igital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Special Situations Special Situations cess Control (MAC) protocol. In cdma2000, the MAC is sent on a dedicated control channel, and in W-CDMA, the MAC is sent using slotted Aloha. Even with the extra overhead of coordination, the reverse shared packet channel allows more efficient high- speed data service than the system would have if we set up high-speed links for short bursts of data. 38.1.3 Time division duplex (TDD) Time division duplex (TDD) allows flexible distribution of the CDMA resource between forward and reverse directions. The TD-SCDMA provider has to estimate the total CDMA traffic in both directions and allocate enough TD-SCDMA carriers for the total traffic. The TD-SCMDA system balances forward and reverse on its own. The TD-SCDMA carrier has seven time slots in its time-division cycle. The system is required to use at least one of them in the forward direction and at least one in the re- verse direction. Use of the five time slots other than the two required in each direction is allocated according to subscriber demand. This allows the TD-SCDMA channel to be highly skewed in either direction. A ratio as high as six-to-one can be supported or a ratio as low as one-to-six (a high ratio in the other direction). If the system has at least two-sevenths of its resource utilization in balanced services such as voice, then the TD-SCDMA channel can be nearly perfectly efficient in allocating forward and reverse resources to subscribers. Unlike the cdma2000 and W-CDMA providers, the TD-SCDMA service provider does not have to decide up front how much capacity to allocate in each direction. This is a tremendous advantage in flexibility because these usage patterns can change. As an ex- ample, a community of workday Web surfers may go on holiday and start snapping MMS pictures of their family activities. Suc