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User and Control plane (U/C-plane) separation is a defining feature of 5G. In terms of nodes within 5G-RAN, CU-CP is solely responsible for control plane communication and signalling.
The logical C-plane interface to/from CU-CP is carried following Stream Control Transmission Protocol (SCTP) protocol.
For example, as shown in Fig. 1 signalling bearers for connections AMF-to-from-CUCP (i.e. NG-C), CUCP-to-from-DU (i.e. F1-C), and CUCP-to-from-DU (i.e. E1) are passed through the SCTP protocol layer at the transport level. (figure References: TS 38.412, TS 38.462, TS 38.472).
Hence, to establish any communication with CU-CP, the respective nodes (i.e. DU, CU-UP, and AMF) must first establish the SCTP link. Let us read an excerpt from 3GPP specification about F1-C logical interface:
F1 Setup: The F1 Setup procedure is used to create a logical F1 connection between the CU-CP and DU. It is necessary to establish an SCTP connection between the CU-CP and DU before the F1 Setup procedure can be initiated.
The same is true for true to create E1 and NG-C links.
In short, once the SCTP link is established, the signalling bearers (c-plane flow) are good to flow between CUCP and other nodes. I will not discuss in detail what SCTP is, readers can refer to excessive material over the web for it. In short, The SCTP is a transport layer protocol that is used for exchanging control messages between NG-RAN entity gNB-CUCP and gNB-CU-UP, gNB-DU, and AMF in 5GC.
Stream Control Transmission Protocol (SCTP) Link Establishment
Figure 2 explains how the SCTP link is established between the gNB-CU-CP and gNB-CU-UP. The procedures for F1-C and NG-C would also follow similar message exchanges.
Step 1:
Step 1:
As shown in the figure, first the 4-way handshake process takes place, initiated by the gNB-CU-UP (SCTP client) by sending an INIT message. CU-CP (SCTP server responds with INIT_ACK message, followed by COOKIE_ECHO and COOKIE_ACK message exchange.
Step 2
Step 2
Once the SCTP link is established, now logical E1 interface could be created using the E1-Setup procedure as shown in the figure.
Step 3
Step 3
After the logical E1 interface is created, signalling messages could flow between CUCP and CUUP. In case no signalling messages are there to be exchanged then, at regular intervals, CUCP and CUUP exchange heartbeat messages to monitor the SCTP link.
The above three steps are followed for F1-C and NG-C interface creation and maintenance as well.
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