, there is a simultaneous use of the uplink as the local computer con- firms receipt of the packets. Capacity calculations for data systems need to take this two-way flow of data used for verification or error correction into account. 1 17.2.3 Data-only carriers All three of the 3G CDMA standards (cdma2000, W-CDMA, and TD-SCDMA) are de- veloping detail standards for DO frequencies that are designed to provide high data bandwidth to support high-speed data transmission.2 For example, the cdma2000 1x EV-DO standard, also known as EV phase 1, supports data rates of up to 2.4 Mbps, whereas the standard for a mixed voice and data carrier, cdma2000 1x, supports a data rate of 144 kbps. The DO carrier can only offer its highest rate by giving the entire car- rier to one subscriber for a period of time. As a result, it would be impractical to sup- port real-time voice on the same carrier. At the present time, creating a DO carrier al- lows data transmission capacity per call to increase by a factor of more than 16. Wideband CDMA (W-CDMA) takes a different approach to managing DO channels. This approach has several elements that are described in Sec. 10.3. The elements in- clude the ability of the user terminal to define its voice and data capacity, the defini- tion of four QoS classes, the use of time division carriers to support high-speed data ser- vices, use of the Aloha protocol to allow user terminals to request supplemental data bandwidth, and the availability of high-speed data transmission on the downlink to support services such as Web browsing that require higher download speeds than up- load speeds. Time division synchronized CDMA (TD-SCDMA) provides the most flexible service, allowing variable data rates on demand in both directions, with rates equivalent to those found in other 3G services. TD-SCDMA has the ability to assign a data channel to the least-interfered time slot and then to select the optimal CDMA code within that time slot. As these approaches to data-only carriers evolve, it will be up to system