DTM is a protocol based on the GSM standard that makes simultaneous transfer of CS and PS data over the same radio channel simpler. Without DTM, the mobile device must be capable of reception and transmission simultaneously (full-duplex) requiring complex and expensive circuitry in the mobile terminal. With DTM this requirement doesn’t exist and makes the device implementation cheaper.GSM/EDGE device with simultaneous CS/PS capability was supported, but only for Class A GPRS devices. With DTM, a mobile phone capable can be engaged in both CS and PS call and simultaneous voice and packet data connection in GSM/EDGE networks with the lower Class B requirements thus having a simpler half duplex circuitry due to co-ordination of the timeslot allocations in uplink and downlink for both voice and data. Older devices in Class B without DTM capability could still receive CS calls while having an active PS session. With these devices, the PS connection is put on hold for the duration of the voice call. After the voice call terminates, the data connection resumes data transfer.but in the C class MS need to select CS or PS services manually.