- Dual Transfer Mode
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). DTM is a 3GPP baseline R99 feature.
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 kbit/s. In 2010, devices with DTM multislot class 32 such as Nokia N900 are available.
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 is started on a mobile phone that is in Packet Transfer Mode(i.e. in a data call), the procedure takes place in three stages. The TBF's (Data "Call") are released. A dedicated connection for the voice call is initiated. Finally, the mobile phone uses DTM for re-establishing the data connection.
3GPP Release 6 specifies the Enhanced DTM CS Establishment and Enhanced DTM CS Release procedures to enable smooth transitions between the packet transfer and dual transfer modes, without having to release the TBF's. This enables continuous data transfer also when calls are set up and released, as well as reduced load on the common control channels of the GSM network. This technology is not yet supported by any operator.
As a side effect, in a network supporting DTM it is possible to have paging coordination (a service already provided by some operators based on a different network setup) for the GSM/EDGE devices engaged in data transfer. This means that incoming calls can be received also during packet data transfer sessions (be it web browsing, file download, or background services such as email), which is beneficial for both the operator (more calls are established) and the subscriber (calls come through persistently, i.e. also in case the device is engaged in data transfer).
Starting late 2009 or early 2010, Vodafone has added DTM support in its network in the UK. In addition to enabling simultaneous call and data transfer in 2G network, DTM -capable network can also secure that incoming calls are received by devices that are transferring packet data (depending on the network implementation, this can also apply to devices that do not support the DTM feature).
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