H. 323 Protocol
H.323 was the first protocol used for VoIP communication. In May, 1996, International Telecommunication Union (ITU) gave H-323 protocol, which defines how voice, video and data traffic should be transported over IP based local area networks.
The purpose for adopting such protocol was to increase internet communication’s ability to handle real time traffic like voice and video chatting reliably. But, this standard was quickly adopted by the VoIP industry as a means of transmitting voice from IP networks to traditional PSTN phones. The story of VoIP actually started by utilizing the H.323 protocol for call signaling when a small Israeli company called Vocaltec released the first VoIP soft phone of the world using H.323.
H.323 protocol was made with the combination of the traditional PSTN protocols and the Internet-related standards. By leveraging from both circuit-switched and packet-switched protocol standards, H.323 is able to smoothly integrate with the PSTN and computer.The H.323 protocol, use the term gatekeeper to media gateway controller (MGC).
When we use to talk in traditional circuit-based telephone or mobile phones network, then we get the signal tones like on-hook, off-hook, ringing tones, busy tones that enables us to know the status of called parties or the status of the network.But, H.323 endpoints like Gateways, Gatekeepers or softswitch are packet-based; rely upon signaling messages, instead of signaling tones to convey information from each others.
The H.323 sets this signaling massage between a Gatekeeper and Gateway. Gatekeeper use a process called RAS, which stands for Registration, Admission, and Status to communicate with a gateway. To understand how it works, let’s make an example of you when making an overseas phone call.
Your call has reached a Gateway from PSTN network. Then, the gateway sends an “Admission Request” to the Gatekeeper. The Gatekeeper then checks the number you have just dialed and decide which Gateway it will use to transfer the call to reach it`s destination.
After finding one, Gatekeeper returns IP address of it`s selected Gateway to your Gateway. Then, your GW will send a call setup massage to that selected GW. The GK will send your gateway’s IP address to that selected gateway to establish the call which is “Admission Control’.
Now you are taking your friend, either of you can disconnect the call by sending a “Disconnect Request’. We actually do it by pressing a button on the phone, and that command goes through GW to the GK. When Gatekeeper receives that request it will send disconnect confirmation to both Gateways.
Later in 2002, IETF published another standard called SIP (Session Initiating Protocol). Since then, H.323 competes with SIP in many aspects.