Where does SIP fit with H323?
H.323 embraces the more traditional circuit-switched approach to signaling based on the ISDN Q.931 protocol and earlier H-series recommendations. However, SIP utilises the more lightweight Internet approach based on HTTP. SIP addresses some of the perceived shortcomings of H.323. For example, establishing an H.323v1 call can take around a dozen packets and about 6 to 7 round-trip times, depending on how connections and packets are overlapped. For a modem connection, where transmission delays are substantial, setting up an H.323 call can take several seconds. However, setting up a SIP call via UDP takes 1.5 round trip times and four packets. H.323v1 allows to address either hosts directly (but without user name), or go through an alias. All aliases have to be resolved through a gatekeeper. SIP destination and forwarding addresses can be any URL, including mailto, phone, H.323 and http URL, affording flexibility in combining SIP with other signaling protocols. Also, email-like names can
