Active Communication

Description

Active Communication is a concept used in 3GPP specifications to describe the state of a User Equipment (UE) when it has an ongoing connection that allows it to exchange user data or signaling with the network. In the context of Circuit Switched (CS) services, a UE is in active communication when it has a CS connection established, such as a voice call. For Packet Switched (PS) services, active communication is defined by the existence of one or more Activated PDP (Packet Data Protocol) contexts. A UE may have either CS or PS active communication, or both simultaneously. This concept is fundamental for understanding UE states and resource management in 2G and 3G networks.

The architecture supporting active communication involves the UE, the Radio Access Network (RAN), and the Core Network (CN). For CS services, the Mobile Switching Center (MSC) manages the connection, while for PS services, the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) handle PDP contexts. The UE transitions between idle and active states based on whether it has active communications. In idle mode, the UE has no active connections and conserves battery by monitoring paging channels. When a communication is initiated, the UE moves to an active state, and radio resources are allocated. The network tracks the UE's active communications to manage mobility, handovers, and resource allocation.

Key components include the UE, the Base Station (NodeB/BTS), the Radio Network Controller (RNC) or Base Station Controller (BSC), the MSC, SGSN, and GGSN. The UE's protocol stack (e.g., RRC, MM, SM) manages the establishment and release of connections. The RRC (Radio Resource Control) protocol handles radio resource allocation, while the MM (Mobility Management) and SM (Session Management) protocols manage CS and PS connections, respectively. The PDP context is a data structure that contains the UE's IP address and QoS parameters, and its activation signifies that the UE is ready to send and receive packet data.

In operation, when a UE initiates a CS call, it sends a request to the network, which establishes a CS connection through the MSC. The UE is then in active communication for CS. For PS services, the UE activates a PDP context by sending a request to the SGSN, which creates the context and establishes a connection to the GGSN. The UE can have multiple PDP contexts active simultaneously, each potentially with different QoS. The network uses the active communication state to decide when to page the UE, when to perform handovers, and how to allocate resources. When the communication ends, the connection is released, and the UE may return to idle mode if no other active communications exist.

The concept of Active Communication has evolved over releases. In Rel-20, the definition was updated to reflect the broader range of connections in modern networks, including PS connections such as PDP contexts and active PDN (Packet Data Network) connections. This change acknowledges the evolution from traditional CS/PS to all-IP networks, where connections are more diverse. The term remains relevant for understanding UE state management and is a prerequisite for many procedures in 3GPP specifications.

Purpose & Motivation

The Active Communication concept was created to provide a clear definition of when a UE is engaged in communication services, which is essential for resource management and mobility procedures. In early 2G networks, CS and PS domains were separate, and the network needed to know whether a UE was actively using services to manage radio resources and paging. The definition allowed the network to distinguish between idle UEs (which can be paged) and active UEs (which have dedicated resources). This distinction is critical for efficient use of spectrum and battery life.

The primary motivation was to enable the network to manage UE states and resources effectively. For CS services, an active communication means a dedicated channel is allocated, and the UE is consuming radio resources. For PS services, active communication via PDP contexts means the UE has an IP address and can send/receive data. The network can use this information to decide when to release resources, when to perform handovers, and how to handle mobility. It also helps in charging and policy control, as active communications are subject to billing and QoS enforcement.

Over time, the concept has evolved to accommodate new technologies. The introduction of always-on connectivity and multiple PDN connections in LTE and 5G required an update to the definition. The Rel-20 change reflects the shift towards all-IP networks where CS domain is phased out, and PS connections are the norm. The concept remains fundamental for understanding UE state transitions and is used in various 3GPP specifications, including 21.810, 21.905, 21.910, 22.261, and 22.861. It ensures consistent terminology across different generations of networks.