GSM Mobile Network – mobile network of 2nd generation
HSCSD Data Transmission in GSM Network

As it was stated in the previous chapter, the maximum available data transfer rate that can be used within one time slot is 14.4 kbps.

The one way how can be further increased bit rate in an existing GSM network is to use multiple time intervals simultaneously for serving a user. This mode of data transmission is abbreviated as HSCSD. It is still based on principle of CSD but compared to conventional CSD it brings significant increase in transmission rate.

Data transmission through HSCSD

As it is shown in the previous figure, communication between the MS and the BS is performed through an associated group of time slots and transferred to the MS. The allocation of timeslots is dependent on the current number of available radio channels and on the capabilities of the MS itself. The data are transmitted by the BS and the BSC (Abis interface) over channels with a bit rate of 16 kbps. Overall 32 channels with the transfer rate 64 kbit/s (i.e., 2 Mbps in total) are available for communication between the BS and BSC. Each of these channels is further divided into four subchannels with transmission rate of 16 kbps.

The channel with transfer rate of 16 kbps can be used to transmit voice (13 kbps) or common data (14.4 kbps). Data are sequentially merged into the standardized channel (64 kbps), which is done in the BSC in TRAU (Transcoder and Rate Adaptation Unit) unit. This unit is designed to convert encoded voice (with rate of 13 kbps) into a standard voice PCM channel (rate 64 kbps) or modify the data transfer rate to 64 kbps. Hence also the maximum transfer rate per one channel, which can be achieved using this technology, is 64 kbps.

Transfer, which is performed this way, can be in most cases asymmetrical. This means, for example, that in the direction from a MS to the network can be assigned three time intervals and in the direction to the MS can be assigned with only one interval. This method of capacity allocation of radio channel is used very often and it is suitable, for example, for connection to the Internet, where the data are transferred mainly from the network to the user. Standard, which defines the mode of HSCSD, divides the available modes up to 18 classes according to how many channels can be used for both directions.