Srtp-Rtp And Srtp-Srtp Transcoding; Multiple Rtp Media Streams Per Call Session - AudioCodes Mediant 800B User Manual

User's Manual
listed in the SDP offer. This feature is referred to as Coder Preference and applies to both
SBC legs:

Incoming SDP offer: The device arranges the coder list in the incoming SDP offer
according to the order of appearance of the Allowed Audio Coders Group that is
associated with the incoming dialog. The coders listed higher up in the group take
preference over ones listed lower down. To configure this, configure the 'Allowed
Coders Mode' parameter (IpProfile_SBCAllowedCodersMode) in the associated IP
Profile to Preference or Restriction and Preference. If you configure the parameter
to to Preference, the coders in the SDP offer that also appear in the Allowed Audio
Coders Group are listed first in the SDP offer, and the coders in the SDP offer that do
not appear in the Allowed Audio Coders Group are listed after the Allowed coders in
the SDP offer. Therefore, this setting does not restrict coder use to Allowed coders,
but uses (prefers) the Allowed coders whenever possible.

Outgoing SDP offer: If only Allowed coders are used, the device arranges the coders
in the SDP offer as described above.
However, if Extension coders are also used, the coder list is arranged according to the
SBCPreferencesMode parameter. Depending on the parameter's settings, the
Extension coders are added after the Allowed coders according to their order in the
Allowed Audio Coders Group, or the Allowed and Extension coders are arranged
according to their position in the Allowed Audio Coders Group.

29.5.7 SRTP-RTP and SRTP-SRTP Transcoding

The device supports transcoding between SRTP and RTP. The device can also enforce
specific SBC legs to use SRTP and/or RTP. The device's handling of SRTP/RTP is
configured using the IP Profile parameter, SBCMediaSecurityBehaviour, which provides
the following options:

SBC passes the media as is, regardless of whether it's RTP or SRTP (default).

SBC legs negotiate only SRTP media lines (m=); RTP media lines are removed from
the incoming SDP offer-answer.

SBC legs negotiate only RTP media lines; SRTP media lines are removed from the
incoming offer-answer.

Each SDP offer-answer is extended (if not already) to two media lines for RTP and
SRTP.
If after SDP offer-answer negotiation, one SBC leg uses RTP while the other uses SRTP,
the device performs RTP-SRTP transcoding. To translate between RTP and SRTP, the
following prerequisites must be met:

At least one supported SDP "crypto" attribute.

The EnableMediaSecurity parameter must be set to 1.
Transcoding where both legs are configured for SRTP is typically required to trans-encrypt
and trans-decrypt. This is relevant when the MKI and Symmetric MKI parameters are
enabled. In other words, both sides need to both encrypt and decrypt the outgoing and
incoming SRTP packets, respectively.
DSP resources are not required for RTP-SRTP transcoding.

29.5.8 Multiple RTP Media Streams per Call Session

The device's SBC application supports multiple RTP media streams per SBC call session.
It supports the negotiation of up to five media streams ('m=' line) in the SDP offer/answer
model per session. The media can include a combination of any of the following types:

Audio, indicated in the SDP as 'm=audio'
Version 7.2 677 Mediant 800B Gateway & E-SBC
29. SBC Overview