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In telecommunications, Bearer Service or data service is a service that allows transmission of information signals between network interfaces. These services give the subscriber the capacity required to transmit appropriate signals between certain access points, i.e. user network interfaces.
The bearer services include the following:
- Rate adapted sub-rate information like circuit switched asynchronous and synchronous duplex data, 300-9600 bits.
- Speech and data swapping during a call, i.e. alternate speech and data.
- Modem selection, i.e. selection of 3.1 kHz audio service when inter-working with ISDN.
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Bearer services GSM specifies different mechanisms for data transmission, the original GSM allowing for data rates of up to 9600 bit/s for non-voice services. Bearer services permit transparent and non-transparent, synchronous or asynchronous data transmission. Transparent bearer services only use the functions of the physical layer (layer 1) to transmit data. Data transmission has a constant delay and throughput if no transmission errors occur. The only mechanism to increase transmission quality is the use of forward error correction (FEC), which codes redundancy into the data stream and helps to reconstruct the original data in case of transmission errors. Depending on the FEC, data rates of 2.4, 4.8, or 9.6 kbit/s are possible. Transparent bearer services do not try to recover lost data in case of, for example, shadowing or interruptions due to handover. Non-transparent bearer services use protocols of layers two and three to implement error correction and flow control. These services use the transparent bearer services, adding a radio link protocol (RLP). This protocol comprises mechanisms of high-level data link control (HDLC), (Halsall, 1996) and special selective-reject mechanisms to trigger retransmission of erroneous data. The achieved bit error rate is less than 10–7, but now throughput and delay may vary depending on transmission quality. Using transparent and non-transparent services, GSM specifies several bearer services for interworking with PSTN, ISDN, and packet switched public data networks (PSPDN) like X.25, which is available worldwide. Data transmission can be full-duplex, synchronous with data rates of 1.2, 2.4, 4.8, and 9.6k bit/s or full-duplex, asynchronous from 300 to 9,600 bit/s (ETSI, 1991a). Clearly, these relatively low data rates reflect the assumption that data services will only constitute some small percentage of the overall traffic. While this is still true of GSM networks today, the relation of data and voice services is changing, with data becoming more and more important. This development is also reflected in the new data services.