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Call processing
Call signaling
Communication Manager implements the gatekeeper routed call model of H.323. The
registration process that is described above allows the endpoint and the Communication
Manager gatekeeper to exchange addresses to establish a TCP connection for a "call signaling"
channel (the H.323/H.225 channel). Once the TCP connection is established for call signaling,
the H.225.0/Q.931 signaling protocol is used over that connection to route the call and
exchange addresses necessary to establish a second TCP connection. This second TCP
connection is used for "media control" (the H.245 channel).
When Communication Manager chooses to route the media flow streams through the switch, it
selects and allocates available media processor resources, and sets the corresponding circuit
packs up to receive and send the media stream or streams from/to the endpoints using the
negotiated capabilities for each terminal. Each terminal is told to send its media stream or
streams to the appropriate Media Processor circuit pack. The switch connects the two media
streams, and thus completes the bearer path between the terminals.
Media stream handling
Media processing
The basic functions of the TN2302 Media Processing (MedPro) circuit pack include:
Taking media streams off the IP network, terminating RTP/UDP (adjusting for variable
delay in arrival rate), and converting them into PCM audio for transmission on the TDM
bus.
Taking media streams from the TDM bus, encoding them with the proper codec, and
transmitting them as RTP packets to an IP endpoint.
Originating and terminating an RTCP control channel for each media stream.
The particulars of the media conversion that is to be performed on each media stream are
controlled by Communication Manager. The Quality of Service (QoS) information obtained from
the RTCP channel is passed from the circuit pack to Communication Manager.
DTMF tone handling
The Media Processor circuit pack listens for and detects DTMF tones coming from the TDM
bus, strips them out of the audio stream, and sends a message to the Media Server indicating
that it has done so. The Media Server in turn generates and sends the appropriate H.245 tone
message to the endpoint that is receiving the audio stream. The receiving endpoint then plays
the specified tone. Compressed codecs, such as G.729, generally do a poor job of passing
DTMF tones. By sending tones out of band, fidelity is maintained. This method is useful when
connecting to a voice mail or an integrated voice response (IVR) system, where DTMF digits
are used to navigate through prompts.
94 Avaya Application Solutions IP Telephony Deployment Guide
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