129 VOICE OVERVIEWIntroduction to VoIP VoIP makes it possible that voice services such as plain old telephone service(POTS) can be carried over the IP network. VoIP is implemented via voice packets.In VoIP, the voice gateway segments the voice signal into frames and stores themin voice packets to transmit.Currently, interworking between PSTN and Internet is implemented via VoIPgateways. Meanwhile, the PC-to-telephone, telephone-to-PC, andtelephone-to-telephone technologies are mature and the call quality has beenimproved greatly. Therefore, VoIP can completely meet the commercialrequirements.H.323 and session initiation protocol (SIP) are two common protocols used in VoIP.For details about H.323 and SIP, refer to “Voice Protocols” on page 2238.VoIP System For POTS, all functions from the call originator to the call receiver are implementedby the public switched telephone network (PSTN). VoIP is different from POTS.Figure 647 VoIP systemIn Figure 647, the VoIP gateway provides interfaces for communication betweenthe IP network and PSTN/integrated services digital network (ISDN), users connectto the originating VoIP gateway through PSTN, the originating VoIP gatewayconverts analog signals into digital signals and compresses them into voice packetsthat can be transmitted over the IP network, and the IP network transmits thevoice packets to the terminating VoIP gateway, which reduces the voice packets torecognizable analog signals and transmits them to the receiver. This is a completetelephone-to-telephone communication process. In practice, a gatekeeper (GK)server or SIP server may be applied in the VoIP system to implement the functionsof routing and access control.Basic VoIP Call Flow The following describes a basic VoIP call flow:1 A user picks up a telephone and the modular voice card detects the user’soff-hook action in real time2 The modular voice card transmits the off-hook signal to the VoIP signal processingmodule on the VoIP gateway.PSTN PSTNIPGateway GatewayTelephoneGK server / SIP serverTelephone
