Cisco WAN Switching Modules

Field Notice: G.729 and G.729a Performance on Universal Voice Module (UVM) Model D with Firmware Revision

December 22, 1998

Products Affected




G.729 and G.729a is supported only on the UVM Model D and with 9.1 switch software.

Problem Description

A bug has been found that affects the voice quality of G.729 and G.729a in some instances.

Problem Symptoms

Speech over G.729 and G.729a permanent virtual circuits (PVCs) may suffer from low quality and exhibit "breaking up" of words, phrases, and sentences.


G.729 and G.729a, also known as CS-ACELP, is a voice compression algorithm that allows for compression of 64 kBps voice into approximately 8 kBps. The UVM Model D, supported in release 9.1 of switch software, implements changes to the digital signal processor (DSP) chips to support G.729 and G.729a compression. Voice compression techniques normally sacrifice some fidelity in order to achieve optimal bandwidth utilization, which in turn reduces cost to users. Compression of voice channels allows a greater number of PVCs to be routed over trunking resources compared to uncompressed (64 kBps) voice channels.


There is no workaround for G.729 or G.729a. Do not use G.729 and G.729a (CS-ACELP) compression. Instead, use LD-CELP compression until the G.729 and G.729a problem is resolved. UVM firmware D.D.C resolves the G.729 and G.729a voice quality issue. Firmware D.D.C is due by December 27, 1998.


To follow the bug ID link below and see detailed bug information, you must be a registered user and you must be logged in.

CSCdk67963 (registered customers only)

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods:

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