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There is a concern that in the GPRS-UMTS packet inter-system handover (ISHO), too many data frames may be lost during handover between these two systems. Similarly in the circuit switched GSM-UMTS inter-system handover too much delay will deteriorate speech quality, as there are no means of buffering data. This static model study estimates ISHO delays and compares the result against QoS objectives, T.S.. Release 99. This paper begins with an elaboration on message sequence charts for UMTS⇔GPRS and UMTS⇔GSM intersystem handover flows. The factors for end-to-end delay are discussed in depth with reference to the message flows. Data from Motorola GSD simulators has been used and delay is calculated for six service scenarios in both circuit switched and packet switched domains. Results show relatively high end-to-end delays with respect to UMTS QoS objectives. A list of potential enhancements is suggested to improve performance.