A.3 The entities of the mobile system

23.0023GPPNetwork architectureRelease 17TS

A.3.1 Gateway Location Register (GLR)

GLR is pseudo-HLR located in visited network. The roamer’s information is stored in it and handles location management of it within the network. Presence of GLR is invisible from home network therefore interface between HLR and GLR is same as one of HLR and VLR. Also, the interface between the VLR and GLR is the same as the one between the VLR and the HLR.

The GLR is a logical node and acts as a VLR for MAP signalling (e.g. PRN) from the HPLMN point of view. The GLR acts as a HLR for MAP signalling (e.g. Register SS) from the VPLMN point of view.

The GLR also acts as an SGSN for MAP signalling only (e.g. ISD) from the HPLMN point of view. This is because MAP operations such as ISD must be terminated at the GLR for the purpose of subscriber data caching.

The GLR shall terminate all TC dialogues and start new dialogues towards the HLR or the VLR. The GLR shall generate SCCP address of the HLR (i.e. E.214 MGT) from IMSI.

A.3.2 Intermediate Mobile-services Switching Centre (IM-MSC)

The Intermediate MSC (IM-MSC) is the logical node, which represent MSCs in the visited network. Some service features use the MSC Number stored in the HLR directly to deliver message from a certain node in home network (e.g. SMS-GMSC) to serving MSC in visited network. In such case, the message is firstly distributed to representative MSC (i.e. IM-MSC) and it relays it to actual serving MSC interrogating routing information to GLR.

– The Intermediate MSC (IM-MSC) is a logical node and represents the VMSC in the GLR equipped VPLMN.

– The IM-MSC acts as the VMSC for the HPLMN in the same way that the GLR acts as a VLR for HPLMN. The IM-MSC terminates MAP signalling from the HPLMN towards the VMSC and forwards the signal to the actual VMSC.

– The IM-MSC has an address interrogation function with which it is able to obtain the actual VMSC Number from the GLR.

– The IM-MSC is implemented in the same physical node as the one in which the GLR is implemented.

– The GLR alters the VMSC Number to the IM-MSC Number within an Update Location message.

– The IM-MSC Number is the E.164 Number assigned to the IM-MSC [53].

– The interrogation function of the IM-MSC is similar to that in the SMS-GMSC.

A.3.3 Intermediate GPRS Serving Node (IM-GSN)

The Intermediate GSN (IM-GSN) is a logical node and represents the SGSN for some GTP signalling termination in a GLR equipped VPLMN.

The IM-GSN acts as an SGSN for only some GTP signalling messages (i.e. PDU_Notification request/response, PDU_Notification_reject request/response) from the HPLMN point of view. The IM-GSN terminates these GTP signalling messages from the HPLMN towards SGSN and forwards the signal to the actual SGSN. The IM-GSN has an address interrogation function with which it is able to request the actual SGSN address from the GLR.

Apart from the case described above (i.e. PDU_Notification request/response, PDU_Notification_reject request/response), all other GTP signalling should be handled directly between the SGSN and the GGSN.

NOTE: MAP signalling towards the SGSN is NOT terminated at the IM-GSN. Instead it is terminated at the GLR.