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 AoE protocols. Protocols can be broadly categorized into Layer-1, Layer-2 and Layer-3 systems based on the 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 rather than the one native to Ethernet, which generally creates compatibility issues and thus requires a dedicated network for the protocol.
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 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
Layer-3 protocols encapsulate audio data in OSI model layer 3 packets. By definition it does not limit the choice of protocol to be the most popular layer-3 protocol, the Internet Protocol. In some implementations, the layer-3 audio data packets are further packaged inside OSI model layer-4 packets, most commonly User Datagram Protocol or Real-time Transport Protocol. Use of UDP or RTP 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 IP packets can traverse the Internet, most layer-3 protocols cannot provide reliable transmission over the Internet due to the limited bandwidth, significant End-to-end delay and packet loss that can be encountered by data flow over the Internet. For similar reasons, transmission of layer-3 audio over wireless LAN are also not supported by most implementations.
RockNet by Riedel Communications, uses Cat-5 cabling. Hydra2 by Calrec uses Cat-5e cabling or fiber through SFP transceivers. MADI uses 75-ohm coaxial cable with BNC connectors or optical fibre to carry up to 64 channels of digital audio in a point-to-point connection. It is most similar in design to AES3, which can carry only two channels. AES47 provides audio networking by passing AES3 audio transport over an ATM network using structured network cabling. This was 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. Some of 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, 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.
