Appendix A: Keys to a Successful Mission-Critical VoIP Installation
While VoIP traffic is unforgiving of packet loss, its payload is thankfully fairly
predictable – a function of the number of calls in progress (see Figure 5A). But data
traffic, although forgi
ving of packet loss, is usually unpredictable (see Figure 5B).
Shared Private vs. Public Networks
The goal of making VoIP reliable is hampered when the bandwidth-to-payload ratio is
unpredictable. The problem with shared private networks (even LANs) is that the
combined payload is not only variable but often unpredictable (see Figure 5C) and the
m
ore applications that share the network, the more unpredictable it is (see the sidebar for
potential solutions to this problem). The problem with public networks (like the internet)
is that the bandwidth is variable – its theoretical maximum reduced by traffic from public
users (see Figure 6). If either the bandwidth or the payload is variable and the degree to
which the
y vary is unknown then the bandwidth-to-payload ratio is unpredictable.
The best network for mission-critical VoIP traffic is one in which both the bandwidth and
the payload are predictable and which has ample spare bandwidth left. Which means
avoiding public networks, and it means knowing or controlling how much peak payload
traffic all applications on a private network are generating. The easiest of all
configurations to reliably support mission-critical VoIP is a private, dedicated (non-
shared) network.
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