h.323 and SIP are signalling protocols. MGCP and Megaco are control protocols. Many group are working on integrating 323 and SIP. H.323 so far has won. SIP is very new and requries a proxy server. (more equipment) SS7 is used as the standard for inter office communications between PSTN offices. MGCP was to use ss7 data setup infrastructure but VoIP for voice transport.