Signalling

Description

Signalling in telecommunications refers to the exchange of control information that governs the establishment, maintenance, and teardown of communication sessions, as well as the management of network resources. Unlike user-plane traffic, which carries the actual payload (voice, data, video), signalling operates in the control plane and is essential for coordinating network behaviour. In 3GPP systems, signalling encompasses a wide range of protocols and procedures across both the radio access network (RAN) and core network (CN).

At the radio access level, signalling includes Radio Resource Control (RRC) messages exchanged between the User Equipment (UE) and the base station (e.g., eNodeB in LTE, gNodeB in 5G NR). These messages handle connection setup, radio bearer configuration, handover commands, and measurement reports. Below RRC, the Radio Link Control (RLC) and Medium Access Control (MAC) layers provide acknowledged-mode and unacknowledged-mode transport services, ensuring reliable delivery of signalling messages when required. The physical layer (PHY) then transmits these messages over the air interface using control channels such as PDCCH, PUCCH, and PBCH.

In the core network, signalling is divided into Non-Access Stratum (NAS) and Access Stratum (AS) domains. NAS signalling, carried between the UE and core network entities like the Mobility Management Entity (MME) or Access and Mobility Management Function (AMF), handles authentication, security mode control, tracking area updates, and session management. AS signalling, on the other hand, manages the radio connection between the UE and the RAN. The separation allows the core network to evolve independently of the radio access technology, a principle that has enabled the smooth migration from 2G to 5G.

Signalling protocols are structured in layers, following the OSI model. In 3GPP, the control plane protocol stack for LTE and 5G includes NAS, RRC, PDCP, RLC, MAC, and PHY. Each layer adds its own header and performs specific functions. For example, PDCP provides ciphering and integrity protection for signalling messages, while RLC ensures reliable delivery through acknowledged mode (AM) when needed. The signalling connection is an acknowledged-mode link between the UE and the core network that transfers higher-layer NAS information.

The efficiency and scalability of signalling are critical to network performance. Excessive signalling can lead to congestion, increased latency, and reduced capacity. Therefore, 3GPP has continuously optimized signalling procedures, introducing features like signalling reduction (e.g., RRC_INACTIVE state in 5G), control plane CIoT optimizations for IoT devices, and network slicing to isolate signalling traffic for different services. Signalling also plays a vital role in mobility management, enabling seamless handovers and location updates as users move across cells and tracking areas.

In summary, signalling is the nervous system of a mobile network, enabling coordination, control, and management of all communication activities. Its design directly impacts network efficiency, scalability, and the user experience.

Purpose & Motivation

Signalling exists to coordinate and control the complex interactions required for mobile communication. Without it, devices could not establish calls, authenticate, move between cells, or manage data sessions. The primary purpose is to enable the network to set up, maintain, and tear down connections dynamically, ensuring that resources are allocated efficiently and that user data reaches its destination reliably.

Historically, early mobile networks like 1G used analogue signalling with limited control capabilities. The introduction of digital signalling in 2G (GSM) brought structured protocols such as SS7 and later SIP, enabling features like roaming, SMS, and call forwarding. However, as networks evolved to 3G, 4G, and 5G, the complexity and volume of signalling grew exponentially due to increased data services, mobility requirements, and the need for fine-grained QoS control. 3GPP standardized signalling protocols to ensure interoperability between vendors and operators, facilitating global roaming and seamless service delivery.

The motivation for continuous signalling evolution includes reducing latency, improving spectral efficiency, supporting massive IoT deployments, and enabling network slicing. For instance, the shift from circuit-switched to packet-switched signalling in 4G eliminated the need for separate voice and data networks, simplifying architecture. In 5G, service-based architecture (SBA) replaces traditional point-to-point signalling with HTTP/2-based APIs, making the core network more flexible and cloud-native. Thus, signalling remains a cornerstone of mobile network design, driving innovation and efficiency.