rvices. With user-based QoS, premium users receive prioritized service in a proportional manner and experience greater data rates than non-premium users. Flow- based QoS goes a step further and differentiates between flows to different applications on the same device. Rev. A does not compromise on VoIP voice quality and it utilizes the same EVRC (Enhanced Variable Rate Codec) circuit switched voice codec as cdma2000 1x, thus maintaining the same audio fidelity. Improved Operation with cdma2000 1X: Rev. A introduces new overhead signaling and messaging to support multi-mode devices (1X and EV-DO dual mode operation). Improved Battery Life: Rev. A introduces a slotted mode operation for the terminals which improves the battery life of the Rev. A terminals. During slotted mode of operation terminal only monitors selected slots. Backward Compatibility: Rev. A system is backward compatible with Rev. 0 system economizing the investment of operators and subscribers. EV-DO Revision B The Rev. B extends to multi carrier operation by serving users simultaneously over multiple 1.25 MHz carriers. Thus, Rev. B becomes the first standard which is truly in line with IMT-MC.3 Rev. B is backward compatible with Rev. A and protects operators' and subscribers' investments. Rev. B technology was standardized as TIA-856-B [5] in October 2007. The key benefits of EV-DO Revision B are discussed in this section. 3 International Mobile Telecommunications 2000 - Multi-carrier (IMT-MC) refers to cdma2000 1X and cdma2000 1xEV (Evolution) radio interfaces. Next Generation Mobile Communications Ecosystem 5.3.1 Multi-Carrier CDMA Before we dive into Rev. B lets briefly look at the fundamental concept of MC-CDMA. There are three methods of MC-CDMA namely MC-DS/CDMA (MC - Direct Spread/CDMA), MC-CDMA and MT-CDMA (Multi-tone CDMA). The Direct Sequence CDMA is the scheme that is used in cdma2000 1xEV-DO systems. The MC-DS-CDMA (Figure 5.6) transmitter spreads the Serial-to-Parallel converted data streams using a spreading code in the time domain S