Choosing a Network Switch for Audio Over IP
AoIP managed network
The solution is a managed switch that keeps track of the multicast traffic and forwards only the streams being used. However, with the hundreds of management protocols available, it's entirely possible to pick managed switches that aren't capable of managing multicast traffic. There's one set of letters to pick out from the acronym soup: IGMP.
Internet Group Management Protocol (IGMP) snooping restrains multicast traffic on a switched network, and was developed to manage multiple videoconferences. AoIP systems are derived from the same standards used in videoconferencing; therefore, the same management system accomplishes the same thing for audio.
There are two flavors of snooping. Passive snooping listens in on the traffic and does not filter or interfere with it in any way. Active or proxy snooping will filter traffic to lessen the load. The switch keeps track of what streams appear. When a user requests that stream, it then forwards that traffic only to the ports that are listening. When that user no longer needs the stream and stops listening, the switch stops sending that data to the port.
There must be one switch on the network that serves an IGMP querier. The querier creates the tables that keep track of the streams. Without this, snooping will not work.
Some switches are capable of snooping but not serving as a querier. These are intended to communicate with a switch that has the feature. If multiple switches with queriers are installed in a facility, only one will serve as the querier, as there can be only one master list.
The heart of the AoIP system
Pretty much any AoIP system can be hooked up as a snake back to back and pass every channel without a problem. When the system increases in scale, it's the network switch that manages the bandwith. In many respects, the maximum number of network channels is not a function of the AoIP appliance but the network switch that ties the appliances together.
-- continued on page 3
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