VoIP and Voice over WLANs
Components of VoIP

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.


Interactive Media Activity

Process Chart: How a VoIP Telephone Call is Made