Editing CdmaOne (section)

====Forward traffic channels====
The Walsh space not dedicated to broadcast channels on the BTS sector is available for ''traffic channel''s. These channels carry the individual voice and data calls supported by IS-95. Like the paging channel, traffic channels have a frame time of 20ms.

Since voice and user data are intermittent, the traffic channels support variable-rate operation. Every 20 ms frame may be transmitted at a different rate, as determined by the service in use (voice or data). P_REV=1 and P_REV=2 supported ''rate set 1'', providing a rate of 1200, 2400, 4800, or 9600 bit/s. P_REV=3 and beyond also provided ''rate set 2'', yielding rates of 1800, 3600, 7200, or 14400 bit/s.

For voice calls, the traffic channel carries frames of ''vocoder'' data. A number of different vocoders are defined under IS-95, the earlier of which were limited to rate set 1, and were responsible for some user complaints of poor voice quality. More sophisticated vocoders, taking advantage of modern DSPs and rate set 2, remedied the voice quality situation and are still in wide use in 2005.

The mobile receiving a variable-rate traffic frame does not know the rate at which the frame was transmitted. Typically, the frame is decoded at each possible rate, and using the quality metrics of the [[Viterbi decoder]], the correct result is chosen.

Traffic channels may also carry circuit-switch data calls in IS-95. The variable-rate traffic frames are generated using the IS-95 ''Radio Link Protocol (RLP)''. RLP provides a mechanism to improve the performance of the wireless link for data. Where voice calls might tolerate the dropping of occasional 20 ms frames, a data call would have unacceptable performance without RLP. 

Under IS-95B P_REV=5, it was possible for a user to use up to seven supplemental "code" (traffic) channels simultaneously to increase the throughput of a data call. Very few mobiles or networks ever provided this feature, which could in theory offer 115200 bit/s to a user.