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Dialogic® Multimedia Platform for AdvancedTCA

SIP Phone Bandwidth and 3G-324M


Summary:
Poor video quality may result if the overall bandwidth of a SIP call exceeds that which is available in a 3G-324M channel.

Symptom:
Degraded Video Quality when streaming data over 3G-324M on Dialogic® Multimedia Platform for AdvancedTCA.

Reason for the Problem:
A gateway application may use a 3G-324M channel tied to an Real-time transport protocol (RTP) audio and video media stream that originates at an audio/video SIP phone. If this is the case, the SIP phone's total bandwidth use must not exceed the capacity of the 3G-324m channel, or degraded video quality will result.

Solution:
A phone should have a configuration setting for video rate control. This establishes a limit as to the maximum bandwidth used by the video part of the media stream. Setting the phone to use Quarter Common Intermediate Format (QCIF) and a low frame rate (i.e., 7-10 fps) is also a good idea.

Technical Discussion:
The total available bandwidth for a broadband IP connection may be several megabits/second or more. This allows a reasonably high quality video image to be presented - CIF, (Common Industry Format, 352x288 pixles) at 30 frames/second, for example.

On the other hand, the available bandwidth for a 3G-324M connection cannot exceed that of a Digital Signal 0 (DS0) connection - 64 kilobits/second - for audio, video and media control. Keep in mind, 3G-324M is for delivering media to mobile phone devices in a QCIF (Quarter CIF) format of 177X144 pixels.

This limits the video that may be supplied from a SIP/RTP connection over IP. The video phone must first be able to supply narrow band Adaptive Multi-Rate Codec(AMR)for audio. This codec achieves a reasonable quaility at a very low bitrate - down to as low as 4.75 kilobits/second. Secondly, the phone needs to limit its H.263 video bandwidth to approximately 37 kilobits/second.


Product Listing:
Dialogic® Multimedia Platform for AdvancedTCA®

Glossary of Acronyms / Terms:
AMR - adaptive Multi-Rate Codec
SIP - Session Initiation Protocol
3G-324M - protocol that allows multimedia communication over circuit switched network.
QCIF - Quarter Common Intermediate Format (176 × 144 video resolution)
RTP - Real-time transport protocol
H.263 - a video codec
DS0 - Digital Signal 0
fps - frames per second






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