CUPS stands for Control and User plane separation. LTE EPC nodes such as SGW and PGW contribute for control and user plane both. In CUPS architecture, both the existing EPC nodes divided into control and user plane nodes i.e. SGW-C, SGW-U, PGW-C and PGW-U. Moreover, two another nodes named as TDF-C and TDF-U introduced.
Because of these separations, new interfaces Sxa, Sxb and Gxc have been introduced. Sxa is the interface between SGW-C and SGW-U. Sxb is the interface between PGW-C and PGW-U and Sxc is the interface between TDF-C and TDF-U.
All the Sxx interfaces uses Packet Forwarding Control Plane (PFCP) protocol which runs over the UDP.
There are few changes in the attach procedure call flow. I am going to list down the changes only.
- There is no change in MME functionality or interfaces at MME.
- Once SGW receives Create Session Request from MME, it perform SGW-U selection procedure. SGW-U selection criteria is completely vendor specific implementation. After SGW-U selection, SGW-C prepares and sends the Sxa Session Establishment Request message to SGW-U.
- SGW-U allocates FTEID-U if it does not come from SGW-C within the Sxa Session Establishment Request message.
Apart from user plane tunnel allocation, SGW-U perform various steps and finally responds to SGW-C by using the Sxa Session Establishment Response message.
- After receiving Sxa Session Establishment Response message from SGW-U, SGW-C prepares and sends Create Session Request as it use to send earlier.
- PGW-C receives the Create Session Request message from SGW-C and performs the PGW-U selection. Similar to SGW-U, PGW-U selection is also vendor specific implementation.
- After PGW-U selection, PGW-C prepares and sends Sxb Session Establishment Request message to PGW-U. The request message may contain TEID-U if it is implemented at PGW-C.
- PGW-U responds to PGW-C using the Sxb Session Establishment Response. The response message may contain FTEID-U if it is allocated at PGW-U.
- PGW-C sends Create Session Response to SGW-C with the allocated FTEID-U. SGW-C sends Sxa Session Modification Request to SGW-U to inform F-TEID-U of PGW-U.
- Now SGW-U and PGW-U both have information of its peer tunnel end points. SGW-U responds to SGW-C with Sxa Session Modification Response message.
- After receiving Modify bearer request from MME, SGW again initiate Sxa Modification procedure to update the eNodeB side S1-U tunnel id at SGW-U.
These are the major changes in attach call flow under the CUPS architecture.
If someone has details on TDF, please share.
