Public Safety / Packet Switched

Description

The term 'PS' in 3GPP standards has two distinct and important meanings, differentiated by context.

As **Packet Switched (PS)**, it refers to the architectural domain and service type where user data is formatted into packets for transmission, as opposed to the Circuit Switched (CS) domain used for traditional voice calls. The PS domain is the foundation of mobile broadband. In 5G, the entire core network is packet-based (5GC), but the term remains relevant historically and in interworking. A PS connection involves establishing a Packet Data Protocol (PDP) context in GPRS/UMTS or a PDN Connection/EPS Bearer in LTE, which defines the data path between the UE and an external packet data network (e.g., the internet). Key network functions in the PS domain include the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) in 2G/3G, the Serving Gateway (S-GW) and Packet Data Network Gateway (P-GW) in LTE/EPC, and the User Plane Function (UPF) in 5GC. These nodes handle packet routing, mobility anchoring, policy enforcement, and charging.

As **Public Safety (PS)**, particularly in specifications related to sidelink (e.g., 23.xxx series for ProSe, 24.xxx for protocols), it denotes applications and network features designed for mission-critical use by first responders (police, fire, ambulance) and other government agencies. Public Safety communications require high reliability, availability, security, and direct mode operation when network infrastructure is damaged or unavailable. 3GPP has standardized features like Proximity Services (ProSe) and Mission Critical Services (MCS) to meet these needs. A key enabler is device-to-device communication over the PC5 sidelink interface, allowing direct UE-to-UE communication (PS-LTE, PS-NR) without traversing the network infrastructure. This includes functions like direct discovery, direct communication, and UE-to-Network Relay.

The two meanings converge in modern networks: Public Safety services increasingly rely on the packet-switched infrastructure for broadband data applications (video, data sharing) and use the PS (Packet Switched) domain for network-connected operations, while also leveraging direct PS (Public Safety) sidelink for resilience.

Purpose & Motivation

The **Packet Switched** meaning originates from the fundamental evolution of mobile networks from voice-centric circuit-switched systems to data-centric packet-switched systems. Early GSM networks were optimized for circuit-switched voice. The introduction of GPRS (General Packet Radio Service) in 2G created the PS domain to enable efficient, 'always-on' internet access by sharing network resources statistically. This solved the problem of inefficient dedicated circuits for bursty data traffic, enabling the mobile data revolution. Each generation (3G UMTS, 4G LTE, 5G NR) has enhanced the PS domain with higher speeds, lower latency, and more sophisticated quality of service (QoS) management.

The **Public Safety** meaning arose from the need for commercial mobile network technology to support the stringent requirements of first responder communications. Traditionally, public safety relied on dedicated land mobile radio (LMR) systems like TETRA or P25. 3GPP standardization, starting in Release 12 and significantly expanding through Releases 13-15, aimed to leverage the economies of scale and advanced capabilities of LTE (and later 5G) to provide broadband public safety services. This addressed limitations of narrowband LMR systems, such as low data rates and lack of advanced multimedia services. The PS (Public Safety) work in 3GPP solves the problem of providing mission-critical push-to-talk (MCPTT), video, and data with high reliability, group communication management, and—critically—the ability to operate directly between devices when network coverage is lost, which is a life-saving requirement in disaster scenarios.