ABSTRACT Universal Mobile Telecommunications System (UMTS) is the third-generation (3G) cell phone technology. UMTS offers telecommunications services (like speech or SMS) and bearer services, which provide the capability for information transfer between access points. It is possible to negotiate and renegotiate the characteristics of a bearer service at session or connection establishment and during ongoing session or connection. Both connection-oriented and connectionless services are offered for Point-to-Point and Point-to-Multipoint communication. The radio interface of UMTS is called UTRAN (UMTS Terrestrial Radio Access Network) which uses W-CDMA as the underlying air interface [19]. 4G technology is also being developed for the heterogeneous networks e.g. WiMax. Today mobile wireless infrastructure is commonly-seen as one of the most advanced form of human communications. The last decade GSM technology has been a leading force in this revolution. Simultaneously with the phenomenal deployment of wireless networks and distribution of user terminals, also the Internet has seen a similar revolutionary growth. Handover means changing/switching of a mobile transmission from one channel to another. The main purpose of handover is to maintain an ongoing call when the hardware changes the channel, whether it is in the same cell or a different cell. Whenever a handover occurs there is always a handover delay which dictates that we cannot guarantee the service continuity. Though the handover time is msec but if there is a long handover latency, it will results in high packet losses and degradation of end-to-end TCP performance in case of packet switched data. Delay sensitive real-time applications demands packet lossless and low latency Quality-of-Service (QOS) guarantee during handover. In this thesis we will find the reasons of delay and packet loss during the handover and effect on the QOS (quality of services).