H.323 is one of the sets of protocols that can be used for VoIP. H.323 is really a collection of other protocols. It also recommends the general network architecture in terms of the elements required in a network.
Check out the H323 Primer. This gives an brief introduction to H.323 and suggests further reading.
An alternative protocol is SIP. I don’t know which one will become dominant though. This is being discussed on another thread on this forum.
Sequencing errors will affect quality, but RTP, the protocol used to transport the digitised payload has mechanisms to assist in timing and sequencing. Both H.323 and SIP use RTP.
To analyse the protocols over an Ethernet LAN, use Wireshark. It’s free.