Audio over Ethernet

In audio engineering and broadcast engineering, Audio over Ethernet (sometimes AoE—not to be confused with ATA over Ethernet) is the use of an Ethernet-based network to distribute real-time digital audio. It is designed to replace bulky snake cables and fixed wiring, and instead use a standard network structured cabling in a facility, providing a reliable backbone for any audio application, such as for large-scale sound reinforcement in stadiums, airports and convention centers, multiple studios or stages.

While on the surface AoE bears a resemblance to Voice over IP (VoIP), AoE is intended for high-fidelity, low-latency professional audio. Because of the fidelity and latency constraints, audio over Ethernet systems generally do not utilize audio data compression. AoE systems use a much higher bit rate (typically 1 Mbit per channel) and much lower latency (typically less than 10 milliseconds) than VoIP. Audio over Ethernet requires a high-performance network. Performance requirements may be met through use of a dedicated local-area network (LAN) or virtual LAN (VLAN), overprovisioning or Quality of service features.

Most AoE systems use proprietary protocol(s) (at the higher OSI layers) which create data packets and data frames that are transmitted directly onto the Ethernet (layer 2) for efficiency and reduced overhead. The word clock may be provided by broadcast packets.

Protocols

There are several different and incompatible protocols for audio over Ethernet. For example, using category 5 cable and 100BASE-TX signaling at 100 Mbits/second, each link can generally transmit between 32 and 64 channels at a 48 kHz sampling rate. Some can handle other rates and audio bit depths, with a corresponding reduction in number of channels.

AoE is not necessarily intended for wireless networks, thus the use of various 802.11 devices may or may not work with various (or any) AoE protocols.

Protocols can be broadly categorized into Layer 1, Layer 2 and Layer 3 systems based on the lowest layer in the OSI model where the protocol exists.

Layer 1 protocols

Layer 1 protocols use Ethernet wiring and signaling components but do not use the Ethernet frame structure. Layer 1 protocols often use their own media access control (MAC) rather than the one native to Ethernet, which generally creates Computer compatibility issues.

Open standards

• AES50
  • SuperMAC, an implementation of AES50^[1]
  • HyperMAC, a gigabit Ethernet variant of SuperMAC^[2]
• MaGIC by Gibson^[3]

Proprietary

• A-Net by Aviom^[4]
• AudioRail^[5]
• RockNet by Riedel Communications^[6]
• Hydra2 by Calrec^[7]

Layer 2 protocols

Layer 2 protocols encapsulate audio data in standard Ethernet packets. Most can make use of standard Ethernet hubs and switches though some require that the network (or at least a VLAN) be dedicated to the audio distribution application.

Open standards

• AES51, A method of passing ATM services over Ethernet that allows AES3 audio to be carried in a similar way to AES47
• Audio Video Bridging (AVB), when used with the IEEE 1722 profile (which transports IEEE 1394/IEC 61883 over Ethernet frames, using IEEE 802.1AS for timing)

Proprietary

• CobraNet
  • RAVE by QSC Audio, an implementation of CobraNet^[8]
• EtherSound by Digigram^[9]
  • NetCIRA, a rebranded EtherSound by Fostex
• REAC by Roland^[10]
• SoundGrid by Waves Audio

Layer 3 protocols

Layer 3 audio over Ethernet protocols encapsulate audio data in Open Systems Interconnection model (OSI model) Layer 3 (Network Layer) packets. By definition it does not limit the choice of protocol to be the most popular Layer 3 protocol - the Internet Protocol (IP). In some implementations, the Layer 3 audio data packets are further packaged inside OSI model Layer 4 (Transport Layer) packets, which is usually the User Datagram Protocol (UDP). Use of the UDP/IP protocol to carry audio data enables them to be distributed through standard computer routers, thus a large distribution audio network can be built economically using commercial off-the-shelf equipment.

Although by definition, IP packets can traverse the Internet, the Layer 3 audio over Ethernet protocols are not designed to traverse the Internet and provide reliable audio transmission due to the limited bandwidth and significant transmission delay usually encountered by data flow over the Internet. Due to similar reason, transmission of Layer 3 audio over wireless LAN are also not supported by most implementations.

Open standards

• AES67^[11]
• Audio Contribution over IP standardized by the European Broadcasting Union
• Audio Video Bridging (AVB), when used with IEEE 1733 or AES67 (which uses standard RTP over UDP/IP, with extensions for linking IEEE 802.1AS timing information to payload data)
• NetJack a network backend for the JACK Audio Connection Kit^[12]
• RAVENNA by ALC NetworX

Proprietary

• Livewire by Axia Audio, a division of Telos Systems
• Dante by Audinate
• Q-LAN by QSC Audio Products
• WheatNet-IP by Wheatstone^[13]

Similar concepts

MADI or AES10 from the Audio Engineering Society is similar in function but uses 75-ohm coaxial cable with BNC connectors or optical fibre with ST1 connectors. It is most similar in design to AES3, which can carry only two channels.

AES47 from the Audio Engineering Society provides linear audio networking by passing AES3 audio transport over an ATM network using structured network cabling (both copper and fibre). This is used extensively by contractors supplying the BBC's wide area real-time audio connectivity around the UK.

Audio over IP differs in that it works at a higher layer, encapsulated within Internet Protocol. These systems are usable on the Internet, but may not be as instantaneous, and are only as reliable as the network route — such as the path from a remote broadcast back to the main studio, or the studio/transmitter link (STL), the most critical part of the airchain. This is similar to VoIP, however AoIP is comparable to AoE for a small number of channels, which are usually also data-compressed. Reliability for permanent STL uses comes from the use of a virtual circuit, usually on a leased line such as T1/E1, or at minimum ISDN or DSL.

In broadcasting and to some extent in studio and even live production, many manufacturers equip their own audio engines to be tied together with Ethernet. This may also be done with gigabit Ethernet and optical fibre rather than wire. This allows each studio to have its own engine, or for auxiliary studios to share an engine. By connecting them together, different sources can be shared among them. Logitek Audio is one such company using this approach.

See also

• Comparison of audio network protocols
• Ethernet Powerlink
• HDBaseT

References