MSC site STM-16 transport STM-16 network BSC site nx STM-1 n X 2M RNC site Combined BSC & RNC site Figure 7.14 An integrated core transport network. In Figure 7.15 [8] leased lines terminate point-to-point between BTS and BSC, while cascaded microwave structures use three sites connected in a cascade, and the structure hub serves five BTSs including itself. In the example links, about half of the hub-to- BSC links are 4 X 2 Mb and other half is 8 X 2 Mb (4 X (2 X 2 M)). The UMTS Network and Radio Access Technology 35 % hub MW links 30 % % Leased Lines 2x2 Mb 2x2 Mb GMS BS 2x2 Mb L/L 2Mb 2x2 Mb 2x2 Mb 4x2 Mb 4x2 Mbi 35 % cascade MW links 4x2 Mb A 2G Access Network attached BSC site to a 3G SDH backbone Figure 7.15 Typical 2G access network attached to a SDH backbone. 7.10.2.1 Co-siting Transmission Sites In a hybrid 2G/3G environment, we would aim to integrate the UMTS network with the existing GSM network infrastructure to re-use the latter to the maximum. Thus, in the macrocell coverage, e.g. we would co-locate WCDMA BSs with existing GSM BSs as much as possible. To achieve this Figure 7.16 [8] illustrates two key options, excluding an initial approach of simply carrying 3G traffic over the existing GMS transmission infrastructure. UTRAN UTRAN UTRAN Circuit ATM on Emulation fractional on ATM UTRAN UTRAN Figure 7.16 2G and 3G transmission site sharing. In the first option, carrying 3G traffic over the BSS, the ATM transmission to/from the 3G BS gets mapped from/to an E1 2 Mbit/s through 64 kbit/s time slots. We call this Deploying 3G Networks ATM on fractional E1. Here we assume the time slot handling DXC function is in- cluded in the GSM BS. At the GSM BSC, we connect the UTRAN time slots to the RNC semi-permanently. This solution may apply while 3G traffic remains low. In the 2nd option, all radio access network transmission gets carried by a common ATM backbone network composed by UTRAN nodes (e.g. RNCs and BSs). The GSM BSs also get connected to the UTRAN BSs. In the UTRAN BS we map Els to an emulated ATM bas