[5gc] Introduction to three SSC (session and service continuity) modes

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I work for an international well-known terminal manufacturer , be responsible for modem Chip development .
stay 5G In the early stage, he was responsible for the terminal data service layer 、 Core network related development work , Current lead 6G Research on technical standards of computational power network .


The content of the blog mainly focuses on ：
       5G/6G Explanation of the agreement
        Explanation of computational power network （ Cloud computing , Edge of computing , End calculation ）
        senior C Language explanation
       Rust Language explanation

5G SSC(Session and Service Continuity) Model is introduced

       In any telecommunications technology , A key feature requirement is data session and business continuity , To ensure the uninterrupted service experience of users , and Not subject to any UE IP Address or Core network anchor The change of . stay 4G In the network ,EPC Through maintenance P-GW And terminal PDU Session to provide IP Continuity of conversation , Regardless of the mobility of the terminal .

       stay 5G System architecture , Support SSC (Session and Service Continuity) function , It can meet various continuity requirements of the terminal for different applications and services .5GC Out of flexibility and considering subsequent evolution , by UE Or the service provides multiple types of SSC Pattern .

SSC Pattern classification

      5G The system architecture provides three types of sessions and business continuity (SSC) Pattern , To meet different applications / Business continuity requirements . once SSC Patterns and PDU Associated with a conversation , So it's in PDU The lifetime of the session does not change . According to the demand of data service ,5G The architecture allows applications to influence SSC The choice of mode , As shown in the figure below ：

SSC Mode 1

SSC Pattern 1 Next , The network provides UE The connection service of does not drop , The user's during the entire session IP The address remains the same , That is, the session anchor does not change , Acting as an anchor UPF Not because UE Mobility 、 Access network technology changes , No matter how the user moves , Whether the user passes 3GPP Access or Non 3GPP Access , Acting as an anchor UPF Will not change , Also assigned to UE Of IP The address will not change .

SSC Pattern 1 It can be applied to any PDU Session type and any access type .

SSC Mode 2

stay SSC Pattern 2 Next , The network can release and UE The connection service and the corresponding PDU conversation , Originally assigned to UE Of IP The address will also be released accordingly .SSC Pattern 2 It can be seen as releasing the original connection when the session anchor changes , Re establishment UE Connection to the new user face anchor , Intermediary business will be interrupted .

stay SSC Pattern 2 Next ,SMF According to the operator's strategy （ for example , come from AF Request or equipment load status （ Load balancing ）） Trigger release PDU Session and indicate UE Immediately establish a new connection to the same data network PDU conversation , Accordingly, it is UE Assign a new IP Address , Does not provide IP Continuity .

SSC Pattern 2 It can be applied to any PDU Session type and any access type .

SSC Mode 3

SSC Pattern 3 Be situated between SSC Pattern 1 and SSC Pattern 2 Between , When the user face changes （ That is, the session anchor changes ）, Build on new before change PDU Connection between session anchors , To ensure business continuity .

SSC Pattern 3 Of PDU conversation ,SMF Support in release UE With the last PDU Before the connection of the session anchor , establish UE And new PDU Connection between session anchors , The new connection also accesses the same DN. new PDU The session anchor is given to UE Assign a new IP Address / After prefix , adopt NAS Signaling indication UE Maintain the old IP Address / Prefix for a while , Then release the old IP Address / Prefix , At the same time, the old PDU Session resources are released accordingly . In the process , User IP The address will change as the user anchor changes , But since a new connection has been established before , There will be no business interruption on the user side .

SSC Pattern 3 Only applicable to IP PDU Type of conversation , The access type can be 3GPP Access or Non 3GPP Access .

SSC The choice of mode

SSC Mode selection is made by SMF Complete based on ：

• Allowed in user signing information SSC Pattern （ Include default SSC Pattern ）;
• PDU Session type and UE Requested SSC Pattern .

SSC The schema is part of the user's subscription information ,SMF from UDM Receive user signings S-NSSAI、DNN Permitted SSC Select the corresponding mode from the list of modes . When the user signs up S-NSSAI、DNN Need to allocate static IP Address / The prefix ,SMF Expected this PDU Conversation selection SSC Pattern 1. Operators can use UE Routing strategy （URSP） towards UE Provide SSC Mode selection strategy （SSCMSP）（SSCMSP yes URSP Part of the rules ）.UE Will use SSCMSP To determine the session type and SSC Pattern , Operators can also update URSP Rules to update UE Of SSC Pattern . When PDU When the session type is non fabric type or Ethernet type ,UE and SMF Should not choose SSC Pattern 3. If UE No SSCMSP, be UE Can be based on UE Local configuration selection SSC Pattern （ If it is configured locally SSC Pattern ）.

SMF choice SSC After the mode is determined , Will inform UE Select for this PDU The session SSC Pattern .

Example

       As shown below ,SSC Schema information as NAS Part of layer signaling . The terminal sends to the core network PDU session establishment request, These include PDU Type of conversation And request SSC mode( Optional ). Reply from the core network PDU session establishmet accept Respond to selected SSC mode, Here's the picture ：

PDU Session Establishment Request

PDU Session Establishment Accept

summary

• 5G Technology allows service providers to provide services for a given PDU Session settings specific SSC Pattern ;
• In a moving scene ,SSC The mode determines the flow of packets in the network ;
• PDU The session is configured to use a specific “ Session and service continuity ”(SSC) Pattern ;
• 3GPP The specification provides three different types of SSC Pattern ;
• UE Can be in PDU Session Establishment Request News SSC Mode Field requests a specific pattern ：;
• SMF Use PDU Session Eslablishment Accept In the news Selected SSC Mode Field specifies the configured SSC Pattern ;
• SSC Pattern 1 and 2 Can be used for any type of PDU, and SSC Pattern 3 It can only be used for IP PDU type ;
• Three SSC Patterns can be used for 3GPP Access or Non 3GPP Access .

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