R2511-HP MSR Router Series Voice Configuration Guide(V5)

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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 "Voice protocols."
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
In Figure 1, the VoIP gateway provides interfaces for communication between the IP network and
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.