The major components of a VoIP network are very similar in functionality to that of a circuit-switched network. VoIP networks must perform all of the same tasks that the PSTN does, in addition to performing a gateway function to the existing public network. Although using different technology and approach, some of the same component concepts that make up the PSTN also create VoIP networks. There are three major pieces to a VoIP network.
- Media gateways
- Media gateway/signaling controllers
- IP network
Media Gateways
Media gateways are responsible for call origination, call detection, analog-to-digital conversion of voice, and creation of voice packets (CODEC functions). In addition, media gateways have optional features, such as voice (analog and/or digital) compression, echo cancellation, silence suppression, and statistics gathering.
The media gateway forms the interface that the voice content uses so that it can be transported over the IP network. Media gateways are the sources of bearer traffic. Typically, each conversation (call) is a single IP session transported by a Real-time Transport Protocol (RTP) that runs over UDP. Media gateways exist in several forms. For example, media gateways could be a dedicated telecommunication equipment chassis, or even generic PC running VoIP software. Their features and services can include some or all of the following.
Trunking gateways that interface between the telephone network and a VoIP network. Such gateways typically manage a large number of digital circuits.
Residential gateways that provide a traditional analog interface to a VoIP network. Examples of residential gateways include cable modem/cable set-top boxes, xDSL devices, and broadband wireless devices.
Access media gateways that provide a traditional analog or digital PBX interface to a VoIP network. Examples include small-scale (enterprise) VoIP gateways.
Business media gateways that provide a traditional digital PBX interface or an integrated soft PBX interface to a voip network.
Network access servers that can attach a modem to a telephone circuit and provide data access to the Internet.
Discreet IP telephones units.
Media Gateway Controllers
Media gateway controllers house the signaling and control services that coordinate the media gateway functions. Media gateway controllers could be considered similar to that of H.323 gatekeepers. The media gateway controller has the responsibility for some or all of the call signaling coordination, phone number translations, host lookup, resource management, and signaling gateway services to the PSTN (SS7 gateway). The amount of functionality is based on the particular VoIP enabling products used.
In a scalable VoIP network, you can breakup the role of a controller into signaling gateway controller and media gateway controller. For calls that originate and terminate within the domain of the VoIP network, only a media gateway controller might be needed to complete calls. However, a VoIP network is frequently connected to the public network. You could use a signaling gateway controller to directly connect to the SS7 network, while also interfacing to the VoIP network elements. This signaling controller would be dedicated to the message translation and signaling needed to bridge the PSTN to the VoIP network.
The services of these devices are defined by the protocols and software they are running. There are several protocols and implementations that any number of vendors could deploy. Knowing the details of how the devices use their suite of protocols is important to designing the IP backbone that is to service the VoIP elements.
IP Network
You can view the VoIP network as one logical switch. However, this logical switch is a distributed system, rather than that of a single switch entity; the IP backbone provides the connectivity among the distributed elements. Depending on the VoIP protocols used, this system as a whole is sometimes referred to as a softswitch architecture.
The IP infrastructure must ensure smooth delivery of the voice and signaling packets to the voip elements. Due to their dissimilarities, the IP network must treat voice and data traffic differently. If an IP network is to carry both voice and data traffic, it must be able to prioritize the different traffic types.
There are several correlations to the VoIP and circuit-switching components, however there are many differences. One is in the transport of the resulting voice traffic. Circuit-switching telecommunications can be best classified as a TDM network that dedicates channels, reserving bandwidth as it is needed out of the trunk links interconnecting the switches. For example, a videophone conversation reserves a single DS-0 channel, and that end-to-end connection is used only for the single conversation.
IP networks are quite different from the circuit-switch infrastructure in that it is a packet-network, and it is based on the idea of statistical availability. Class of service (CoS) ensures that packets of a specific application are given priority. This prioritization is required for real-time applications to ensure that the voice service is unaffected by other traffic flows.
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