Enabling Media Services in a VoIP Network
The VoIP or Packet Communications Reference Architecture defines two functional entities related to providing media services. One is the application server; the other is the media server. Both of these entities reside in what is called the "application plane," because their role is to deliver application services within the network. The same architecture also defines the "services layer" and the "media layer." Residing in the services layer, the application server runs applications, which in the voice space would include voice messaging, voice response and conferencing. These applications are implemented in languages and APIs such as VXML, SALT, JAIN and others. A protocol such as Session Initiation Protocol (SIP) is used as a text message-based protocol to set-up, define, control and tear down a call between two user agents, which could be the application server, the terminal device (IP phone) or a media gateway controller (or softswitch). The application server also uses SIP to maintain a session relationship with the media server. The role of the media server (residing in the media layer) is to receive, manipulate and transmit RTP media streams for the delivery of services. The application server provides the application intelligence, while the media server provides the media stream delivered to the endpoint in RTP steams. The following diagram shows the relationship of these entities via protocols:
It should be noted that in the above diagram, network entities are often encompassed together. An application server is often part and parcel of a media server. A media server is often part and parcel of a media gateway. And subsets of these entities are often grouped into "solutions" such as IP-PBXs, Class 5 replacement switches or enhanced services platforms. However, the media server function remains the same - to provide media services. Let's examine specific media server functions in detail and the services they enable.
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