First, a Windows network today is almost certainly running TCP/IP protocol over Ethernet cable and electrical signaling. So a more accurate question would be: which VoIP protocol is best for Ethernet.
There are three primary VoIP protocols being used today:
– H.323, the oldest, was used for MSN Messenger when that first came out. It is a good bet if your phone system is using ISDN circuits, since the call setup is handled in a similar fashion. Strike one.
– Media Gateway Control Protocol (MGCP), is similar to H.323 but allows for much less intelligent devices (phones) devices but putting giving greater control to the central gateway device. It may end up supporting cell phones, or it may be relegated to transmitting stereo signals over your home wireless network. Strike two.
Session Initialization Protocol (SIP) is the defacto standard for VoIP service today. Cisco, Avaya, Nortel (oh wait, that’s Avaya now, and LG/Ericsson, and . . .) all these hardware and software makers support it. Asterisk open source PBX supports it.
All VoIP service providers support it, and Windows won’t care.
An important distinction is that SIP only handles call setup and teardown (hangup), while the audio conversation is handled by Real Time Protocol (RTP) over UDP.
So SIP is your best bet today, in my humble opinion (IMHO).
I don’t want to forget to mention one more current protocol:
IAX2 – Inter-Asterisk eXchange protocol
This is a very efficient protocol that Asterisk implementations can use in place of SIP/RTP. It is very router/firewall friendly and bandwidth efficient, but it is not supported by many other VoIP hardware or service providers.