AES67 Practical Guide
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All remaining traffic should have no priority tagging, which is BE (0) for
best effort
(economy
class)
Since the network may be used to transport other types of traffic which needs certain prioritization,
(i.e. voice data or video flows), the network administrator may have to change these values or adopt
the switch configuration accordingly. Since not all AES67 devices support DSCP reconfiguration (and
/ or use other values by default), other strategies may have to be applied (see guidelines section).
1.4
Session information
In order to connect to an available stream and process its audio data, a node needs technical
information about the stream. This is called session description data (SDP
7
) and contains information
about the multicast address of the stream, the encoding format and packet setup (i.e. bits per sample,
sampling frequency, number of channels in stream, number of samples in packet) and its relationship
to the reference time. Without this information, a receiver would not know how to connect to the
stream and how to decode the packet content.
While AES67 clearly defines any required SDP attribute and their allowed parameter ranges, it is
silent on the required method to convey this information. Session discovery (which would allow for
system-inherent detection of available streams) has also been deliberately excluded from the
standard requirements. While a number of protocols exist to announce available streams and
transport the related SDP data, the creators of the AES67 standard felt that it would have been too
stringent to actually mandate for a specific method; instead, they decided to just mention some
widely used protocols and to leave it to the device manufacturer to decide on and implement those
methods which would best suit their typical application environments
8
.
While most devices support mDNS/RTSP
9
(the default RAVENNA method) and SAP
10
(Dante devices
in AES67 mode), not all devices support both methods, and some don’t even offer manual read-out /
entry of SDP data. If there is no common method of sharing the required SDP information between
two devices, stream connection setup may be impossible or at least very difficult. The guidelines
section provides hints on how to potentially circumvent this problem.
7
Session Description Protocol -
https://en.wikipedia.org/wiki/Session_Description_Protocol
8
The AES67 standardization Task Group simply assumed that with no mandatory protocol in place a device
would provide a method to read / enter the SDP data by manual or other device-specific means (i.e. GUI or
other configuration interface).
9
mDNS = multicast DNS (also known as Bonjour) for service discovery, RTSP = real-time streaming protocol
for SDP transport
10
Session Announcement Protocol, an experimental multicast protocol for periodic session announcement
and SDP transport