Cisco 521SG - Unified IP Phone VoIP Administration Manual page 119

Configuring SIP, SPCP, and NAT
Configuring SIP
Cisco Small Business SPA300 Series, SPA500 Series, and WIP310 IP Phone Administration Guide
Parameter
Set G729 annexb
Description
G.729 Annex B (G.729b) that provides silence
compression by enabling a voice activity detection (VAD)
module. It uses 2-byte Silence Insertion Descriptor (SID)
frames transmitted to initiate comfort noise generation
(CNG). If transmission is stopped and the link goes quiet
because of there is no speech transmitted, the receiving
side might assume that the link has been cut. By inserting
comfort noise, analog hiss is simulated digitally during
the silence to assure the receiver that the link is active
and operational.
none—do not enable.
no—turn on but do not silence the VAD.
yes—enable.
Not applicable to the Cisco SPA525G or
Cisco SPA525G2.
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