Signalling Link

Description

The signalling link in 3GPP is a lower-layer protocol entity that provides an acknowledged-mode link layer service for transferring signalling messages between the User Equipment (UE) and the radio access network (RAN), as well as between the UE and the core network (via the signalling connection). It is a fundamental building block of the control plane, ensuring that critical control messages are delivered reliably and in order. The signalling link operates at the access stratum level and is typically implemented by the Radio Link Control (RLC) sublayer in acknowledged mode (AM), although other layers such as PDCP may also contribute to reliability.

In the protocol stack, the signalling link sits below the Radio Resource Control (RRC) layer and above the Medium Access Control (MAC) layer. When the RRC layer needs to send a message (e.g., an RRC Connection Setup or a measurement report), it passes the message to the signalling link (RLC AM), which segments it into Protocol Data Units (PDUs), adds sequence numbers, and ensures retransmission if the peer does not acknowledge receipt. This acknowledged-mode operation is crucial for signalling because control messages must not be lost or corrupted. The signalling link also handles in-sequence delivery and duplicate detection, providing a reliable stream to upper layers.

The signalling link is used for both UE-RAN signalling (e.g., RRC messages) and UE-core network signalling (NAS messages). NAS messages are encapsulated within RRC messages and carried over the signalling link. Thus, the signalling link acts as a transport for both access stratum and non-access stratum signalling. In LTE, the signalling link is established when the UE enters RRC_CONNECTED state, and it is maintained as long as the RRC connection exists. In 5G NR, similar principles apply, with the signalling link being part of the radio bearer configuration for signalling radio bearers (SRBs).

Key characteristics of the signalling link include: acknowledged-mode transmission with retransmissions, segmentation and reassembly of large messages, flow control, and support for multiple logical channels. It is configured with specific parameters such as maximum retransmission threshold, polling timers, and buffer sizes, which are optimized for signalling traffic (typically low volume but high priority). The signalling link is also designed to support mobility: during handover, the signalling link context is transferred from the source to the target RAN node, allowing seamless continuation of signalling.

The signalling link is distinct from the signalling connection, which is an end-to-end concept between the UE and core network. The signalling link is hop-by-hop, specifically between the UE and the RAN. However, the two are closely related: the signalling connection relies on the signalling link to transport NAS messages over the radio interface. Together, they form the complete control plane path. The signalling link also differs from user-plane data radio bearers, which may use unacknowledged mode (UM) for low-latency services like VoIP. Signalling, by contrast, always requires reliability, hence the use of acknowledged mode.

In terms of architecture, the signalling link is managed by the RRC layer, which configures the RLC entities for signalling radio bearers (SRB0, SRB1, SRB2 in LTE; SRB0, SRB1, SRB2, SRB3 in 5G). SRB0 uses transparent mode (TM) for initial access, while SRB1 and SRB2 use acknowledged mode for reliable delivery. The signalling link is thus a configurable entity that adapts to different signalling needs.

Overall, the signalling link is a critical component of the 3GPP control plane, providing the reliable transport necessary for all radio-level signalling and for carrying NAS messages between the UE and core network. Its design ensures that control messages are delivered accurately and efficiently, enabling the network to manage connections, mobility, and security.

Purpose & Motivation

The signalling link exists to provide a reliable, acknowledged-mode transport service for control plane messages over the radio interface. It solves the problem of ensuring that critical signalling messages—such as RRC connection requests, handover commands, and NAS authentication messages—are delivered without loss or corruption, even in adverse radio conditions. Without a reliable signalling link, the network could not coordinate radio resources, manage mobility, or establish secure connections.

Historically, in early mobile networks like GSM, signalling was carried over dedicated control channels with limited reliability mechanisms. As networks evolved to 3G and 4G, the need for more robust and flexible signalling transport grew. 3GPP introduced the RLC acknowledged mode as the foundation for the signalling link, allowing retransmissions and segmentation to handle varying message sizes and error rates. This was a significant improvement over previous approaches, which often relied on simple repetition or lacked error recovery.

The motivation for a dedicated signalling link includes: (1) ensuring high reliability for control messages, which are essential for network operation; (2) supporting variable message sizes through segmentation and reassembly; (3) enabling efficient multiplexing of multiple signalling flows over a single radio bearer; and (4) facilitating mobility by allowing the signalling link context to be transferred during handover. In 5G, the signalling link continues to evolve with features like duplication for ultra-reliable low-latency communication (URLLC) and enhanced security. Thus, the signalling link remains a cornerstone of 3GPP radio protocol design, ensuring that the control plane operates reliably and efficiently.