An inter
system handovers in a mobile telecommunication
system is performed when a
dual mode user equipment (UE) covered by both
GSM / GPRS network and UMTS network connects a dedicated channel and sets up a call in a BSS region where the
GSM / GPRS provides coverage, and then moves to a UTRAN (
UMTS Terrestrial Radio Access Network) where the UMTS provides coverage, wherein the method includes the steps of: if the
dual mode UE receives an inter
system handover command, i.e. from the BSS to the UTRAN, requesting
GSM / GPRS
data link layer to suspend a GSM / GPRS
data link by a sublayer RR of GSM / GPRS
network layer in the UE, requesting a GSM / GPRS
physical layer to release a physical channel of the GSM / GPRS, and sending the inter system
handover command to the UTRAN for authorizing a sublayer RRC of UMTS
network layer in the UE to continue a call; requesting, at the RRC in the UE, a UMTS physical channel to be configured as a UMTS physical channel, and monitoring if the UMTS
physical layer succeeds to have the configuration of the UMTS physical channel as requested; if the UMTS
physical layer succeeds to have the configuration of the UMTS physical channel, requesting, at the RRC in the UE, a UMTS
data link layer to configure a UMTS data link, and conveying information to the UTRAN through a UMTS channel that the
handover between systems from the BSS to the UTRAN has been successfully performed; and sending, at the RRC, a GSM / GPRS resource release message to RR / GRR, thereby resetting the GSM / GPRS physical layer and the GSM / GPRS
data link layer.