 coding option and that it will additionally use multiple time slots. To see how this might operate requires a basic knowledge of the physical structure of a traffic channel on the air interface. Downloaded from Digital 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. Future Enhancements and Services Chapter 11 Figure 11-3 Evolution of mobile 3G by international standards. GSM Operator GSM 900 GSM 1800 Circuit-Switched Packet-Switched The uplink and downlink of a GSM traffic channel take place on differ- ent frequencies. Also, the uplink and downlink time slots occur at different times in the eight time slot frame. Additionally, when engaged in a traffic channel, a GSM mobile station must constantly be monitoring downlink power levels from neighboring cells as part of the handover process. Over an eight time-slot frame therefore a mobile will Receive a downlink burst. Transmit an uplink burst. Monitor a downlink transmission from a neighboring cell. Two Time Slots One restriction that HSCSD places upon multiple time slot links is that the time slots allocated must be consecutive. As seen in Figure 11-4, the use of two time slots is relatively simple to implement. The mobile is still able to run through its standard routine of receive, transmit, and monitor a neighbor within an eight time slot frame. With three or more time slots being used, an overlap occurs between the receive and transmit times and implementation of this involves substantial hardware changes in the mobile station, that is, the use of a radio frequency (RF) duplexer. (At first sight, it looks as if no overlap occurs when using three time slots, but overlap does occur due to the timing advance applied to the uplink.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the 