HSCSD (High Speed Circuit Switching Data)

Hi this is one data transfering technology used in the GSM mobiles for internet usage.

High-speed circuit switched data (HSCSD) is a feature that enables the co-allocation of multiple full rate traffi c channels (TCH/F) of GSM into an HSCSD confi guration. The aim of HSCSD is to provide a mixture of services with different air interface user rates by a single physical layer structure. The available capacity of an HSCSD confi guration is several times the capacity of a TCH/F, leading to a significant enhancement in air interface data transfer capability.

Ushering faster data rates into the mainstream is the new speed of 14.4 kbps per time slot and HSCSD protocols that approach wireline access rates of up to 57.6 kbps by using multiple 14.4 kbps time slots. The increase from the current baseline of 9.6 kbps to 14.4 kbps is due to a nominal reduction in the error- correction overhead of the GSM radio link protocol (RLP), allowing the use of a higher data rate.

For operators, migration to HSCSD brings data into the mainstream, enabled in many cases by relatively standard software upgrades to base station (BS) and mobile switching center (MSC) equipment. Flexible air interface resource allocation allows the network to dynamically assign resources related to the air interface
usage according to the network operator’s strategy, and the end-user’s request for a change in the air interface resource allocation based on data transfer needs. The provision of the asymmetric air interface connection allows simple mobile equipment to receive data at higher rates than otherwise would be possible with a symmetric connection.

For end-users, HSCSD enables the roll-out of mainstream high-end segment services that enable faster web browsing, fi le downloads, mobile video-conference and navigation, vertical applications, telematics, and bandwidth-secure mobile local area network (LAN) access. Value-added service providers will also be able
to offer guaranteed quality of service and cost-effi cient mass-market applications, such as direct IP where users make circuit-switched data calls straight into a GSM network router connected to the Internet. To the end-user, the value-added service provider or the operator is equivalent to an Internet service provider that offers a fast, secure dial-up Internet protocol service at cheaper mobile-to-mobile rates. HSCSD is provided within the existing mobility management.  Roaming is also possible. The throughput for an HSCSD connection remains constant for the duration of the call, except for interruption of transmission during handoff. The handoff is simultaneous for all time slots making up an HSCSD connection. Endusers wanting to use HSCSD have to subscribe to general bearer services. Supplementary services applicable to general bearer services can be used simultaneously with HSCSD.

Firmware on most current GSM PC cards needs to be upgraded. The reduced RLP layer also means that a stronger signal strength is necessary. Multiple time slot usage is probably only effi ciently available in off-peak times, increasing overall off-peak idle capacity usage. HSCSD is not a very feasible solution for bursty
data applications.

object: BSC [BASICS]
range: TRUE, FALSE
default: FALSE

Enable HSCSD, this parameter specifies whether the feature 'High Speed Circuit Switched Data (HSCSD)' is enabled for the BSC or not.

1) This parameter enables HSCSD for the BSC base only. To activate it, however, it must be explicitly enabled for each BTS (see CREATE BTS [BASICS]: BTSHSCSD).

2) As a mandatory precondition for HSCSD the features 'early classmark sending' (see SET BTS  [OPTIONS]:EARCLM) and 'pooling' (see parameter ENPOOL) must be enabled! 

Principle: HSCSD is a feature which allows the 'bunching' of up to 4 consecutive radio timeslots for data connections of up to 38,4 (= 4 x 9,6) kbit/s (multislot connections). The data rate depends on the bearer capability requested by the MS and the negotiation result between MS and MSC. Each HSCSD connection consists of 1 main TCH which carries the main signalling (both FACCH and SACCH) and further 1..3  secondary TCHs. All radio timeslots used for one connection are FR timeslots located on the same TRX and
use the same frequency hopping mode and the same TSC. 

Connection modes: There are 2 types of multislot connections:
Symmetric and asymmetric ones. In symmetric mode all secondary TCHs are bi-directional (UL and DL) and in asymmetric mode the secondary channels are only uni-directional (DL) TCHs or can be a mix of  bi-directional and uni-directional TCHs (example: One 'HSCSD 3+2' call consists of: one main TCH, one secondary bi-directional TCH and one secondary uni-directional TCH). The downlink based asymmetry allows the use of a receive rate higher than the transmission rate and is thus very typical for Internet  applications. The asymmetric mode is only possible for non-transparent data connections.

Resource allocation: The BSC is responsible for the flexible air resource allocation. It may alter the number of TCH/F as well as the channel codings used for the connection. Reasons for the change of the resource allocation may be either the lack of radio resources, handover and/or the maintenance of the service quality. The change of the air resource allocation is done by the BSC using 'service level upgrading and downgrading' procedures. For transparent HSCSD connections the BSC is not allowed to change the user data rate, but it may alter the number of TCHs used by the connection (in this case the data rate per TCH changes). For non-transparent calls the BSC is also allowed to downgrade the user rate to a lower value.


In symmetric mode individual signal level and quality reporting for each used channel is applied. For an asymmetric HSCSD configuration individual signal level and quality reporting is used for the main TCH. The quality measurements reported on the main channel are based on the worst quality measured on the main and the unidirectional downlink timeslots used. In both symmetric and asymmetric HSCSD configuration the neighbour cell measurements are reported on each uplink channel used. All TCHs used in an HSCSD connection are handed over simultaneously. The BSC may alter the number of timeslots used for the connection and the channel codings when handing the connection over to the new channels. All kinds of inter-cell handovers are supported, intracell handover is possible only with cause 'complete to inner' or 'inner to complete'.

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