When a mobile subscriber moves from a VLR area to another VLR area, a Location Registration procedure will happen using a TMSI. This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR.
GTP' has been designed to deliver the CDR(s) from the CDF, which generates CDRs to the CGF(s). This protocol is
required if the CGF resides outside the CDFs. It utilizes some aspects of GTP (defined in
TS 29.060), which is used for packet data tunnelling in the backbone network.
GTP' operates on the Ga interface and does not imply the use of any specific backbone network.
GTP' performs the following functions:
CDR transfer between the CDF and the CGF.
Redirection of CDRs to another CGF.
Detect communication failures between the communicating peers, using echo messaging.
Advertise to peers about its CDR transfer capability (e.g., after a period of service downtime).
Prevents duplicate CDRs that might arise during redundancy operations.
If so configured, the CDR duplication prevention function may also be carried out by marking potentially
duplicated CDR packets, and, delegating the final duplicate deletion task to a CGF or the Billing Domain
(instead of handling the possible duplicates solely by GTP' messaging).
This optional signalling path may be used by the GGSN to retrieve information about the location and supported services for the mobile subscriber, to be able to activate a packet data network address.
There are two alternative ways to implement this signalling path:
If an SS7 interface is implemented in the GGSN, signalling between the GGSN and the HLR uses MAP.
If there is no SS7 interface in the GGSN, any GSN in the same PLMN and which has an SS7 interface installed can be used as a GTP to MAP protocol converter, thus forming a signalling path between the GGSN and the HLR.