 Due to the fact that the uplink and downlink timeslot usage was not co-ordinated between the voice and data connections, the class A devices in practice had to be capable of transmit and reception simultaneously.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.