How Frame Formats in CDMA
Once the analog voice is compressed by one of the vocoder processes, some additional data is added to produce a frame. Each frame in CDMA is 20 milliseconds regardless of the data rate used. This figure shows all of the possible frame configurations for both the 8 kbps and 13 kbps vocoders.
In the case of the 8 kbps vocoder running at full rate, each frame consists of a mixed mode bit, 171 vocoder bits, 12 bits of CRC, and 8 encoder tail bits. The result is a frame of 192 bits which produces a continuous date rate of 9,600 bps ( 192 bits x 50 frames/sec = 9,600 bps).
The mixed mode bit indicates if the frame is pure channel data or if it contains at least some signaling. The CRC bits allow the mobile to verify that it has correctly decoded a frame. The encoder tail bits provide 8 zeroes in a row to flush the contents of the convolutional coder in preparation for processing the next frame of data.
For the CDG 13 kbps coder, the frame is still 20 milliseconds, but a total of 288 bits are sent to produce a data rate of 14,400 bps ( 288 bits x 50 frames/sec = 14,400 bps).
The 14,400 bps channel has the mixed mode bit, CRC bits, and encoder tail bits like the 9,600 bps channel. However, the 14,400 bps channel includes a CRC for all data rates while the 9,600 bps channel only provides a CRC check for full and half rate frames.
The 14,400 bps channel also includes 1 bit that is reserved in the forward link but is used by the mobile in the reverse link to indicate a frame erasure (the CRC check did not pass). This assists the base station in performing forward link power control efficiently.