The main principle is that whenever a RAB is assigned for a PDP context (or re-assigned) the SGSN decides whether to use a direct user plane tunnel between RNC and GGSN, or, if it needs to handle user plane data and use two tunnels as in earlier 3GPP releases. Further, when a Direct Tunnel was in use and the RAB assigned for a PDP context is released (i.e. the PDP context is preserved) the GTP-U tunnel is (re)established between the GGSN and SGSN in order to be able to handle the downlink packets.
The SGSN needs to decide before every RAB assignment and every SRNS relocation if direct tunnel will be used or not.
In the Intra SGSN Inter-system Change procedure from 3G to 2G, the SGSN needs to perform a PDP Context Update Procedure to establish the tunnel between the 2G SGSN and the GGSN and release the Direct Tunnel between RNC and GGSN.
In the Intra SGSN Inter-system Change procedure from 2G to 3G, the SGSN provides to the RNC the GGSN's Address(es) for User Plane and TEID(s) for Uplink data in the RAB Assignment Request during the Service Request Procedure. Then the SGSN performs a PDP Context Update Procedure to establish the Direct Tunnel between the RNC and the GGSN and release the tunnel between SGSN and GGSN.
Toggling between 2G and 3G might (or might not) be frequent. This may effect the Gn signalling when Direct Tunnel is deployed. It is implementation specific how this is addressed.
In order to inform the GGSN of the IP address of the RNC and the TEID for the active PDP context, the SGSN will send an Update PDP Context Request message to the GGSN during every PDP Context Activation procedure for which a Direct Tunnel is established. This message, which is an optional message in two-tunnel system, increases the signalling load on SGSN and GGSN.
RAB release and re-establishment procedures, which become visible to the GGSN in a Direct Tunnel system, also increase the signalling load on SGSN and GGSN. If the radio-link is bad, and the RNC releases the Iu connection, the frequent RAB release and re-establishment increases the signalling load on SGSN and GGSN. One consequence is that it is useful if the RNC utilises URA-PCH state.
Some Intra SGSN procedures, such as Intra SGSN inter RNC, Intra SGSN Inter-system change, and possibly other procedures, which are invisible to the GGSN in two-tunnel system, will become visible to the GGSN and increase the signalling load on both SGSN and GGSN.
Receipt of the Direct Tunnel Indicator makes the GGSN aware that the SGSN is using a Direct Tunnel.
When using Direct Tunnel, if the GGSN receives a GTP-U Error Indication (from the RNC) it shall not release the related PDP context locally but the GGSN shall notify the SGSN that the RAB in RAN is invalid. This needs the GGSN to be updated.
If, when using two tunnels transport network QoS was used on Iu-PS but not on Gn then moving to Direct Tunnel requires the GGSN to implement transport network QoS.
RNCs that only support ATM transport should support IP transport for the Iu-PS interface. Alternatively a solution as described below in clause 6.4
can be used.
Due to this Direct Tunnel feature, the Error Handling Behaviour of Rel-7 RNC is changed.
If, when using two tunnels transport network QoS was used on Gn but not on Iu-PS then moving to Direct Tunnel requires the RNC to implement transport network QoS.
Within the network area where Direct Tunnel is used, the Iu-PS and Gn transport networks must be made visible to each other, this is a network configuration issue.
This is minor security drawback. The risk could be reduced by using firewall between Iu-PS and Gn networks instead of simple router (this might impact user-plane latency). Only IP traffic to predefined IP-addresses/ports from predefined addresses/ports would be allowed between Iu-PS and Gn networks. Considering network security, a firewall can be deployed between the RNC and the IP network.
For an ATM based Iu-PS, the ATM connection needs to be terminated and "interworked"
to IP. The location of the interworking may be within the Iu-PS network or at the edge of the Iu-PS network.
shows the connectivity between the Iu-PS network and the Gn (backbone) network.
The Iu-PS and Gn transport networks need to be using the same IP version.
Currently, UMTS QoS architecture is defined in TS 23.107
, which is based on the two tunnel architecture. However, it is believed that this QoS architecture should be updated due to this Direct Tunnel feature.
The deployment of Direct Tunnel requires consistent QoS treatment on Iu-PS and Gn interfaces.