scope:

General Description

Service Option 56, the Selectable Mode Vocoder (SMV), provides two-way voice communication between the base station and the mobile station using the dynamically variable data rate speech codec algorithm described in this standard. The transmitting speech codec receives voice samples and generates an encoded speech packet for every Fundamental Channel Traffic Channel frame.^† The receiving station generates a speech packet from every Fundamental Channel Traffic Channel frame and supplies it to the speech codec for decoding into voice samples.

SMV communicates at one of four rates: 9600 bps, 4800 bps, 2400 bps and 1200 bps.

^† IS-2000-A uses the term "frame" to represent a 20 ms grouping of data on the Fundamental Channel. Common speech codec terminology also uses the term "frame" to represent a quantum of processing. For Service Option 56, the speech codec frame corresponds to speech sampled over 20 ms. The speech samples are processed into a packet. This packet is transmitted in a Traffic Channel frame.