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The UMTS 3G system (3 rd generation cellular system) is designed to offer a packet switched mode for data transfers in parallel to a circuit switched mode. This feature was planned since the beginning of 3G studies, contrary to 2G ones (such as the GSM system). To provide reliable data transfers, protocols implementing ARQ (Automatic Repeat reQuest) techniques are needed. In UMTS, up to three ARQ protocols can simultaneously be active at different levels of the protocol architecture. In this thesis, we focus on RLC (Radio Link Control), TCP (Transmission Control Protocol) and MAC-hs (Medium Access Control-high speed) protocols. The stacking of ARQ mechanisms represents a risk of interactions between them. We study studies some of these interactions. Reliable data transfers on the UMTS radio interface are provided by the RLC protocol, precisely through RLC entities that are configured to operate in an acknowledged mode (AM). The RLC protocol is highly adaptable and specific primitives allow the tuning of the protocol for different QoS requirements. We study different aspects of the RLC mechanisms such as a SDU discard function. Number of UMTS services are expected to be close to Internet applications such as, webbrowsing, e-mail, file transfer, etc. In Internet, these services are provided via the traditional and imperative TCP/IP stack. Protocol mechanisms of TCP are compared with RLC ones. Only a part of TCP extensions for wireless networks consider a radio link with an ARQ protocol at layer 2. We analyze the TCP behavior over RLC and we discuss the usefulness of some of the proposed TCP extensions when used in the UMTS environment. The introduction of RLC below the TCP/IP stack is useful for the overall performance; however RLC parameters have to be carefully set up. High flexibility of RLC leads to a long list of adjustable RLC parameters. One of signific ant parameters is the buffer size of the considered RLC entity. Impact of different TCP protocol soft states on the RLC buffer occupancy has to be considered. We investigate how the RLC buffer occupancy changes for different values of wired RTT (Round Trip Time) and BlER (Block Error Rate) on the radio interface. In addition, we an alyze the impact of different RLC buffer sizes on the TCP performance. To overcome some drawbacks of TCP/RLC stacking, we propose to introduce between RLC and TCP a specific mechanism called Early Error Notification (EEN). The EEN mechanism is based on the RLC SDU discard function and on a capability of exchanging control messages between TCP and RLC entities. The EEN mechanism is proposed for uplink data transfers. A possibility of using EEN for downlink data transfers is also discussed.