(GIX). A GIX can be compared with an international exchange or a national gateway. 15.12 MOBILITY MANAGEMENT 15.12.1 ROAMING AND HANDOVER Handover from one technology/system in one cell to another technology/system in another cell is highly desirable, since it permits support from one network to the other one. The UMTS systems can for example be implemented more flexibly with good coverage from the start. This will of course also improve the UMTS business case. Let us look at a handover from UMTS/WCDMA to GSM. We assume that the mobile station has a single radio receiver. The mobile station must in any case be able to commu- nicate with both technologies to perform the handover. Further we look at the handover from WCDMA to GSM. When there is a need for a handover to the GSM system the WCDMA device can for example use a technique called compressed mode. See Figure 15.17. Like the GSM system the WCDMA system has to measure the signal strength in the GSM cells while still communicating in WCDMA. By compressing some of the sent frames a small free time is created which can be used for GSM signal measurements. Provided the measure- ments are successful the GSM base station controller is informed via WCDMA radio network controller. The handover from WCDMA to GSM is illustrated in Figure 15.18. Bit rate=2+R Bit rate=R Bit rate=R Radio frame Idle time for GSM measurements Figure 15.17 Compressed mode creates gaps or idle spaces in time that WCDMA mobile termi- nals use to perform measurements on GSM cells. Real-time services cannot use the sudden bit rate changes around the idle WCDMA time slot used for measurements of GSM Understanding Changing Telecommunications - Building a Successful Telecom Business Common WCDMA WCDMA handover and GSM part of message the terminal mobile WCDMA info GSM-specific GSM handover part of the message WCDMA-specific part mobile terminal of the mobile terminal GSM handover message is extracted in WCDMA part. and passed on to GSM part GSM handover message Figure 15.18 WCDMA orders handover to