 basic network topologies for constructing the UTRAN: chain, star, ring, and tree structure. Chain Node B Node B Node B Switch / Node B Router Node B Node B Node B Node Bi Switch / Node B Node B Router Node B Switch Node B Router Node B Switch / Router Node B Node B Figure 4.2: Basic network topology configurations for UTRAN Usually the network is designed as a mixture of the above topologies. Figure 4.3 shows an example of a UTRAN network topology. A Node B can either be directly connected to the RNC, or act as a traffic concentrator for a set of Node Bs, or it can be connected to an intermediate router or switch. An intermediate router or switch can be used to achieve a higher degree of traffic concentration, which might be used to connect several Node Bs to a common and distant RNC, or to aggregate the traffic between groups of RNCs and the core network. Choosing an appropriate network topology has a great impact on the traffic patterns in the network and as a consequence, affects the allocation of the link capacities, link costs and total network costs. Different network topologies indicate different degrees of traffic aggregates, and thus the resultant multiplexing gain and cost savings differ from one another. For example, the star topology can achieve high multiplexing gain as a result of high concentration of traffic on the backbone link, i.e. the link between RNC and the switch or router which connects all Node Bs. 4 Framework of UMTS Network Dimensioning Suburban Rural Node B Network Router Node B Node B Urban Node B Node B Figure 4.3: UTRAN network topology example 4.2.3 Traffic Control Traffic control is used to distinguish different traffic streams and provide differentiated treatment to the corresponding packets such as packet buffering, scheduling, shaping, and forwarding. From a network dimensioning perspective, it is important to consider the impact of the traffic control schemes on the traffic carried in the network, and thus on the link dimensioning. For example, in the UMTS access network, to 