduced to zero for a tromboned call.
3 TAT improves voice quality. If a call is tromboned using a G.729 codec,
multiple transcodings can diminish voice quality. Since each transcoding
introduces errors for a G.729 codec, the goal is to eliminate as many hops
as possible. TAT provides the means to accomplish this.
TAT call Scenario
The following call scenario helps to understand TAT.
1 Site 1 and Site 2 both have an IP Trunk 3.01 (or later) node installed. IP
Trunk 3.01 (and later) is used for trunking between the two sites.
2 Telephone A at Site 1 calls Telephone B at Site 2. Telephone B answers
the call and decides to transfer the call to Phone C which is located at
Site A.
3 Telephone C answers the call transferred from Telephone B at Site 2.
4 After the call has been answered by Telephone C, Site B sends a TAT
Invoke message to Site A. Site B only sends a TAT Invoke message if
the Tromboned Trunks belong to the same D-Channel and Customer. If
a customer has multiple DCHIP cards in their node, The first leg of the
call could be associated with one D-Channel and the second leg of the
call associated with another D-Channel. In this case, TAT will not be
invoked.
To prevent problems, the following recommendations are made:
• The use of multiple DCHIPs in a node or the use of multiple IP Trunk
3.01 (and later) nodes in a system must be implemented with caution. It
can lead to poor voice quality in certain call scenarios.
ITG engineering guidelines
IP Trunk Description, Installation, and Operation
Page 157 of 644