Sizing the Ethernet Switch

Sizing Switches Illustration

Whenever we sit down with a broadcaster to design an audio-over-IP network, it doesn't take long before we get into Ethernet switches. The right switch is that important, second only to the type of cable used.

We maintain a list of Ethernet switches by make and model that we've tested and approved for our WheatNet-IP network, so all the necessary requirements like whether or not it can query IGMP (it should) are covered. Other decisions, like the size of the switch, are not so obvious.

You might have been told in the past to go with the biggest switch you can find to support the largest number of multicast groups, or IP address groupings that represent channels on a console, for example. But you might not have to, so why spend extra for something you don't need? A lot of older AoIP system designs don't manage the multicast streams, and actually keep the pathway open for them, which takes up valuable, expensive resources in the switch. But because the WheatNet-IP system continually responds to IGMP queries checking for usable source groupings on the network, any unneeded multicast streams are pruned as a result.

Pruning means that, barring any major changes, the switch you get today for your WheatNet-IP system will not fall short as time goes on and as old channel assignments lose their relevancy.

The quick math for determining how to size a switch at the core or at the edge of your WheatNet-IP Intelligent Network is to count each source as a multicast group. Later, we'll get out the precision math and count wire runs, switch locations, number of PCs, number of I/O BLADEs, and the sources for each console. But as a general rule you can use this formula to give you a ballpark figure that builds in some extra headroom for growth.

For example, if you have an E-6 console with 24 bus minuses, 4 program busses, 4 aux busses, 4 mix-minuses, and 4 monitor outputs, that's 40 multicast groups for that particular console. Do this for each console plus add any hybrids, PC drivers, satellite drivers, mic channels and processing feeds for the core switch plus any switches you'll need at the edge of your network. If the resulting number is well within 255, you can be fairly certain that a switch specified for 255 multicast groupings will work. If your number bumps up closer to 1,000, you will want to consider a larger switch.

In addition to sizing the switch, our systems engineer Kelly Parker suggests you look carefully at the services supported by the switch or stack of switches. If you plan to take advantage of Power over Ethernet found in our TS-22 Talent Station, for example, you'll want to determine how many PoE ports are available, if any. But be forewarned. PoE can double the cost of the switch, depending on how many devices you're going to power, according to Parker. He recommends broadcasters spring for PoE in the switch if it can save them the cost, hassle and eyesore of mounting a power strip under studio furniture for several talent stations; one Ethernet cable strung along the bottom of the countertop is much, much easier to deal with in those cases.

Other considerations for selecting an Ethernet switch are level of operation, such as if it needs to support intra-VLAN routing between buildings (although you might do better to connect two physical networks) and whether or not you'll go with managed or unmanaged, which is dependent on budget and the extent of hardware you will need it to support. Unmanaged switches are less expensive, so it's tempting to go this route – and many stations do. Managed switches are a little more, but are necessary for a larger, more complex system that will need to be set up to specifications with a software interface of some kind (e.g. telnet, web, terminal).

For more information on selecting the right switch, download Kelly's WheatNet-IP Switch Selection Document. To determine bandwidth considerations for I/O, switch topology and more, download Kelly's paper Network Design Considerations for Ethernet Audio.


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