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Latency and reliability are really directly related to more than just the VoIP system. If implementing this or any other converged application, a company needs to be sure that their house is in order FIRST. This should include an infrastructure audit (cabling, electronics, etc.) to be sure that no errors are present that would carry over to additional network devices and compound the problem. You will want the highest bandwidth available in your electronics and cabling. Traffic shapers and Quality of service have improved over the last year, but if your network is a mess, it won't really matter. The more bandwidth available the better off you will be. While the electronic vendors may tell you that category 5e is all that they require, you will have a much better system with category 6 due to higher bandwidth and better immunity to noise and other throughput bandits.
Outside of the actual infrastructure, the software has come a long way. H.323 was really developed for video conferencing and as such, has a lot in it that is not really needed for VoIP. SIP, on the other hand, helps quite a bit. This protocol allows a connection to be established for the conversation, similar in concept to a circuit being set up in the PSTN. This helps to assure that all packets arrive in order and on time (maybe the post office could benefit from this, but I digress!). Older versions operated in the "connectionless" IP layer with no session creation and as such, latency could be a problem (especially over pubic Internet lines). SIP operates at the Application layer and provides mapping services through upper layer "connection" oriented services.
All in all if PROPERLY planned and installed on a properly performing network, things should work smoothly. This industry has learned a lot, and has grown into a rather mature market place.
One final note: If you will be using Power over Ethernet for your phones, be careful to pay attention to which cabling pairs will supply the power. There are different configurations allowed in the standard and not all manufacturers utilize the same pairs.
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