The DTM handover is a special handover type in GERAN in which the source BSS (or source RNS) requests the target BSS to allocate both circuit-switched (CS) and packet-switched resources. This is useful in scenarios when the UE has, for instance, a conventional (CS) voice call ongoing and a packet session active (e.g. for email sync up or file download) and a handover is required to a GERAN cell that supports DTM operation.
The DTM handover is essentially composed of two handover procedures (one in the CS domain and another in the PS domain), which are synchronized and conducted in parallel. The DTM handover is initiated by the source BSS by sending a PS Handover Required (with a CS Indication) to the SGSN and a Handover Required (with a PS Indication) to the MSC over the Gb and A interfaces respectively. The CS and PS Indicators are used to inform the target BSS that there is an ongoing CS and PS handover in parallel. These indications require the target BSS to wait for both the HANDOVER REQUEST message (from the MSC) and PS HANDOVER REQUEST message (from the SGSN) before responding.