Packet Core 5G Packet Core Network Network Network IP Address IP Address IP Address IP Address IP Address IP Address a.b.c.d a.b.c.d a.b.c.d a.b.c.d a.b.c.d a.b.c.d Figure 2.20 - SSC modes 5G identifiers The SSC mode is associated with every PDU session, and it does not change during the lifetime of the PDU session. The following are the various modes of session continuity - for example, in a UE mobility use case when the user is traveling on a high-speed train: SSC Mode 1: In this case, the network needs to preserve the connectivity of the service provided to the UE. As an example, for a PDU session of the IPv4, IPv6, or IPv4v6 type, the IP address needs to be preserved. SSC Mode 2: In this case, it is not mandatory to maintain the connectivity. When the UE moves from one region to another or undergoes a handover procedure triggered by any other reason, the network can "break-before-make" in this case. Hence, the user connectivity or PDU sessions can be released. As an example, if the UE is allocated an IPv4/IPv6 address during its initial attachment, the address can be released and a new IP address can be assigned to the UE. Hence, a disruption to the session continuity is accepted with SSC Mode 2. SSC Mode 3: As we learned, in SSC Mode 2, the technique of break-before-make is employed and accepted, and SSC Mode 3 is an enhancement of that. A new PDU session is set up before the already established PDU session is torn down. Hence, it is a "make-before-break" procedure. The IP address, in this case, is also not preserved, as the new session may have a different IP address assigned. However, this allows better service continuity than SSC Mode 2 due to the make-before-break principle. In the previous section, we learned about the key concepts of PDU sessions, 5G QoS, and SSC in 5G. These concepts will be used widely by network engineers when designing and operating 5G networks. 5G identifiers In telecommunication systems, the network operator allocates a SIM card to every device. This SIM card is associated not ju