AudioCodes Mediant 800B - Media Anchoring

Version 7.2 669 Mediant 800B Gateway & E-SBC
User's Manual 29. SBC Overview
Typically, the device does not change the negotiated media capabilities (mainly performed
by the remote user agents). However, it does examine and may take an active role in the
SDP offer-answer mechanism. This is done mainly to anchor the media to the device
(default) and also to change the negotiated media type, if configured. Some of the media
handling features, which are described later in this section, include the following:

Media anchoring (default)

Direct media

Audio coders restrictions

Audio coders transcoding

RTP-SRTP transcoding

DTMF translations

Fax translations and detection

Early media and ringback tone handling

Call hold translations and held tone generation

NAT traversal

RTP broken connections

Media firewall
• RTP pin holes - only RTP packets related to a successful offer-answer
negotiation traverse the device: When the device initializes, there are no RTP pin
holes opened. This means that each RTP\RTCP packets destined to the device
are discarded. Once an offer-answer transaction ends successfully, an RTP pin
hole is opened and RTP\RTCP flows between the two remote user agents. Once
a pin hole is opened, the payload type and RTP header version is validated for
each packet. RTP pin holes close if one of the associated SIP dialogs is closed
(may also be due to broken connection).
• Late rogue detection - once a dialog is disconnected, the related pin holes also
disconnect.
• Deep Packet inspection of the RTP that flows through the opened pin holes.
29.5.1 Media Anchoring
By default, the device anchors the media (RTP) traffic. In other words, the media between
SIP endpoints traverses the device. You can change this default mode by enabling direct
media between SIP endpoints. Media anchoring may be required, for example, to resolve
NAT problems, enforce media security policies, perform media transcoding, and media
monitoring.
To enforce RTP traffic to flow through the device, the device modifies all IP address fields
in the SDP:

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
The device uses different local ports (e.g., for RTP, RTCP and fax) for each leg (inbound
and outbound). The local ports are allocated from the Media Realm associated with each
leg. The Media Realm assigned to the leg's IP Group (in the IP Groups table) is used. If not
assigned to the IP Group, the Media Realm assigned to the leg's SIP Interface (in the SIP