High speed circuit switched data


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High Speed Circuit Switched Data ( HSCSD ), German as fast circuit-switched data transmission is an extension of GSM - mobile telephony standards CSD to achieve faster data transfer. By bundling several data channels, data transmission rates of up to about 115.2 kbit / s (= 8 × 14.4 kbit / s) can theoretically be achieved.

Technically, it is a bundling of several neighboring time slots on a logical connection. In the GSM , eight time slots are transmitted with a time delay per frequency . Theoretically, all eight radio slots could be assigned to one connection. However, this would only work if two antennas were used, one each for uplink (sent data) and one for downlink (received data). With the usual one antenna, the mobile phone switches to reception after the transmission pulse and needs a certain amount of time to adapt. In fact, only a maximum of four channel slots can be used, with a division of 2: 2, 3: 1 or 4: 1 for the downlink: uplink being available. These then act as a multiplier of the basic data rate per slot of 9.6 kbit / s or 14.4 kbit / s, with a practical maximum of 4 × 14.4 to 57.6 kbit / s.

HSCSD enables transparent and non-transparent data transmission. In the case of transparent transmission, error checking is left to the application. This enables a steady stream of data, which is necessary for real-time applications (such as video transmissions). Small errors in the picture are not so annoying, which makes HSCSD more suitable than GPRS for this type of application . In the case of non-transparent data transmission, the network takes over the error correction. This is used when surfing and emailing.

Practical use

Not every GSM network operator offered channel bundling via HSCSD. In Germany it is only Vodafone (and only in contract tariffs, not with CallYa), because E-Plus switched off HSCSD on August 1, 2010. Vodafone allows a transmission speed of 14.4 kbit / s per channel.

Since only terminal devices are available on the German market that can handle a total of four channels, there are two variants in practice:

  • The bundling of two channels each for uplink and downlink results in 28.8 kbit / s in each direction.
    Vodafone only allows this 2: 2 bundling with the ISDN interface e.g. B. analog protocol V.34 or digital protocol V.110, which must then be set to 38.6 kbit / s.
  • When three are bundled into one channel , 43.2 kbit / s and 14.4 kbit / s are available. This speed is only offered by E-Plus, but requires the digital protocol V.120 in the ISDN interface for full use. The analog protocol is limited to 33.6 kbit / s. As mentioned above, this offer is no longer available.

If the device is moved, the bundle often cannot be taken over in the event of an HSCSD handover and the connection falls back to CSD with only one channel.

future

HSCSD is a very inefficient data service. When using HSCSD, several radio channels are reserved even if no data is being sent or received. This is why HSCSD has been switched off by most mobile network providers since the beginning of the 2010s or is at most still supported for older machine-to-machine (M2M) applications. New machine-to-machine (M2M) applications can be implemented using the packet-oriented data service (PSD) GPRS or LTE .

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