Choosing a Network Switch for Audio Over IP


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All network switches are not created equal. While all carry Ethernet packets, there are limits to how many packets a switch can handle. Even an expensive business-class gigabit Ethernet switch may not be up to snuff.

Audio-over-IP systems use multicast streaming technology to send audio around the facility. Multicast streaming was designed for applications like videoconferencing. In a typical office where one or two conferences may happen for an hour or two at any given time, the excess load is minimal. However, in a radio station where many streams run non-stop, it does not take long to overload the network.

AoIP unmanaged network

Each audio device has its own private IP address, and each device is sending and receiving multiple streams of audio. Those streams are assigned Class D multicast addresses. This address range is between 224.0.0.0 to 239.255.255.255. Most of that range is reserved for WAN and Internet specific uses by the Internet Assigned Numbers Authority. However, there is a block between 239.192.0.0 - 239.251.255.255 reserved for organizations to use locally; this block is used by AoIP devices. Every audio stream gets its own multicast address assigned automatically by its sender, and all network switches will pass that data.

The problem is that once all of the audio devices are powered up and sending anywhere from 8 to 64 different streams out to the network the switch will dutifully pass that traffic to every port -- along with the rest of the data flowing across the network -- whether the other ports need all of that data or not. This is called flooding. Like in a denial of service attack, once you fill the pipe with enough unwanted traffic, needed packets won't get through, and the end result is clicks, pops and holes in the audio.

Unless you are building a very small AoIP system, an unmanaged switch will simply not work because the streams that are transmitting but not being used at the moment will flood out the on-air streams.

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