Voice Overview; Introduction To Voip; Voip System; Basic Voip Call Flow - HP FlexNetwork MSR Series Configuration Manual

Voice overview

Introduction to VoIP

Voice over IP (VoIP) enables IP networks to provide voice services such as plain old telephone
service (POTS). In VoIP, the voice gateway encapsulates voice signals into packets to transmit. IP
telephony is a typical VoIP application.
Interworking between PSTN and IP is implemented through VoIP gateways. VoIP meets the
commercial requirements for PC-to-telephone, telephone-to-PC, and telephone-to-telephone
technologies.
H.323 and Session Initiation Protocol (SIP) are two common protocols used in VoIP. For information
about H.323 and SIP, see

VoIP system

For POTS, all functions from the call originator to the call receiver are implemented by the public
switched telephone network (PSTN). VoIP functions differently from POTS, as described in this
section.
Figure 1 VoIP system
Figure 1, the VoIP gateway provides interfaces for communication between the IP network and
In
PSTN/ISDN. Users connect to the originating VoIP gateway through PSTN. The originating VoIP
gateway converts analog signals into digital signals and compresses them into voice packets that
can be transmitted over the IP network. The IP network transmits the voice packets to the terminating
VoIP gateway, which converts the voice packets back into recognizable analog signals and then
transmits them to the receiver. This is a complete telephone-to-telephone communication process. In
practice, a gatekeeper (GK) server or SIP server can be applied in the VoIP system to implement
functions such as routing and access control.

Basic VoIP call flow

The following describes a basic VoIP call flow:
1. A user picks up a telephone, and then the modular voice card detects the user's off-hook action
in real time.
2. The modular voice card transmits the off-hook signal to the VoIP signal processing module on
the VoIP gateway.
3. The VoIP signal processing module generates dial tones.
4. The user hears dial tones played by the session application and begins dialing before the dial
tone timer expires.
protocols."
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