The HSCSD configuration is a multislot configuration using the TCH/F data channel mapping described in TS 45.002. Two types of HSCSD configurations exist, symmetric configuration and asymmetric configuration. For both types of configurations the channels may be allocated on either consecutive or non-consecutive time slots taking into account the restrictions defined by the classmark. An example of the HSCSD operation with two consecutive time slots is shown in Figure 2.

A symmetric HSCSD configuration consists of a bi-directional FACCH and co-allocated bi-directional TCH/F and SACCH channels. An asymmetric HSCSD configuration consists of a bi-direction FACCH and co-allocated uni directional or bi-directional TCH/F and SACCH channels. A bi-directional channel is a channel on which the data is transferred in both uplink and downlink directions. On uni-directional channels for HSCSD the data is transferred in downlink direction, only. In both symmetric and asymmetric HSCSD configurations one bi-directional channel, the main channel, carries a FACCH used for all the signalling not carried on the SACCH(s). For HSCSD configuration all SACCHs are synchronized so that idle frames for each time slot coincide. The classification of mobile stations used for HSCSD shall be based on Multislot classes, described in detail in TS 45.002. Further classification shall be based on the Mobile Station Classmark depending on the supported modulations. The same frequency hopping sequence and training sequence is used for all the channels in the HSCSD configuration. The same channel coding is used for all the channels in the HSCSD configuration, though in the enhanced modulation mode, for non-transparent services, it is possible to have one channel coding used in the downlink and another channel coding used in the uplink. Different channel codings for up- and downlink could be applied in three cases, see TS 22.034:

1. If the mobile station only supports enhanced modulation in the downlink direction.
2. If the mobile station supports enhanced modulation in both directions, but the user indicates preference for uplink or downlink biased channel coding asymmetry.
3. If the mobile station supports enhanced modulation in both directions, and the user indicates preference for channel coding symmetry, but the link conditions justifies different channel coding in uplink or downlink.

For Mobile Stations supporting 8-PSK modulation additional channel codings shall apply. The change between different TCH/F channel codings can be provided in A/Gb mode with the RR Channel Mode Modify or the Configuration Change procedure, and in GERAN Iu mode with the Radio Bearer Reconfiguration procedure. It shall be possible to change between channel codings of different modulation schemes. In symmetric HSCSD configuration individual signal level and quality reporting for each HSCSD channel is applied. For an asymmetric HSCSD configuration individual signal level and quality reporting is used for those channels, which have uplink SACCH associated with them. The quality measurements reported on the main channel are based on the worst quality measured among the main and the uni-directional downlink time slots used. In both symmetric and asymmetric HSCSD configuration the neighbouring cell measurement reports are copied on every uplink channel used. See TS 45.008 for more detail on signal level and quality reporting. In A/Gb mode, separate ciphering keys are used for each HSCSD channels. The ciphering keys used on different channels are derived from the Kc. See TS 43.020 for more details. In GERAN Iu mode, the same ciphering architecture is used as in UTRAN Iu mode. See TS 43.051 and TS 33.102 for more details.

The procedures discussed in this clause follow the procedures described in detail in TS 48.008, TS 25.413, TS 25.415, and TS 24.008. Modifications are referred with text in brackets and conditional procedures with dashed line. Normal signalling or signalling presented earlier in the document is drawn with ovals.