23.0343GPPHigh Speed Circuit Switched Data (HSCSD)Release 17Stage 2TS
The air interface user rate in the original GSM data transmission is limited to 9,6 kbps with the 12 kbps air interface rate. The HSCSD described in the present document Stage 2 description allows higher air interface user rates to be used for transparent and non-transparent data services.
NOTE: In the present document the term "air interface user rate" corresponds to the transfer rate in radio interface for user data and "air interface rate" includes additional data related to transmission protocols.
HSCSD is a feature enabling the co-allocation of multiple full rate traffic channels (TCH/F) into a HSCSD configuration. The aim of HSCSD is to provide a mixture of services with different air interface user rates by a single physical layer structure. Further improvements in data rates are achieved through enhancement of the radio interface (modulation and coding schemes), which allows higher bit rates per one GSM time slot. The available capacity of a HSCSD configuration is several times the capacity of a TCH/F, leading to a significant enhancement in the air interface data transfer rate.
Figure 1 represents the network architecture to support HSCSD in A/Gb mode based on the concept of multiple independent channels in one HSCSD configuration. Figure 1a represents the network architecture to support HSCSD in GERAN Iu mode.
NOTE: 3GPP TS 43.051  does not specify the GERAN internal interface between BSC and BTS.
In case when enhanced modulation is used the number of time slots in the radio interface may not correspond to the number of data streams in the network side, for example a 28,8 kbps service may be offered through one air interface time slot, but it requires two 14,4 (16 kbps) Abis channels. Another example is bit transparent 56 kbps to 64 kbps service where two air interface time slots of 32 kbps are multiplexed onto one 64 kbps data stream on the network side.
Figure 1: Network architecture for supporting HSCSD in A/Gb mode
Figure 1a: Network architecture for supporting HSCSD in GERAN Iu mode
A new functionality is introduced at the network and MS to provide the functions of combining and splitting the data into separate data streams which will then be transferred via n channels at the radio interface, where n = 1, 2, 3, … 8. Once split, the data streams shall be carried by the n full rate traffic channels, called HSCSD channels, as if they were independent of each other, for the purpose of data relay and radio interface L1 error control, until to the point in the network where they are combined. However, logically the n full rate traffic channels at the radio interface belong to the same HSCSD configuration, and therefore they shall be controlled as one radio link by the network for the purpose of cellular operations, e.g. handover. This requires a new functionality in BSS.
The different user data substreams carried on the radio channels (one substream being the data flow over a single TCH) shall be mapped over the A interface or GERAN Iu interface, and vice versa, following the rules defined in 3GPP TS 24.008  and 3GPP TS 48.020 .
In A/Gb mode, the use of resources on the A and E interfaces is restricted to one 64 kbps circuit by multiplexing the data streams into one A interface circuit (see ITU‑T Recommendation I.460 ).
In GERAN Iu mode, the user plane at the Iu interface shall comply to the Iu UP protocol (3GPP TS 25.415 ). For transparent calls the Iu user plane is operated in transparent mode, version 1, for non-transparent calls it is operated in support mode for predefined SDU sizes, version 2.
After an inter-MSC SRNS relocation the user plane between the anchor MSC or MGW and the target MSC or MGW shall comply to the Nb UP protocol (3GPP TS 29.415 ), if the anchor MSC or MGW and the target MSC or MGW are connected via an ATM interface or IP interface. The NbUP shall be configured in support mode, the data is transported in a 64 kbit/s bit stream (for details see 3GPP TS 29.007 ).
If both MSCs are connected via a TDM interface the use of resources on the E interface is restricted to one 64 kbps circuit (see 3GPP TS 29.007 ).
4.1 HSCSD service aspects
At call setup a user indicates a maximum number of TCH/F, acceptable channel codings (including extensions to acceptable channel codings for ECSD channel codings), possible other modem type, and fixed network user rate values. For non-transparent HSCSD connection, in addition, wanted air interface user rate is indicated and the network resource needs, if user wishes to make use of the user initiated modification of the maximum number of TCH/F and/or wanted air interface user rate (user initiated service level up- and downgrading described in subclauses 5.2.4 and 5.2.4a) during the call. In case the indicated acceptable channel coding(s) implies that enhanced modulation is possible, the user may indicate a preference for channel coding asymmetry, i.e. downlink biased channel coding asymmetry, uplink biased channel coding asymmetry or channel coding symmetry. Together these parameters describe the HSCSD characteristics and network uses them to allocate an appropriate HSCSD connection.
For both transparent and non-transparent HSCSD connections the call can be established with any number of TCH/F from one up to the maximum number of TCH/F, i.e. the minimum channel requirement is always one TCH/F.
If the wanted air interface user rate requirement cannot be met using a symmetric configuration, an asymmetric configuration can be chosen. The network shall in this case give priority to fulfilling the air interface user rate requirement in downlink direction.
For non-transparent HSCSD connection the network can use dynamic allocation of resources, i.e. TCH/F, as long as the configuration is not in contradiction with the limiting values defined by the MS and the mobile equipment is capable of handling the allocated channel configuration. For transparent HSCSD connection the dynamic resource allocation is applicable, if the air interface user rate is kept constant. The change of channel configuration within the limits of minimum and maximum channel requirements is done with resource upgrading and resource downgrading procedures (described in subclauses 5.2.3 and 5.2.3a) during the call.
The MS may request a service level up- or downgrading during the call, if so negotiated in the beginning of the call. In the user initiated modification procedure, the user can modify the channel coding asymmetry preference when enhanced modulation is indicated. This modification of channel requirements and/or wanted air interface user rate and/or channel coding asymmetry preference is applicable to non-transparent HSCSD connections only.
4.2 HSCSD service aspects in UTRAN Iu mode
The multislot mechanism is not needed in UTRAN Iu mode, as one bearer can provide all needed data rates. In UTRAN Iu mode, consequently the parameters required for setup of a multislot call are not needed in a call setup, and the MSC shall ignore the parameters.
The parameters which are specific to multislot are (all contained in the Bearer Capability Information Element):
– Maximum number of traffic channels.
– Acceptable Channel coding(s).
– UIMI, User initiated modification indication.
– Acceptable Channel Codings extended.
4.2.1 UTRAN Iu mode to GERAN Iu mode or A/Gb mode handover
In case of handover from UTRAN Iu mode to GERAN Iu mode or A/Gb mode the multislot parameters are required in the middle of an ongoing call. A multi system mobile station supporting UTRAN Iu mode and at least one GERAN mode shall therefore always include the multislot parameters in the setup, also in UTRAN Iu mode.