ed 2 Mbit/s line, and transmit towards the RNC on a common backbone to- gether with ATM based UTRAN traffic from the UTRAN BSs. We call this circuit emulation on ATM. In the RNC E1s from the GSM BS get re-established and sent to/ from the GSM BSC node. This option applies when projecting large UTRAN traffic. 7.10.2.2 The Backhaul Network A SDH regional backhaul infrastructure aims to connect the BSCs with the switch sites in the main cities (i.e. cities A-C in our example). The backhaul network will provide distributed high capacity and high availability infrastructure, from where the last mile access can proceed over microwave links or leased lines. We co-locate an ATM/AAL2 switch with Frame Relay (FR) encapsulation capabilities at each BSC site. The ATM switch and the BSC inter-connection supports fractional ATM flow, thereby allowing AAL1 circuit emulation traffic onto the ATM and TDM domain for a pure separated ATM and TDM domain upstream in the network. GSM voice traffic use TDM transport form the BSC whereas UTRAN traffic and GPRS packet data share common ATM transport resources. We may assume that on average each regional ring terminates 4-6 BSCs to the switch sites. We can also assume that for the size of our example network, six regional rings can terminate in switch site of city A, and four rings can terminate in the switch sites of city B and city C, respectively. Capacity projections in our example shows an average of 15 Els required per ring in 2000 based on GSM traffic, and about 80 E1s in 2005 including 3G traffic. Thus, any rings with STM-1 capacity will need an upgrade before 2005. Consequently, a mix of STM-4 rings for larger traffic and STM-1 rings for lower traffic areas will result in the ideal backhaul network. 7.10.2.3 The National Backbone Network To take the traffic flow from STM1-4 rings, a high capacity STM-16 ring will inter- connect the main BSC sites of each region with the switching sites in cities A-C. In addition, we would expect a high-capacity backbone ATM switch in each switch 