AudioCodes Mediant 500 E-SBC, User Manual

Page: 451 / 920

User's Manual 27. SBC Overview
Version 6.8 451 Mediant 500 E-SBC

No Media Anchoring: The RTP packet flow does not traverse the device. Instead, the
two SIP UA's establish a direct RTP/SRTP flow between one another (see ''No Media
Anchoring'' on page
451 ).
27.4.1 Media Anchoring without Transcoding (Transparent)
To direct the RTP to flow through the device (for NAT traversal, firewall and security), all IP
address fields in the SDP are modified:

Origin: IP address, session and version id

Session connection attribute ('c=' field)

Media connection attribute ('c=' field)

Media port number

RTCP media attribute IP address and port
Each SBC leg allocates and uses the device's local ports (e.g., for RTP\RTCP\fax). The
local ports are allocated from a Media Realm associated with each leg. The legs are
associated with a Media Realm as follows: If the leg's IP Group is configured with a Media
Realm, then this is the associated Media Realm; otherwise, the leg's SRD Media Realm is
the associated one. The figure below illustrates an example of SDP handling for a call
between a LAN IP Phone 10.2.2.6 and a remote IP Phone 212.179.1.13 on the WAN.
Figure 27-3: SDP Offer/Answer Example
27.4.2 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