User Equipment Associated Signalling

Description

UE-associated signalling refers to the class of signalling messages in 3GPP networks that are tied to a specific User Equipment (UE) through a UE-associated logical connection. This concept is fundamental to the control plane protocols used in radio access networks, such as S1-AP, X2-AP, NG-AP, F1-AP, and E1-AP. The definition has evolved over releases to encompass different interfaces and network architectures, reflecting the progression from LTE to 5G and the disaggregation of network functions.

In the context of the E1 interface between the gNB-CU-CP (Central Unit Control Plane) and gNB-CU-UP (Central Unit User Plane), UE-associated signalling uses the UE-associated logical E1-connection to associate messages to a specific UE. Similarly, for the F1 interface between gNB-DU (Distributed Unit) and gNB-CU (Central Unit), it uses the UE-associated logical F1-connection. In LTE, the S1 interface between eNB and EPC (Evolved Packet Core) used the UE-associated logical S1-connection. These logical connections are established during UE context setup and are identified by a pair of identifiers: the gNB-CU-UE-F1AP-ID and gNB-DU-UE-F1AP-ID for F1, or the gNB-CU-CP-UE-E1AP-ID and gNB-CU-UP-UE-E1AP-ID for E1.

The primary function of UE-associated signalling is to ensure that control messages are delivered to the correct UE context in the receiving node. When a message is sent, it includes the appropriate UE-associated logical connection identifiers, allowing the receiving node to route the message to the correct UE instance. This is critical for procedures such as UE context setup, modification, release, and handover.

UE-associated signalling is distinct from non-UE-associated signalling, which is used for global procedures that are not tied to a specific UE, such as interface setup, reset, or error indication. The separation allows for efficient handling of both UE-specific and common procedures.

The evolution of UE-associated signalling mirrors the architectural changes in 3GPP networks. In LTE, it was primarily associated with the S1 interface. With the introduction of 5G and the disaggregated RAN architecture, the concept expanded to F1 and E1 interfaces. The definition in the specifications has been updated to reflect these new interfaces, as seen in the changes from Rel-15 to Rel-17 and Rel-19.

In summary, UE-associated signalling is a cornerstone of control plane protocols, enabling reliable and unambiguous communication between network nodes regarding specific UEs. Its proper implementation is essential for the correct operation of mobility, session management, and other UE-related procedures.

Purpose & Motivation

UE-associated signalling exists to provide a reliable mechanism for associating control plane messages with a specific UE across various network interfaces. It solves the problem of message routing ambiguity in networks where multiple UEs are served simultaneously. Without a clear association, a receiving node would not be able to determine which UE a message pertains to, leading to errors in UE context management, handover, and session establishment.

The concept was introduced in Release 8 for LTE, where the S1 interface between eNB and EPC needed a way to identify UE-specific signalling. The UE-associated logical S1-connection was defined, using identifiers such as eNB-UE-S1AP-ID and MME-UE-S1AP-ID. This allowed the MME and eNB to manage UE contexts independently and reliably.

With the advent of 5G and the disaggregation of the base station into CU and DU, the same principle was extended to the F1 and E1 interfaces. The F1 interface between gNB-DU and gNB-CU uses the UE-associated logical F1-connection, while the E1 interface between gNB-CU-CP and gNB-CU-UP uses the UE-associated logical E1-connection. This ensures that the control plane can manage user plane functions in a disaggregated architecture.

The motivation for UE-associated signalling is to enable scalable and efficient control plane operations. By tying messages to a specific UE, the network can support a large number of UEs with minimal overhead, as each message only needs to carry the relevant identifiers rather than full UE context information. This also facilitates load balancing and redundancy, as UE contexts can be moved between nodes.