Cisco ASR 5x00 Administration Manual page 95

PDN Gateway Overview
Figure 14.
Table 7. Subscriber-initiated Attach (initial) Call Flow Description
Step Description
1 The UE initiates the Attach procedure by the transmission of an Attach Request (IMSI or old GUTI, last visited TAI (if
available), UE Network Capability, PDN Address Allocation, Protocol Configuration Options, Attach Type) message
together with an indication of the Selected Network to the eNodeB. IMSI is included if the UE does not have a valid GUTI
available. If the UE has a valid GUTI, it is included.
2 The eNodeB derives the MME from the GUTI and from the indicated Selected Network. If that MME is not associated
with the eNodeB, the eNodeB selects an MME using an "MME selection function". The eNodeB forwards the Attach
Request message to the new MME contained in a S1-MME control message (Initial UE message) together with the
Selected Network and an indication of the E-UTRAN Area identity, a globally unique E-UTRAN ID of the cell from where
it received the message to the new MME.
Subscriber-initiated Attach (initial) Call Flow
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