Now that you understand more about GKs, let’s cover in more depth how H.323 communicates.
Figure 1.1. Call Control and Setup Using a Gatekeeper
One way to memorize steps of H.323 call setup using a GK:
You get RAS-ed by a 225-lb. bully (H.225), which you have no control over (UDP) (port 1719).
RAS messages are passed through the GK to each device.
If no GK is present, H.225 messages are sent between endpoints.
You call 931 (Q.931) (H.225) to set up a call with your friends. You do not control them (UDP).
Your 245-lb. friend (H.245) takes control of everyone on the call (TCP) (port 1720). Negotiations ...