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IP Audio Begins Interoperability Journey
Closely related to the network time that is continually adjusted by way of PTP, is the function of the media clock. On the input side of the AoIP system, the media clock sets the sampling rate for the ADC at one of the standard's supported frequencies, 44.1-, 48- or 96kHz. Likewise, on the output side, the media clock is used by the DAC. The media clock has a fixed relationship to the network (grand-master) clock; for example, the media clock used for an ADC at a 48kHz sampling rate will advance 48,000 samples for each elapsed second on the network clock.
In the AES67 standard, media packets are transported using IP version 4, though care has been taken in its design so that the standard can facilitate IPv6 at a later date.
All participating devices must support IGMPv2 and optionally IGMPv3. IGMP (Internet Group Management Protocol) will be used for devices requesting multicasts, such as the PTP messages, media streams, and discovery (more on that a little later).
Propagation time through a network must necessarily be low for media packets that comply with AES67. For this reason, quality of service (QoS) becomes a necessary feature in the network. All devices will use the DiffServ method, meaning that the DSCP (Differentiated Services Code Point) field in the header of each media packet will be marked according to their traffic class, allowing the network to recognize the need of said packets for preferential treatment. All devices will tag outgoing packets with their appropriate DSCP values.
Encoding and Streaming
Several audio payload formats are supported in AES67: L16 (16-bit linear) and L24 (24-bit linear). All devices must support a 48kHz sample rate, and should support 44.1- and 96kHz sample rates.
Multicasting of streaming data is an efficient means by which audio information can be distributed on a one-to-many basis. Multicasting is also an important component in connection management (described later). All receivers must be able to receive multicast and unicast streams. In AES67 only one device will send to a multicast destination. The destination address for a particular stream is configured in the management interface of the sending device.
Discovery is used by participants on the network to build a list of other participants or sessions available on the network. The list is then available to end-users to facilitate connection management. Connection management requires a Session Initiation Protocol URI (uniform resource identifier) or a Session Description Protocol description. Although no discovery service is mandated by this standard, these can be delivered via one of a number of means, including: Bonjour (for Apple); SAP version 2; Axia Discovery Protocol; or Wheatstone's Wheatnet IP discovery protocol.
Sessions descriptions are used by discovery, and in connection management (described a little later in this article). Session descriptions specify critical information about each stream including network addressing, encoding format and origination information. Interoperability imposes additional SDP requirements and recommendations for the following aspects:
■ Packet time
■ Clock source
■ RTP clock and media clock offset
■ Payload type
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