Dual Transfer Mode
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Dual Transfer Mode (DTM) is a protocol based on the GSM standard that allows simultaneous transfer of Circuit switched (CS) voice and Packet switched (PS) data over the same radio channel (ARFCN).
A mobile phone which is DTM capable can be engaged in both CS and PS call and simultaneous voice and packet data connection in GSM/EDGE networks. The DTM Capability and the multislot capability of a mobile phone can be found in its technical specifications.
One common class implemented by mobile phone vendors is the DTM Multislot Class 11. For example, the technical specification of Nokia N95 states a speed of DL/UL 177.6/118.4 kbps.
A simultaneous voice and data call implies that a data call might start on an ongoing voice call or a voice call might start on an ongoing data call.
In case, a voice call has be start on a mobile phone in Packet Transfer Mode(i.e. in a data call), this happens in three stages. TBF (Data Call) is released. A dedicated connection for the voice call is initiated. Finally, the mobile phone uses DTM for re-establishing the data connection.
