If the existing LAN is made up of standards-based Ethernet switches, it is fairly straight forward to set up VoIP on the network. Most small companies do not configure QoS on the LAN. The most straight forward deployment of VoIP for a company that affords the most control is to use an IP PBX.
You plug the IP PBX into the network and configure the IP phones from the IP PBX. You can then plug the IP phones into the network switches and they will register with the IP PBX. You plug AC adapters into the wall to power the phones unless your switches support IEEE 802.3af for delivering Power-over-Ethernet wires.
You can then select a SIP service provider (ITSP) and have SIP service running the same day or work with a traditional phone company which may take a couple weeks or more to get phone service turned on for your business.
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This was first published in July 2006