1. Core network architecture based on CUPS
At the 76th 3GPP CT meeting in June 2017, 3GPP completed the CUPS standard in R14, which is the “technical standard for the separation of the EPC user plane and the control plane”. The agreement, namely
In the 23.214 protocol, the servicing gateway (S-GW), packet data network gate-way (P-GW) and traffic detection function in the evolved packet core network (EPC) are proposed. traffic detection function (TDF). The first is to provide an enhanced architecture. The second is to realize the independent scaling/expansion deployment of user plane functions and control plane functions without affecting the existing node functions (centralized deployment or distributed deployment), thereby realizing flexible deployment and operation of EPC.
The main reason why we consider separating the user plane and the control plane is that in the future 5G network, data traffic will grow rapidly, which places high demands on the carrying capacity of the mobile communication network, that is, the requirements for delay are even higher. Strict. After the core network adopts this user plane and control plane separation technology, it can reduce the delay of mobile application services. For example, we can choose EPC user plane nodes close to 4G/5G base stations or EPC that is more suitable for mobile terminal usage. User plane nodes transmit data to mobile terminals. Compared with the original structure that combined the user plane and the control plane, this separated structure is more flexible and can also reduce latency. This architecture is the key to future 5G communications. One of the architectures, Figure 11 shows the reference architecture defined by 3GPP.
1) Schematic diagram of the separation architecture of user plane and control plane
Figure 11 shows the independent CP/UP modules of S-GW, P-GW and TDF. U represents the user plane and C represents the control plane.
Compared with the architecture that does not separate the control plane/user plane (CP/UP), S-Gw, P-GW, and TDF are all separated into user plane and control. Secondly, there are changes in the interface.
Add Sx interface between CP/UP: S-GW, P-GW, TDF, each has its own Sx interface;
The original interface will be separated due to the separation of CP and UP: S2a becomes S2a-U and S2a-C, control interface and user interface;
3. Some interfaces do not have functional differentiation and are interface functions that completely inherit the original unseparated structure. For example: S12 interface is completely inherited by S-GW-U; Gx, Gy, Gz, Gw, SGi interfaces, the functions are completely inherited by P -GW-C inheritance (between P-GW-U and TDF-U) etc.;
In addition, Figure 11 shows a special case, that is: S-GW, P-GW, and TDF all undergo CP/UP separation. In fact, the following combinations may also exist: S-GW separates, but P-GW does not separate. This situation is also supported.
In addition, there is a structural type that merges S-GW and P-GW, that is, the user plane and control plane of the separated S-GW and P-GW can be combined.
The noodles are merged and become a unified entity. The structure diagram is shown in Figure 12. At the same time, the merged and unified Sx interface also appeared.
2) Changes in the functions of S-GW, P-GW and TDF after the user plane and control plane are separated.
In addition to interface changes, S-GW, P-GW and TDF also have certain functional changes. Some functions disappear and are inherited by other modules, and some functions are added. For a detailed function list, see Table 4.3 of the 23.214 protocol. 2-1-Table 4.3.2-3.
After S-GW separates the control plane and user plane, all functions on S-Gw-U are controlled by S-GW-C. LoRa gateway
Original functions of S-GW:
Load/overload control function, repair and recovery function, operation administration and main-tenance (OAM), GPRS tunneling protocol
protocol, GTP) load and path management, the function will not be carried in the SGW after the separation of CP/UP (it will not be inherited in the P-GW).
UE mobility endpoint: In 3GPP and non-3GPP access, changing the endpoint of the target GTP-U is not supported after separation (P-GW inherits this part of the function).
The remaining functions are completely divided according to the control and user planes, and there are no new functions.
After P-GW separates the control plane and user plane, all functions on P-GW-U are controlled by P-GW-C.
P-GW original functions:
Support for UE mobility. Forwarding of end tags + forwarding cached packets. These two functions do not exist (completely inherited by S-GW).
②The load/overload control function, repair and recovery function, operation and maintenance management interface (OAM), GTP load and path management will not be carried in the P-GW after the CP/UP separation (it will not be inherited in the S-Gw) .
3Sl release/cache/downlink data notification. Sub-functions 1, 2, and 3 under this main function are all fully inherited by S-GW (P-GW only inherits its own sub-function 4, and the P-GW billing process is suspended. The rest of the content is inherited by S-GW).
4PCC (Policy and Charging Control) sub-function 9+13 disappears (PCC is completely carried by P-GW).
TDF also has similar function inheritance and differentiation. For details, please refer to the 23.501 protocol.
3) User plane function selection
Since the user plane and the control plane are separated, certain rules need to be formulated for how to select the user plane for the control plane.
First of all, S-GW-C can only select S-GW-U. Similarly, P-GW and TDF can only select the user plane of the corresponding module. Therefore:
S-GW-C selects S-GW-U;
P-GW-C selects P-GW-U; 3TDF-C selects TDF-U.
For the case of CP merger (merger of S-GW-C and P-GW-C): you can choose the merged UP or an independent UP. The selected UP may not necessarily include all functions, but may only be a subset.
The CP should consider when selecting UP: the location information of the UE (the location may not be selected if the location is too far away); the capacity of the UP, such as: what functions it can support, etc.; and the characteristics required by the UE, which should be selected according to the characteristics of the UE. User plane functions; Finally, the deployment situation of UP should be considered (distributed, centralized or on the access network side).