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VoIP at HQ
If the voice network at your company's headquarters is anything like most large companies' networks, odds are you don't have a prayer of running VoIP over your existing data network. The bandwidth and frame-forwarding requirements of a very large-scale VoIP deployment are likely too much for your data infrastructure.

Both bandwidth and frame-forwarding can foil even the most ambitious network design, mostly because of inconsistent codec (coder/decoder) implementations in first-generation products. Different vendors use different codecs for low-bandwidth VoIP, so there isn't much hope for anything other than G.711 across vendor lines. With G.711 generating 64 Kbps of continuous traffic at each end when there is any sound, you're forced to plan for 128 Kbps of constant utilization for every VoIP call.

The other side of the coin is the rate at which this traffic travels over your data network. Codecs sample audio, then send the samples in small IP packets. A key part of the utilization problem stems from the frequency of sample transmission. Some vendors sample every 60 milliseconds, resulting in lower frame rates (and overall quality), while others sample as often as every five milliseconds, which sends 300 packets per second over your network.

Even if your infrastructure can handle the data traffic, it may not keep up with the frame-forwarding rates. Taken together, these factors can easily become overwhelming. You also must consider that the cost of building a data network that can handle these levels of traffic for hundreds or thousands of users--including all the new Fast Ethernet switches, the gigabit core switches, the OC-12 backbone wiring, the maintenance agreements and so on--could easily exceed the cost of building an equivalent PBX-based infrastructure.

That's why we feel VoIP deployments should be limited to sites with only 100 or so users on contained networks. The largest working installations we know of are in the 500-seat range on networks designed to handle the traffic generated by VoIP. But even though you may not yet be able to deploy VoIP across your large-scale backbone, this does not preclude its use in smaller installations. If you have a floor, wing or isolated department with a couple hundred users and a state-of-the art infrastructure, you could easily deploy VoIP there, bringing those users into the corporate fold through an H.323 gateway to your existing PBX infrastructure. In fact, we recommend this sort of trial deployment to measure usage and other issues in your environment.