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Paging

Overview​

Paging is a functionality which allows the mobile network to find devices without an active connection (typically in a power saving mode) so that data can be transferred to such devices. Some typical use cases of paging are: Mobile Teminated (MT) calls, SMS delivery and emergency alerts.

Note that most Internet traffic is started by mobile devices themselves and when the traffic starts from the device there is no paging. Paging is needed when the traffic comes downstream (from a server) to the device, and the exact location of the device is not known to the network (location in this context typically refers to a serving cell). Paging is used depending on the device state, as perceived by the network. In the case of 5G actually, two different connection states are relevant:

  • RRC state (RAN) – Paging is needed to transfer data to the mobile in two states: RRC_IDLE and RRC_INACTIVE. Paging can be skipped in RRC_CONNECTED for user traffic
  • CM state (Core) – paging is needed in CM_IDLE the network CM_CONNECTED state the connection to the 5G core is maintained

Classically, paging comes from the core network, where some activity (such as an incoming call) is started towards the mobile and there is no active connection between the mobile and core, i.e. the device state is CM_IDLE.

A quite special functionality of 5G compared to previous generations is that the CM_CONNECTED state can be maintained while the device releases the RRC connection and enters the RRC_INACTIVE state. This introduces the concept of RAN-initiated paging

In RRC_CONNECTED paging can still be used for two other use cases: informing devices of System Information updates and informing devices about emergency situations.

Paging on specifications:​

  • TS 38.300: NR and NG-RAN Overall description
    • section 9.2.5 - General description of paging (from RAN perspective)
    • 9.2.2.4 - Overview of network triggered transition from RRC_INACTIVE to RRC_CONNECTED
    • Minor descriptions of paging related concepts throughout the text
  • TS 23.501: System architecture for the 5G System (5GS)
    • General descriptions of paging (throughout the spec)
    • Section 5.4.3 - Paging Strategy Handling
  • TS 23.502: Procedures for the 5G System (5GS)
    • Section 4.2.3.3 Paging is in essence described as extra steps needed to accomplish the "Network Triggered Service Request" when the device is in CM_IDLE
  • TS 38.304: User Equipment (UE) procedures in Idle mode and in RRC Inactive state
    • section 7 - Description of Idle Mode DRX for Paging monitoring, including the formulas to determine paging frames and paging occasions
    • Minor descriptions how paging relate to camping ( a camping UE will normally monitor paging) and how paging can be used to receive system information notifications
  • TS 38.321: Medium Access Control (MAC) protocol specification
    • 5.5 - PCH reception
    • Table 7.1.2 Definition of P-RNTI / PCH / PCCH
  • TS 38.331: Radio Resource Control (RRC) protocol specification
    • section 5.3.2 - Description of RRC procedure when the UE receives a paging message
    • 6.2.2 - Paging Message definition
    • DownlinkConfigCommonSIB::PCCH-Config - Main paging parameters (broadcasted in SIB-1)
  • TS 24.501:Non-Access-Stratum (NAS) protocol for 5G System (5GS)
    • 5.6.2 Short description of paging from NAS signaling perspective, including timers and expected behavior after paging.
  • TS 38.413: NG Application Protocol (NGAP)
    • 8.5 Paging procedures (from the perspective of AMF / gNB communication)
  • TS 38.423: Xn Application Protocol (XnAP)
    • 8.2.5 RAN Paging
  • DRX
  • Camping
  • RRC_IDLE
  • RRC_INACTIVE
  • Tracking Area
  • RAN notification Area