trip delay time and low bit error rate. Background class. This is targeted at services which are flexible to delay or delay vari- ation, such as email, file downloading, etc. This class is serviced with the least priority. Additionally, QoS classes in UMTS can be further differentiated by parameters alloca- tion and retention priority (ARP) and traffic handling priority (THP) [1]. In practice, in 3G mobile networks not all QoS classes are used because typically telephony is provided via the CS part, while only Internet access service over the mobile network is based on IP connectivity to which the QoS classes refer. However, they exist as means for QoS differentiation when different carrier-grade services are provided by telecom operator, such as carrier-grade VoIP (with QoS guarantees) or video streaming services with QoS guarantees. The QoS support in mobile networks standardized by 3GPP is accomplished by the definition of bearers. What is a bearer in UMTS (the 3G network from 3GPP)? The bearer service is an enabler for any data transmission between two defined endpoints in the mobile network, including the mobile RAN and mobile core network. In UMTS the bearer may refer to control (i.e. signaling) or data information. For example, there is the SMS bearer and USSD (unstructured supplementary service data) bearer for SMS and USSD messaging, respectively, both provided via GSM signaling. Then there is a voice CS bearer for voice services and a PS bearer in UMTS for Internet access service. In all cases, the bearer specifies the configuration on layer 2 and layer 1 (i.e. the physical layer) of the protocol stack, with the aim to have defined QoS attributes and traffic flow characteristics. The concept of bearer services continues in the same manner in the 4G standards. The UMTS bearer architecture is shown in Figure 5.6. It consists of three main bearers: Radio bearer service (RAB) between the mobile terminals and NodeB (base stations in UMTS), i.e. UTRAN. Iu bearer service between UTRAN and core network gateway,