For CSG aware UEs (Rel-8 and onwards) the CSG subscription data is permanently stored in the HSS/HLR. Retrieved by MME .
FAP-GW will be having information about FAPs being configured to FAP GW by Means of S1 Interface.
The access control list, i.e., the list of IMSIs of all the subscribers for a CSG, is also
stored in an OAM function and pushed to or retrieved by the HNB-GW.
When a UE accesses a CSG cell, the MME/SGSN/MSC/VLR shall check that the CSG ID
of the CSG cell corresponds to a CSG ID in the CSG subscription data, and that the
expiration time (if present) is still valid.
- If the CSG ID of the CSG cell is not present in the UE’s CSG subscription data or the timer
has expired, then the MME/SGSN/MSC/VLR shall send a reject message with the error
code #25 (Not authorized for this CSG). The UE shall remove the entry for this CSG from
the Allowed CSG list if present.
For CSG cells, the network performs access control based on the CSG ID advertised by the
CSG cell and the CSG subscription data of the UE stored in the network. For hybrid cells, the
network verifies whether the UE is a CSG member or not.
1.) The UE initiates the NAS procedure ( Attach,Detach or Service Request) by sending, to the H(e)NB, the appropriate NAS Request.
2.) The H(e)NB forwards the NAS Request message together with the Cell Access Mode,CSG ID, and other information of the cell from where it received the message to the SGSN/MME Via H(e)Nb -GW.
The CSG ID is provided by the H(e)NB if the UE sends the NAS Request
message via a CSG cell or a hybrid cell. The Cell Access Modes is provided by the H(e)NB if the UE sends the NAS Request message via a hybrid cell.
In addition, when a UE accesses a HNB, the HNB performs the Registration procedure (not shown) to inform the HNB-GW of the specific HNB where the UE is located. The HNB sends the UE Register Request message to the HNB-GW to register the UE at the HNBGW if the identity of the UE (provided during RRC Connection Establishment) is unknown at the HNB being accessed, i.e. no Context id exists for the UE in the HNB.
3.)If the UE initiates the NAS procedure at a CSG cell, the SGSN/MME shall check whether the CSG ID is contained in the CSG subscription and is not expired. If the CSG ID is not present or expired, the SGSN/MME shall send the corresponding NAS reject message to the UE with the cause value #25 (Not authorized for this CSG).
4.) The UE shall remove the CSG ID from its Allowed CSG list if present
For an Emergency Attach the SGSN/MME shall not check for access restrictions, regional restrictions or subscription restrictions (e.g. CSG restrictions).
Similarly, if the UE has ongoing emergency bearer services no CSG access control shall be performed.
If all checks are successful then the SGSN/MME constructs a context for the UE.
- The SGSN/MME sends a NAS Accept message to the UE.
If the NAS procedure is initiated due to manual CSG selection and occurs via a CSG cell,
the UE upon receiving the NAS Accept message shall add the CSG ID to its Allowed CSG
list if it is not already present.