VoIP enables a network device to carry voice traffic over an IP network. In
VoIP, a digital signal processor (DSP) segments the voice signal and stores it
in voice packets. These voice packets are transported using IP. Because
multimedia applications are delay-sensitive, a well-engineered, end-to-end
network is needed to successfully use VoIP, as shown in Figure
. Fine-tuning a
network to adequately support VoIP involves a series of protocols and features
to improve quality of service (QoS). Traffic shaping considerations must also
be taken into account to ensure the reliability of the voice connection.
The major components of a VoIP network are similar to those in a
circuit-switched network. In addition to connecting users to the PSTN, VoIP
networks must perform all of the same tasks as the PSTN. There are three major
components to a VoIP network:
- Media gateways
- Media gateway controllers
- The IP network
It is important to realize that terminology is not always used
consistently, especially in the case of emerging technologies. This section
presents the most commonly used terms, but other terms may also be used.
Media Gateway
Media gateways are responsible for call
origination, call detection, analog-to-digital voice conversion, and creation
of voice packets, or compress-decompress (codec) functions. Media gateways also
have optional features, such as voice compression, echo cancellation, silence
suppression, and statistics gathering. The media gateway forms the interface
for voice to the IP network. Each call is usually a single IP session that is
transported by an RTP over the User Datagram Protocol (UDP).
Media
gateways can be dedicated telecommunication equipment or a generic PC that is
running VoIP software. Some additional features and services supported include
functions such as trunking to the PSTN, providing an analog or digital Private
Branch Exchange (PBX) interface, or integrating a soft PBX. The media gateway
can also be used as an IP telephone unit. The media gateway is also called a
Voice Agent.
Media Gateway Controller
Media gateway
controllers contain the signaling and control services that coordinate the
media gateway functions, as shown in Figure
. The
media gateway controller is responsible for some or all of the call signaling
coordination, phone number translation, host lookup, resource management, and
interfacing with the PSTN Signaling System 7 (SS7).
In a scalable VoIP
network, two devices can perform the functions of the controller. The first is
a signaling gateway controller and the second is a media gateway controller. If
a call both originates and terminates within the same VoIP network, a media
gateway controller may be the only device needed to complete the call. However,
a VoIP network is frequently connected to the PSTN. The signaling controller
would be dedicated to the message translation and signaling needed to connect
to the PSTN.
The signaling controller is also referred to as a call
agent in a centralized architecture. It is known as a gatekeeper in an H.323
network and a proxy or redirect server in a SIP network.
The definition
of a softswitch has caused some confusion. The International Softswitch
Consortium limits the term softswitch, to the media gateway controller. Some
vendors include the media gateway or signaling gateway as part of the
softswitch. At the most basic level, a softswitch is media gateway controller
software that provides the call control and resource management for a media
gateway. Softswitches will perform a significant function in converging
services.
IP Network
It is possible to view the VoIP network as one logical
switch. However, this logical switch is a distributed system, rather than a
single switch. The IP backbone provides connectivity among the distributed
elements. This entire system is sometimes collectively referred to as the
softswitch architecture.
The flowchart in this section shows the events
that occur when a call is placed using VoIP and either H.323 or SIP.