Lower overhead costs on O&M and spare parts as well as less restriction of site space. With the trend towards more packet data traffic in the UTRAN, a transport network with centralized RNCs will require less bandwidth due to lower overhead over the lub interface. About half of the traffic can be dropped locally at the RNC/MGW site thereby re- ducing the traffic in the core backbone network to a minimum. We do not need Iu transmission resources between RNC and MGW, neither do we need Iur transmission resources for co-located RNCs. 7.9.1.3 Network Overheads While looking at the traffic flow, we also need to see the corresponding overheads in the various interfaces. In the RNC different overheads apply to the Iu and lub interfaces. Table 7.22 illustrates a non-exhaustive summary of the RNC overheads. Table 7.22 Summary of RNC Overhead Assumptions User data Voice activity factor (%) Voice bit rate 10.35 Voice codec (AMR) Retransmission (%) Signalling (%) Frame OH, CS (%) Frame OH, PS (%) O&M (%) We can summarize more details on the overhead assumptions considered for the CN dimensioning as follows. Packet data over Iu (GTP-U/UDP/IP/AAL5/ATM) has 72% OH, assuming an average packet size of 125 bytes, and about 10% signalling OH on top of user data on the Iu and CN. In addition, there is 21% OH on circuit data over Iu and core network, AAL2/ATM. Overhead in PS interfaces: about 35% OH on packet data for Gb over ATM network, including BSSGP/FR/AAL5/ATM, about 35% OH on packet data on Gn interface, in- cluding GTP-U/UDP/IP. Very low OH on Gi interface, on top of the IP address applica- tion. The UMTS Network and Radio Access Technology We assume 0.2% grade-of-service for UTRAN links with 90% load factor on the physi- cal payload on ATM links, and 0.1% grade-of-service for voice and circuit data over the CN with also 90% load factor on physical payload on ATM links. We add 10% retransmission OH on packet data over the Iub on PS user data, assuming 70-75% load factor on IP links in the CN. We estimate 66% OH, i.e. 10.35 kb