23.0343GPPHigh Speed Circuit Switched Data (HSCSD)Release 17Stage 2TS
5.3.1 Numbering of data substreams
In transparent data transmission the V.110 data frames on the HSCSD channels carry data substream numbers to retain the order of transmission over GSM, between the split/combine functions. Between these functions a channel internal multiframing is also used in order to increase the tolerance against inter channel transmission delays. In A/Gb mode, depending on the location of the access point to external networks the split/combine functionality is located in the BSS or in the IWF on the network side, and at the mobile station. In GERAN Iu mode, the split/combine functionality on the network side is always located in the BSS.
A detailed description of the numbering scheme is given in 3GPP TS 44.021 .
HSCSD also supports user rates which are not multiples of rates provided by one TCH/F.
If the selected user rate requires n TCH/F channels but is less than the total rate that can be achieved with these n TCH/F then in the first n‑1 channels the data frames carry user data on all D bits. In the n th channel the unneeded D bits of the V.110 frames are padded with fill bits.