Digital Routing and Mixing

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Here's what to consider in making the TDM vs. AoIP decision.

The way AoIP systems distribute signals is via the wire. A source is available to any destination on the network because the source is multicast onto the network. Let me explain a little bit about what that means.

TDM spoke

Recall that with a Layer 2 switch (like you see in use for the office network) any device can have a unicast connection to any other device that connects to the same switch. (Those two devices are going to need to be in the same subnet and the same VLAN as well.) A broadcast is a message that the Layer 2 switch floods throughout that entire VLAN. A multicast is a message that is meant to be heard by many hosts on the subnet (as opposed to a unicast, which is a one-to-one communication - and broadcast - which is heard by every host on the subnet). If every stream that represents an audio source is multicast on the local subnet, then any member of the multicast group will be able to not only hear that stream, but to further process it for use. This is fundamentally how the AoIP system types distribute audio. Whereas a TDM system has time-slots that are assigned to sources, and wired connections that carry specific signals to and from the router core, an AoIP system distributes audio over the shared network fabric.

Differ in practice

The sharing of the medium is the basic advantage of Ethernet (and IP communications in general). Fundamentally, networking multiple sources together is simplified by the use of Ethernet, mainly because of that fact that Ethernet is a standard. (This is to say the carrier - Ethernet - is a standard. The AoIP message within the Ethernet carrier is proprietary to the manufacturer.) The fact that it is used so much means that many sources are available for its components, and many sources means lower prices. That's just economics.

There is a caveat with the shared medium though. Whereas a TDM system has a throughput delay that is insignificant, AoIP systems do have a very small amount of delay because of the shared nature of the medium. It takes a finite amount of time to build up the packets, for the Layer 2 switch to process them, and then for the destination device to reassemble all the packets in the correct sequence (because it is possible for them to get out of sequence as they transit the network). The amount of delay that is objectionable to the human ear has been studied thoroughly in VoIP engineering and what has been learned in that field applies to AoIP as well. While those delays are not insignificant they are manageable, and minimized in the system engineering.

AoIP spoke

Both TDM and AoIP systems are scalable in that they can be increased in size after the original design and installation is done. In a TDM system, typically another input card or output card will be installed in a frame when and if more inputs or outputs are needed. Those new cards essentially live in the hub of the system. AoIP differs here in that the inputs/outputs are never part of the hub of the system; they are always at ends of the spokes. A new input/output module would be added to an AoIP system by putting it in a rack somewhere in the facility, and then making it part of the network by adding a single Ethernet cable from it to the local switch.

In terms of wiring one type of system versus the other, it really comes down to your wire management style. Typically TDM systems are based on card-cages, with connectors that carry dozens of circuits at a time. For this reason they are typically landed on external punch blocks because the only real practical way to deal with those large connectors is to terminate all of their associated circuits at the same time during the installation phase. AoIP systems are based around smaller nodes that have fewer inputs and outputs (usually eight pairs of inputs and/or outputs). The installer could chose point-to-point wiring on all those nodes; or, like with a TDM system, all those circuit connections could be landed externally on punch blocks. It really depends upon how you want to organize the wiring. In either case, the number of ingress/egress points is obviously going to be the same for systems that have the same capability (i.e., 128 by 128 TDM vs 128 by 128 AoIP). In other words, wiring is wiring. Both systems can be as simple or complex to install as the installer desires.

-- continued on page 4

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