Huawei v200r001 user manual VoIP Principle, Basic composition, II. H.323 protocol stack

User Manual - Configuration Guide (Volume 3)	Chapter 1
Versatile Routing Platform	VoIP Configuration

1.1.1VoIP Principle

I.Basic composition

IP network

IP voice gateway

IP voice gateway

PSTN		PSTN
Telephone	GateKeeper	Telephone

Figure VC-1-1Basic composition of the VoIP system

For the plain voice service, all the functions from the caller to the called are implemented by PSTN, but IP voice service is quite different.

In the above figure, the IP voice gateway provides the port between the IP network and the public telephone networks (PSTN/ISDN), and user is connected to the IP voice gateway through the PSTN local loop. The IP voice gateway is responsible for converting the analogue signals to digital signals, compressing and packetizing so that they become packet voice signals that can be transmitted over the IP network. Then, they are sent to the user gateway and the IP voice gateway at the called end reverts the packets to recognizable analogue voice signals. Once these signals arrive at the called terminal through PSTN, a communication process from telephone to telephone completes. In real VoIP networking, you may need gatekeeper to accomplish functions such as routing and access control.

VoIP uses UDP (User Datagram Protocol) in the Transport Layer. Since the UDP provides connectionless and unreliable datagram service, it is not very appropriate for real-time application. The current approach is to run the RTP (Real-time Transport Protocol) over the UDP to enhance function of real-time application.

IP header

UDP header

RTP header

Voice load

Figure VC-1-2VoIP packet format

II. H.323 protocol stack

To realize the VoIP, currently almost all the manufacturers adopt the ITU-T standard protocol family H.323. The H.323 protocol is implemented in the Application Layer, which mainly describes the terminals, device and services for multimedia communication in local area network without quality of service (QoS) guarantee, including H.225.0, H.245, G.729, G.723.1, G.711, H.261, H.263 and T.120 series, etc.

G.723.1, G.729 and G.711 are audio codec protocols, H.263 and H.261 are video codec protocols, H.225.0 and H.245 are system control protocols, and the T.120 series are multimedia data transport protocol.

RTP and its controlled protocols RTCP (RTP Control Protocol) together ensure the real-timeliness of voice message transmission. The function of RTP is enhanced via RTCP. RTCP is used to give feedback for the quality of data dispatch. With this

1-2