It is always said H.323 is complex and SIP is simpler. Why?
1. The component
there are new components in H.323. but there are also new components like proxy server, redirect server in SIP.
2. The protocols
H.323 could co-operate with other ITU standard like H.225,H.245, H.450. But SIP could also work together with other IETF protocols like RSVP, RSTP, HTTP,etc.
3. It is said H.323 is the umbrella specification. "WHY?"
4. In H.323 system, it is said gatekeeper is opertional. but if without gatekeeper, how do two terminals talk to each other in the same LAN where gateway is not necessary? The terminals can just talk to each other, send request directly to the other terminal? (so confused about this)
Yes. In general H323 is more complex than SIP. H323 has been created by ITU (and in my opinion, everything created by ITU is complex ;)), SIP is created by IETF.
So... simply said - H.323 has been created by telecom addicted engineers, whilst SIP has been created by IP network engineers.
2. H323 can not operate with other ITU standarts. This is said in a wrong manner. H323 is a sort of protocol stack. This protocol stack inludes in it - H.225, H.235, H.245, H.450... ...
This is like to say "Car can work with tires, fuel pumps and brakes". No. Tires, fuel pumps and brakes are parts of the car. They make the car - a car :)
3. H323 is the umbrella specification, because it acts like an umbrella covering the actual standarts - H225, H235 and H245. Read the previous paragraph :)
4. Two H.323 units (not all though, some IP phones or SoHO gateways can operate only through a gatekeeper) can speak direct to each other, without involving a "RAS conversation" with a gatekeeper. This is called direct H225 signalling. They send H.225 / Q.931 signalling messages to the opposite 1720 port directly.
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