Internet Telephony Protocol | H.323

Internet Telephony Protocol is also called Voice Over IP . It is a real-time interactive audio/video application. As from the name Internet Telephony, it uses internet as a telephone network with additional features. To communicate between two parties, it uses packet-switched Internet.

There are two different types of protocols that enable Internet Telephony.

1. H.323
2. Session Initiation Protocol (SIP)

H.323 :
Basically, H.323 may be a standard set by ITU which allows telephones on the general public telephone network to speak to computers connected to internet. The architecture of H.323 is shown as shown as:

H.323 Architecture Model

At the center, there is gateway placed which connects the internet to the telephone network. Gateway translates a message from one protocol to another. It speaks H.323 protocol on Internet site and PSTN i.e. Public Switched Telephone Network protocol on telephone side. It will act as both computer and telephone. A LAN may have a gatekeeper which controls the terminals under its jurisdiction calledzone.

Protocols of H.323 :
To maintain audio or video communication, H.323 uses a number of protocols. The various protocols used are as shown in the following figure.

H.323 Protocol Stack

1. G.711 or G.723.1 –
   These protocols are used for compression.
2. H.245 –
   In this, multiple compression protocols are permitted, H.245 allows the terminals to negotiate which protocol to use for compression. H.245 also negotiates bit rate.
3. RTCP –
   It is used to control RTP channels.
4. Q.931 –
   This protocol is required for establishing and releasing connections, providing dial tones, making ringing sounds.
5. H.225 –
   This protocol enables the terminals to talk to gatekeepers. This protocol also manages PC-to-gatekeeper channel called RAS channel. This channel allows the terminals to join and leave the zone, request and return bandwidth and provide status updates.
6. RTP –
   It is used for actual data transmission.