About Encoding/Decoding Protocols; About Audio Transcoding - Cisco 3521 Administrator's Manual

Chapter 3 Configuring the Cisco IPVC 3500 Series Gateway

About Encoding/Decoding Protocols

A number of video conferencing terminal applications require the G.722 and G.722.1 audio compression
codecs to provide high quality voice communications. The G.722 and G.722.1 formats, using a digital
sampling rate of 7 KHz, provide higher quality voice sampling with a greater dynamic range. The
gateway does not transcode G.722 or G.722.1, but supports them transparently. Since the G.722 codec
is of a much higher audio quality than other codes and requires higher bandwidths, the gateway supports
G.722 and G.722.1 at the following call bit rates:
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Both endpoints in a call must support G.722 and G.722.1 audio codecs.

About Audio Transcoding

The gateway can support audio transcoding through the Audio Transcoder Module (TCM). The TCM is
a PCI mezzanine card (PMC) that implements Digital Signal Processing (DSP). The TCM has a
processing capacity of up to 20 channels for audio transcoding in video calls.
The Gateway TCM can perform audio transcoding between the following types of audio protocols:
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When your unit includes both a gateway and an Cisco IPVC 35xx MCU, G.728 transcoding is supported
Note
on the Cisco IPVC 35xx MCU only.
Each audio codec differs in the audio quality, compression, and bit rates that it provides. G.711 provides
toll quality audio at 64 Kbps, G.728 provides near toll quality audio at 16 Kbps, and G.723.1 provides
voice quality audio at 5.3 or 6.4 Kbps.
ISDN endpoints usually support the G.711 and G.728 codecs. Endpoints on IP networks support G.711
and G.723.1 codecs. By performing transcoding between these audio protocols, the gateway can support
communication between endpoints with codecs that are incompatible with each other.
Audio transcoding can also optimize the audio bandwidth usage either on the IP network (G.723.1 < >
G.711) or on the ISDN network (G.728 < > G.711). Transcoding is particularly useful for ISDN codecs,
where bandwidth can be limited to 128 Kbps for a video call. For example, when G.728 < > G.711
transcoding takes place, the audio bandwidth usage is compressed to 16 Kbps. This provides an
additional 40 Kbps of bandwidth to the existing video bit rate on the ISDN network, contributing to
improved video quality.
The Cisco IPVC 3500 PRI Gateways automatically perform A-Law G.711-to-µ-Law G.711 translation
Note
between the IP and ISDN sides if needed.
You can configure the gateway to prioritize the transcoding, giving preference to a particular codec that
is applied to calls, thus optimizing the resource allocation utilized by each call.
Administrator's Guide for Cisco IPVC 3521 BRI Gateway, Cisco IPVC 3526 PRI Gateway, and Cisco IPVC 3540 PRI Gateway
OL-7995-01
G.722 is supported in calls at 224, 256, 336, 384, 448, 512 Kbps (all gateways) and 768, 1472, and
1920 Kbps (PRI gateways only).
G.722.1 is supported in calls at 64, 2B, and 128 Kbps.
G.711 (ISDN) to G.723.1 (IP)
G.723.1 (IP) to G.711 (ISDN)
G.728 (ISDN) to G.711 (IP)
G.711 (IP) to G.728 (ISDN)
Configuring Gateway Settings
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