High-Speed Circuit Switched Data

The HSCSD feature allows subscribers to access higher data rates with multiple TCH/Fs . HSCSD has defined mechanisms to make efficient and flexible use of radio resources for HSCSD. Combining/splitting functionality is defined in the IWF and terminal to separate a data stream into n TCH/F, where n ranges from 1 to 8. These n TCH/Fs, carrying a data stream, are referred to as HSCSD channels. The HSCSD channels carry the substreams as if they were independent of each other. However, the HSCSD channels, serving one stream, are controlled as one radio link. Any cellular operation (e.g., handover), treats the HSCSD channels of one stream as a single entity.

Before HSCSD, the air interface user rate in the original GSM data transmission was limited to 9.6 Kbps. The HSCSD made it possible to have user data rates up to 57.6 Kbps, with the same GSM RF equipment. The data rate is limited by the per-channel capacity on the A interface, which is 64 Kbps.To carry more than 64 Kbps, another split/combine function has to be defined and implemented for the terrestrial circuits on the terminating party. This is not realistic and not needed for circuit switched data.HSCSD doesn't impose any new requirement for the interconnection with PSTN, ISDN, CSPDN, and PSPDN. The subscriber uses general bearer services as defined in GSM TS 02 series, and the HLR stores the bearer capability information as part of the service profile.
HSCSD serves both transparent and nontransparent connections. In transparent data transmission, the data frames on the HSCSD channels carry data substream numbers to retain the order of transmission between the split/combine functions. Between these functions, channel internal multiframing is also used in order to increase the tolerance against inter-channel transmission delays. A transparent connection may request a data rate that is not a multiple of rates provided by one TCH/F. In such a case the data bits in the nth TCH/F need to be padded with fill bits. In nontransparent connection, the RLP and L2R are modified to support multiple parallel TCH/Fs instead of only one TCH/F. Also, the RLP frame sequence number range is increased to accommodate the enlarged data transmission rate.
HSCSD provides both symmetric and asymmetric connection setup. In the symmetric connections, the number of HSCSD channels is same in both uplink and downlink directions. In asymmetric connections, the number of HSCSD channels is different. The network usually allocates asymmetric connections when the desired air interface user rate requirement cannot be met using a symmetric configuration. The network in this case gives priority to fulfilling the air interface user rate requirement in the downlink direction. This is in consideration that the downlink bandwidth is needed more than the uplink bandwidth for most data applications. Note that the asymmetric connections are only given for nontransparent HSCSD.