s.orange.ch 172.19.52.92 Firewalls - These protect the IP network against external attack (e.g. from hack- ers). The firewall might reject all packets that are not part of a GPRS subscriber ini- tiated connection. Border Gateway - This is a router providing, e.g. a direct GPRS tunnel between different operators' GPRS networks via an inter-PLMN data network, instead via the public Internet. 4. Charging Gateway - GPRS charging data are collected by all the SGSNs and GGSNs in the network. The charging gateway collects all this data together, proc- esses it and passes it to the billing system. 3.2.1.2 Open Interfaces To practically visualize the inter-operating environment we will take as reference a GSM Network in transition towards a 3G system. Evolving CN elements, e.g. will concurrently support interfaces (thereby signalling) for both 2G and 3G radio networks, i.e. existing elements will be enable through field up- grades with Iu interface towards high-capacity 3G. The UMTS Network and Radio Access Technology The 3G RAN, e.g. will connect to a GSM CN via the Iu interface. This interface pro- vides a logical separation between CS and PS signalling giving the possibility to physi- cally separate the interfaces, i.e. Iu-CS interface for circuit-switched traffic, based on the ATM transport protocol, Iu-PS interface for packet-switched traffic, based most likely on IP over ATM. The Iu interfaces above assume that: the MSC can also multiplex the Iu-PS interface to the SGSN with only one physical interface from RNC to the core network, and that the MSC will get an ATM module to interact with the ATM based RAN. A second new interface besides the Iu in the CN concerns IP links. It is foreseen that by the time UMTS is deployed, MCSs will support IP connections. Thus the solution can be envisaged as follows: a new feature in the MSC, will be the integrated IP function protocol between two MSCs signalling and user data between CS and PS; the integrated IP function will introduce a new type of trunk signalling to the MSC swit