AudioCodes Mediant 3000, User Manual

Page: 413 / 872

Version 6.6 413 Mediant 3000
User's Manual 28. SBC Overview

PRACK and UPDATE transactions can also be used for initiating subsequent
offer\answer transactions before the INVITE 200 OK response.
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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 or Anti-Tromboning feature enables the use of SBC signaling
capabilities without handling the RTP/SRTP (media) flow between remote SIP user agents
(UA). The RTP packet flow does not traverse the device and instead, the two SIP UAs
establish a direct RTP/SRTP 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. This is done in order to:

Solve NAT problems

Enforce media security policy

Perform media transcoding between the two legs

Media monitoring
However, since media packets traverse the SBC, media quality may degrade, for example,
due to packet delay.
In some setups, 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
RTP to flow between the UAs without traversing the SBC.

Internal LAN calls via WAN: In this setup, the SBC dynamically identifies that the call
is between UAs located in the same network (i.e., LAN) and thereby, directs the RTP
to flow between these UAs without traversing the SBC
In contrast to the regular SBC implementation, the No Media Anchoring feature:

Does not perform any manipulation on SDP data (offer/answer transaction) such as
ports, IP address, coders

Opening voice channels and allocation of IP media ports are not required
The No Media Anchoring feature is typically implemented in the following scenarios:

SBC device is located within the LAN.