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Basic Principles of AES67
AES67 is intended to run on standard packet switching networks build from COTS
configured properly, other traffic can share the same network without degrading the audio streaming
experience.
AES67 builds on these fundamental principles:
•
Synchronization
•
Multicast packet transport
•
Quality of Service
•
Session information
While other AoIP solutions offer enhanced functionality (i.e. stream & device discovery, GPIO
transport etc.), AES67 has deliberately not defined any requirements in this respect, because they are
not required to establish interoperability on the most basic level. Furthermore, various industry
standards covering these functions already exist or are emerging
applicable.
1.1
Synchronization
Synchronization is based on distribution of a common wall clock time to all participating nodes with
sufficient precision. AES67 specifies the IEEE1588-2008 standard (also known as PTPv2 - Precision
Time Protocol version 2)
compatible with PTPv1 (IEEE1588-2002). PTPv2 includes a Best Master Clock Algorithm (BMCA)
which ensures that the best available master clock is elected to serve as the Grandmaster for all
participating AES67 nodes. In the guidelines section, hints are provided on how the Grandmaster
selection can be modified, if required. Once a node is synchronized to the wall clock time served by
the Grandmaster, any desired media clock can be generated locally. If the synchronization precision
is accurate enough, all locally generated media clocks will have the same frequency (i.e. 48 kHz) and
they may even be accurately phase-locked to each other.
With PTP it is possible to achieve accuracy in the sub-microseconds range (deviation of local clock
with respect to the Grandmaster). However, in most cases this requires the deployment of PTP-aware
switches. Fortunately, for most audio applications, single-digit microsecond accuracy is still good
enough, which usually is achievable with standard, non-PTP-aware switches.
1.2
Multicast packet transport
While PTP is based on multicast packet transport, AES67 also mandates for multicast support of
audio stream packets. While basically any COTS switch supports multicast traffic, only managed
switches provide multicast management to effectively avoid network flooding. Unmanaged switches
(or improperly configured managed switches) will treat multicast traffic like broadcast traffic,
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Conventional-Off-The-Shelf
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DNS-SD / mDNS, NMOS IS-04 etc.
3
https://en.wikipedia.org/wiki/Precision_Time_Protocol
3
to be used for time distribution. Note that PTPv2 is not backward-
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AES67 Practical Guide
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infrastructure. If
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and can be implemented if
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