CPM can be used in Downlink (DL) only, Uplink (UL) only, or both DL and UL, depending on UE capabilities. It is used as soon as the UE requires CPM for any of the GSM bands that it supports to avoid having to check and possibly start/stop CPM at AS updates.
Two different CPM methods, SF/2 or Higher Layer Scheduling (HLS), are used to create idle periods. The method used depends on the radio bearer combination, see Figure 8.