sed on the basis of traffic type, e.g. O&M data are on the O&M VLAN, while charging data are on the Ga VLAN, user data on the Gn VLAN and external data on the Gi VLAN. 4.13.2 Iu-PS Interface The Iu-PS interface (between the RNC and SGSN) has both the user plane and control planes. The sig- nalling procedures between the SGSN and RNC are carried by the signalling plane, while the tunnelled subscriber data from the RAN to the SGSN are carried by the user plane. The user plane of the Iu-PS interface consists of the PDP context data, which are tunnelled over the ATM using the GTP Protocol. Both types of traffic are carried over the ATM interface between the SGSN and RNC. However, many of the signalling messages related to mobility and session management, SMS transfer, etc., are exchanged between the SGSN and the UE without any processing by the RNC. RANAP and SS7 are used for sig- nalling. RANAP contains messages used for the mobility and session management procedures, such as relocation (ofRNC), RAB management, location and paging, etc., while SS7 is used to carry the signalling over the ATM connections. SS7 is a protocol stack that fills the functionality between the RANAP and the low level ATM adaptations that fill the ATM cells. The protocol stack contains the SCCP (signalling connection control part), MTP-3b (message transfer part layer 3b), NNI (network-to-network inter- face) SSCF (service specific coordination function) and SSCOP (Service Specific Connection Oriented Protocol). Protection of the Iu-PS Interface This is achieved by the appropriate configuration of the tunnelling units, which depends upon the ATM transport between the 3G SGSN and the RNC. This means that if the PDP context is lost due to failure of the tunnelling units, the subscriber should be able to reconnect to the network using different link and tunnelling units. To achieve this, it is important to limit the rate of data that arrives at the tunnelling unit. 4.13.3 Gn Interface Planning Gn Interface and Protocols The Gn interface carries b