Multicast DHCP

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Put in practice

In an IT context, let's say we have a printer in the sales office. Over time, it wears out and you replace it. In a Multicast DNS environment, you plug in a new printer and give it the same name. It's a different piece of hardware, it has a different address on the network, but you don't need to change 30 different PCs, because when someone in the sales office goes to print, he sees his printer automatically.

That list of printers with human-friendly names such as “Sales Office” and “Newsroom” is Multicast DNS in action. Like the global DNS system that translates Internet IP addresses into friendlier names like, Multicast DNS translates those names into IP addresses without needing to maintain a large DNS server.

In a radio station, we're dealing with streaming audio sources. When a device enters the network, it asks what else is out there and compiles a list of other streams. When the other devices ask what is out there, it replies in kind, and it gets added to their list of streams.

Think of an instant messaging system. When you want to chat, you're not interested as much in who all of your friends are as you are interested in who is available to talk right now and when they are no longer available. There isn't always a server keeping track of which friends are online; your computers can do it directly with Multicast DNS.

Rather than requiring an engineer to enter audio sources and their related IP address by hand, modern IP audio systems use Multicast DNS to keep track of what streams are available and where they are located automatically.

A common myth about Multicast audio systems is that a Multicast DNS component uses too much bandwidth on the network and ultimately interferes with the audio streams. That's not the case. Just as an office filled with PCs and other peripherals doesn't grind to a halt from the multicast traffic, a radio station using a modern IP audio system will continue to run. Because all devices are listening to the same multicast channel, they hear the same answers. When the first device asks for a list of sources, the others listening will take the answer that the responding device gives and use it to fill in whatever they do not have. This keeps redundant information from being transmitted repeatedly.

The final leg of the stool is DNS Service Discovery. Multicast DNS fills the need for small systems to operate without adding a domain server to the facility. However, in a large studio complex, a domain controller will make things easier to manage. If a Multicast DHCP AoIP system finds a suitable DNS server, it will use it. If it's not there (or if it's down) the system can function using Multicast DNS. Like link-local addressing, Multicast DNS offers both a safety net in large facilities and simplicity in small ones.

The key is that we are browsing for services, not addresses; the user knows the equipment by name and not IP address. Addresses can change, and even the equipment can be repaired or swapped out, but every time someone looks for a source, they'll get the audio they're looking for.

Thanks to networking principles proven in the IT world, Multicast DHCP simplifies IP audio networks and makes them faster to install and easier to maintain.

Davis works in tech support for Logitek Electronic Systems, Houston.

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