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5G System Connection Management: CM-Idle and CM-Connected

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The 5G system defines two primary states for a user equipment (UE) in terms of its connection status: Connected Mode (CM) and Idle Mode. Let's delve into each of these states and their underlying technical details.

1. CM-Idle:

Idle State:

  • When a UE is not actively communicating or exchanging data with the 5G core network (5GC), it is considered to be in the idle state.
  • In this state, the UE is registered with the network but is not attached to a specific session or data path.

Tracking Area:

  • In 5G, the concept of Tracking Areas (TA) is introduced, replacing the Location Areas (LA) from earlier generations like 4G/LTE.
  • A TA is a group of cells, and the UE in idle mode tracks its location based on the TA. The network keeps track of which TAs a UE is camping on.

Connection Management in CM-Idle:

  • Tracking Area Update (TAU): When a UE moves to a new TA, it performs a TAU to update its location information with the 5GC. This ensures that the network knows where to route incoming data or calls.
  • Paging: When there's incoming data or a call for an idle UE, the network initiates a paging procedure to locate and notify the UE.
  • Network Attachment: Even in idle mode, the UE remains attached to the network and periodically sends signals to indicate its presence.

2. CM-Connected:

Connected State:

  • When a UE establishes a data or voice session with the 5GC, it enters the connected state. This state involves active data exchange and communication with the network.

Connection Management in CM-Connected:

  • Initial Registration: When the UE powers on or moves to a new area without a valid registration, it initiates an initial registration procedure to establish a connection with the 5GC.
  • Session Management: Once connected, the UE can set up, modify, or terminate various types of sessions (e.g., data, voice). Session management procedures ensure that the UE and network synchronize their states and configurations.
  • Mobility Management: In connected mode, as the UE moves across different cells or even different Radio Access Technologies (RATs) within 5G (like NR-NR, or LTE-NR handovers), mobility management procedures ensure seamless handovers, maintaining the ongoing sessions.
  • QoS (Quality of Service) Management: The 5G system allows for dynamic QoS adjustments based on network conditions, application requirements, and user needs. The UE and network communicate to ensure that the required QoS parameters are met.

Data Transfer:

  • In connected mode, data transfer occurs using dedicated bearers established between the UE and the 5GC. These bearers can have different characteristics based on the service requirements (e.g., latency-sensitive, bandwidth-intensive).

Conclusion:

The 5G system employs CM-Idle and CM-Connected states to manage the connection status of UEs effectively. While CM-Idle focuses on efficient location tracking and minimal signaling to conserve battery and network resources, CM-Connected facilitates active data and voice communication, ensuring optimal performance and user experience. The transition between these states involves various procedures and mechanisms to maintain connectivity, manage sessions, and adapt to changing network conditions.