stem. The PEI can assume different formats for different UE types and use cases. The UE pre- sents the PEI to the network along with an indication of the PEI format being used. If the UE supports at least one 3GPP access technology, the UE must be allocated a PEI in the IMEI format. In 3GPP Rel-15, the only format supported for the PEI parameter is an IMEI. 5G Globally Unique Temporary Identifier is allocated by an AMF to the UE that is common in both 3GPP and non-3GPP access. A UE can use the same 5G-GUTI for International Mobile Subscriber Identity (IMSI) is used as a unique identification of mobile subscriber in the 3GPP networks. The IMSI consists of three parts: (1) mobile country code consisting of three digits. The mobile country code uniquely identifies the country of residence of the mobile subscriber; (2) mobile net- work code consisting of two or three digits for 3GPP applications. The mobile network code identifies the home public land mobile network of the mobile subscriber. The length of the mobile network code (two or three digits) depends on the value of the mobile country code; and (3) mobile subscriber identification num- ber identifying the mobile subscriber within a public land mobile network. Network access identifier defined by IETF RFC 7542 is a common format for user identifiers submitted by a client during authentication. The purpose of the network access identifier is to allow a user to be associated with an account name, as well as to assist in the routing of the authentication request across multiple domains. Note that the network access identifier may not necessarily be the same as the user's email address or the user identifier submitted in an application-layer authentication. 5G Network Architecture 149 accessing 3GPP access and non-3GPP access security context within the AMF. The AMF may assign a new 5G-GUTI to the UE at any time. The AMF may delay updating the UE with its new 5G-GUTI until the next NAS signaling exchange. The 5G-GUTI comprises a GUAMI and a 5G-TMSI, where GUAMI id