No Media Anchoring - AudioCodes Mediant 3000 User Manual

User's Manual
If the above prerequisites are not met, the SDP offer is sent without the extended coders
list. The coders from the extended list are added after the ones from the original offer
(decreases transcoding probability). Coders common between the extended coders list and
those in the original SDP offer are not added. Transcoding may be performed even in
scenarios when the same coder has been chosen - this occurs if the coders use different
coder parameters (e.g. rate and packetization time).
The device also supports early media, whereby the first offer\answer transaction is finalized
and the media flow starts before the SIP call is connected (before the INVITE 200 OK
response). The offer and answer options can be included in the following SIP messages:

Offer in first INVITE, answer on 180, and no or same answer in the 200 OK

Offer in first INVITE, answer on 180, and a different answer in the 200 OK (not
standard)

INVITE without SDP, offer in 180, and answer in PRACK

PRACK and UPDATE transactions can also be used for initiating subsequent
offer\answer transactions before the INVITE 200 OK response.

In a SIP dialog life time, media characteristics after originally determined by the first
offer\answer transaction can be changed by using subsequent offer\answer
transactions. These transactions may be carried either in UPDATE or ReINVITE SIP
transactions. The media handling is similar to the original offer/answer handling. If the
offer is rejected by the remote party, then no media changes occur (e.g. INVITE
without SDP, then 200 OK and ACK, offer\answer within an offer/answer, and Hold
ReINVITE with IP address of 0.0.0.0 - IP address is unchanged).

28.4.3 No Media Anchoring

The No Media Anchoring (commonly referred to as Anti-Tromboning) feature enables the
use of SBC signaling capabilities without handling the media (RTP/SRTP) flow between
remote SIP user agents (UA). The media flow does not traverse the device. Instead, the
two SIP UAs establish a direct media flow (i.e., direct call) between one another. Signaling
continues to traverse the device with minimal intermediation and involvement to enable
certain SBC abilities such as routing.
By default, media packets traverse the device to solve NAT problems, enforce media
security policy, perform media transcoding between the two legs, and media monitoring. In
certain deployments, specific calls do not require media anchoring, for example, when
there is no need for NAT, security, or transcoding. This is typical for calls between users in
the LAN:

Internal LAN calls: When the SBC routes a call between two UAs within the same
LAN, the SBC can forward the SDP directly between caller and callee, and direct the
media to flow between the UAs without traversing the SBC.

Internal LAN calls via WAN: In this setup, the SBC dynamically identifies the call as
between UAs located in the same network (i.e., LAN) and thereby, directs the media
to flow between these UAs without traversing the SBC.
The No Media Anchoring feature is typically implemented in the following scenarios:

The device is located within the LAN.

Calls between two SIP UAs in the same LAN and signaling is sent to a SIP proxy
server (or hosted IP PBX) located in the WAN.
Version 6.8 495
28. SBC Overview
Mediant 3000