Section: 4.3.20.2.3 Attach for RN operation
Content: To start relay operations, the normal attach procedure, with the following exceptions, is applied: - The RN and the USIM-RN perform local security operations (e.g. establishment of a secure channel between them) as specified in TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [41]; - The RN selects a cell from the list acquired during Phase I; - The RN establishes an RRC connection with the DeNB, indicating that the connection is for a RN; - The DeNB is aware of the MMEs that support RN functionality. In all cases when the RN indication is received, the DeNB shall ensure that the current or (re)selected MME supports RN functionality; NOTE 1: The RN follows normal UE behaviour, e.g. the RN's NAS may use either an IMSI or a GUTI. Also, the RN's NAS may or may not provide an S-TMSI to the RN's AS, and hence, the RRCConnectionRequest may either contain an S-TMSI or a random value. - In the S1 interface Initial UE Message, the DeNB sends the RN indication to the MME. This message also carries the IP address of the S-GW/P-GW function embedded in the DeNB; - The subscription data supplied to the MME by the HSS for USIM-RN includes an indication that the subscription is permitted to be used by a RN. - If the S1 interface Initial UE Message indicates that this is a RN, but the subscription data does not indicate that the subscription includes a permission to operate as a RN, then the MME shall reject the NAS procedure (e.g. Attach Request, Tracking Area Update Request, Service Request, etc) with an appropriate cause value (e.g. one that avoids retries on this PLMN yet does not harm a RN that has unexpectedly performed PLMN reselection). NOTE 2: It is anticipated that the MME checks that the HPLMN of the USIM-RN is authorised to attach RNs to this MME. - The MME and RN perform the normal EPS Authentication procedures. - MME (RN) selects the S-GW/P-GW in DeNB for the RN based on the IP address included in the Initial UE Message (i.e. all GW selection and APN related procedures are bypassed during this phase). The MME performs S11 interface signalling with the S-GW/P-GW located in the DeNB; - The MME accepts the attach procedure and sets up an S1 context with the DeNB. When relay function is enabled, MMEs in a pool should all have the same relaying function capability in order to have consistent support for functions such as redundancy, load balancing. Figure 4.3.20.2-1: RN attach procedure The detach procedure for the RN is the same as the normal UE detach procedure, though the RN should ensure that no UE is connected to the RN cells before detaching. It is up to RN implementation how it ensures no UE is connected.