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Let us consider the establishment of a PDU session, shown in Figure 3.13 below, a procedure that is initiated from a UE to the 5G core Session Management Function (SMF) network function. A PDU session is activated and is assigned to a particular network slice of a UE as part of its initial 5G UE registration procedure with a 5G CN. The establishment of a PDU session is part of the 5G Session Management (5GSM) NAS protocol. Figure 3.13 shows only the 5GSM message names without showing the contents of each message.

Figure 3.13 Illustration: NAS layer messages for a 5G PDU session establishment procedure.

In GPRS and UMTS networks also, an MS/UE registers with its CN using the ATTACH procedure for PS services. However, as far as the implementations are concerned, there are differences in the GPRS/UMTS and LTE/EPS ATTACH procedure. For example, unlike the GPRS system, an LTE/EPS ATTACH request message also contains a piggybacked session activation request to the CN.

Example 3.9 illustrates the typical messages flows associated with an SM layer (NAS) PDU session establishment procedure in the case of the 5G system.