Since AoIP systems such as the Logitek JetStream make it a snap to send audio all over a facility over the LAN, it’s a snap to share audio over a WAN between studios, transmitters, or other cities, right?
Yes… and no.
Some background is in order. Popular Audio over IP systems such as the Logitek JetStream are using multicast streaming systems designed for video conferencing to send low latency audio across a local area network (LAN). We just aren’t sending any video along with the audio streams.
Every multicast packet has a “time to live.” This is effectively a boundary control. We know how long it would take for a packet to cross a router and get outside of a LAN, so the streaming protocol states that the time to live is shorter than it would take to make it through a router, which keeps your audio streams from going places where they do not belong.
So if you can get your hands on a piece of dark fiber between point A and point B, as long as there isn’t a router in the middle, it’s possible to put an IGMP aware switch on each end and connect two far away studios or a studio and transmitter site and have it all appear to be one big LAN.
What is this IGMP that everyone talks about? In AoIP, we use multicast packets to stream audio. These packets aren’t recognized by regular Ethernet switches, and when an Ethernet switch sees a packet it doesn’t know how to handle, its default move is to just let it through to every port, turning your office supply store “Smart Switch” into an early 90’s era Ethernet hub. IGMP, or the Internet Group Management Protocol recognizes these multicast packets; IGMP switches use what’s called IGMP Snooping to recognize the multicast streams and uses an IGMP Querier to keep track of which port on the switch is using which stream and thus regulates the traffic.
Using the IGMP Querier, the switch on one end of the fiber makes sure that only the streams being requested on the other end are going down the pipe, which ensures that we don’t flood the data link between the two points.
As a result, we can have JetStream at a transmitter site receive program audio from the studio and send back to the studio audio from satellite receivers or RPU units. We can send audio from a studio in one city to another. We can also send this audio back and forth between sites using a microwave IP radio.
The caveats: you will be limited in the number of streams you can have going back in forth by the bandwidth of your link, and you will want to use high latency streams between the two points to allow for any transmission delays across your link. If you are working with an ISP and fiber, make sure they understand that they cannot throw a router in the middle and that it must remain a point to point connection. If you are using a microwave IP radio, be aware that rain fade will reduce your available bandwidth, which will affect the number of channels that you are sending back and forth.
This setup isn’t for every station. If you only need to send a few channels in one direction you may be better served by a set of codecs. But if you need to send 5 to 8 stereo pairs and change the routing on the fly, talk to us about your needs, because this might work great for you.