LTE Paging Procedure.

This article is about basic paging procedures of LTE network and how they are supported by MME.

Paging Procedure Overview.

The paging procedure in LTE networks is initiated by the Mobility Management Entity (MME) to establish a NAS (Non-Access Stratum) signaling connection with the User Equipment (UE). This process occurs when the MME receives a notification from the Serving Gateway (SGW) that data packets need to be delivered to the UE. The primary goal is to locate the UE and deliver the incoming data.

Paging Types:

The MME supports various types of paging, each tailored to different network scenarios and requirements:

  • Basic Paging: This is the default paging type used unless other specific types are provisioned.
  • SGs_CS and SGs_PS Paging: Applied to UEs connected in UMTS circuit-switched or packet-switched networks.
  • QCI-Based Paging (QCI_1 through QCI_9): Allows the MME to select different paging strategies based on the Quality of Service Class Identifier (QCI). This helps differentiate between best-effort and dedicated bearer calls, ensuring appropriate paging for each service type.

Paging Methods and Strategies:

To optimize the paging process, the MME can use different methods and strategies, which are provisioned by the service provider. These methods consider the UE’s last known location to enhance efficiency:

  1. Last Seen eNodeB: Pages the eNodeB that last sent a Tracking Area Update (TAU) Request or Service Request.
  2. Last Seen Tracking Area (TA): Pages all eNodeBs in the TA associated with the last TAU Request or Service Request.
  3. Last Seen Tracking Area plus Neighboring Tracking Areas: Pages all eNodeBs in the registered Tracking Area Identity (TAI) list, which includes the last seen TA and its neighboring TAs.

Procedure Execution:

The paging procedure starts with the MME sending paging messages to the set of eNodeBs based on the current paging method and the UE’s last known location. When the UE receives the paging message, it initiates the Network Triggered Service Request Procedure to establish communication and deliver the data packets.

Configuration and Attributes:

The service provider can configure various attributes related to the paging procedure, including the type of paging, paging method, related timers, and the number of attempts. These configurations are essential for managing the network resources effectively and ensuring that UEs can be promptly reached when needed.

Paging last seen eNodeB.

The Mobility Management Entity (MME) uses the S-Temporary Mobile Subscriber Identity (S-TMSI) to page the User Equipment (UE). The S-TMSI is a shortened version of the Globally Unique Temporary Identifier (GUTI), constructed from the MME code and the M-TMSI. This temporary identity ensures efficient and secure identification of the UE during paging.

Paging last seen eNodeB. LTE Paging Procedure.

Paging Strategies and Flexibility:

The MME can provision and enforce various paging strategies to optimize network performance. These strategies can be applied to successive paging attempts (up to four attempts) or consistently across all attempts. The illustrations on the following sections demonstrate the MME’s flexibility in this regard.

  1. Effectiveness of Paging:
    • Reducing Failed Connection Attempts: The aim is to minimize the number of failed connections resulting from paging failures.
    • Minimizing Paging Attempts: Ensuring that the UE responds with the fewest possible paging attempts enhances efficiency and user experience.
  2. Efficiency of Paging:
    • Minimizing Paging Messages: Reducing the number of paging messages sent for each UE decreases forward link (FL) traffic, helping to avoid overloads on the downlink.
    • Avoiding Downlink Overloads: Effective paging strategies are crucial to prevent congestion on the downlink, ensuring smooth network operations.
  3. Efficiency of UE Location Updates:
    • Reducing Tracking Area Update (TAU) Requests: Minimizing the number of TAU requests generated by each UE reduces reverse link (RL) traffic and helps to avoid uplink overloads.
    • Handling Uplink Overloads: A comprehensive strategy is needed to manage uplink overloads effectively when they occur.

Illustrative Strategies:

The MME can employ different paging methods based on the UE’s last known location and the success of previous paging attempts. These methods include:

  • Last Seen eNodeB: Pages the eNodeB that last sent a Tracking Area Update (TAU) Request or Service Request. This method targets the most recently known location of the UE, increasing the likelihood of a successful connection.
  • Last Seen Tracking Area (TA): Pages all eNodeBs in the TA associated with the last TAU Request or Service Request, broadening the search area for the UE.
  • Last Seen Tracking Area Plus Neighboring TAs: Pages eNodeBs in the registered Tracking Area Identity (TAI) list, including the last seen TA and its neighboring TAs, further extending the search scope.

Paging last seen TA.

If Last Seen eNB = 107 and Last Seen TA = 2 and Method = ‘Last Seen TA’
Resulting paging list is:
eNB 104 – TA 2
eNB 107 – TA 2
eNB 109 – TA 2

Paging last seen TA.

Paging last seen TA + neighbors.

If Last Seen eNB == 107 and Last Seen TA = 2 and Method = ‘LSTA+N’.

Resulting paging list is:
eNB 101 – TA 1
eNB 102 – TA 1
eNB 103 – TA 1
eNB 104 – TA 2
eNB 105 – TA 1
eNB 107 – TA 1, 2, 4
eNB 108 – TA 1, 4
eNB 109 – TA 2
eNB 110 – TA 4
eNB 112 – TA 4
eNB 113 – TA 4
eNB 114 – TA 4.

NOTE: Assume TA 2 is defined to have neighbors TA 1 & TA 4.

Basic Paging Procedure.

Basic Paging Procedure.

Tasks Performed by MME.

  1. Initial Paging Message: The MME initiates the paging procedure by sending an initial Paging message using the S-Temporary Mobile Subscriber Identity (S-TMSI).
  2. Paging Timer and Attempts: If the Paging timer expires without a response, the MME increments the number of attempts and uses the Paging Attempt Count to select the method for the next page.
  3. Service Request from UE: Upon receiving the Page, the UE responds with a Service Request.
  4. No Response Handling: If the UE does not respond after the provisioned number of attempts, the MME sends a Downlink Data Notification Failure to the Serving Gateway (SGW).

Paging Procedure as Part of Network Triggered Service Request:

When a packet arrives for an idle UE, the following steps are executed:

  1. Packet Arrival at SGW: A packet destined for the UE arrives at the SGW.
  2. Notification to MME: The SGW notifies the MME that a packet has arrived for an idle UE and queues the packet.
  3. Initial Paging Request: The MME sends an initial paging request message to each eNodeB within the UE’s registered tracking area(s). This can involve a large number of eNodeBs.
  4. Page Broadcasting by eNodeBs: Each eNodeB broadcasts the paging message. The UE will respond to one of these pages.
  5. Service Request from UE: The UE requests service from the MME and establishes a radio bearer with the eNodeB.
  6. Bearer Path Signaling: The MME signals the bearer path to the SGW.
  7. Packet Forwarding to UE: The SGW forwards the packet to the UE.

Paging Timers and Policies:

  • Wait for Page Response Timer: The MME sets this timer based on the value provisioned within the Paging Policy.
  • Paging Request Count: The MME maintains a current page request count for each Network Triggered Service Request procedure. This count is used to determine the current paging method and timer value for the ongoing page attempt. There can be up to four attempts.

Separate Strategies for Paging: Separate paging strategies can be provisioned for basic paging and circuit-switched paging over the SGs interface. Additionally, paging can be limited during UE load balancing tasks to manage network resources effectively.

S-Temporary Mobile Subscriber Identity (S-TMSI): The S-TMSI is used for paging the mobile subscriber. It is constructed from the MME code and the M-TMSI and serves as a shortened version of the Globally Unique Temporary Identifier (GUTI). This identity helps maintain privacy and security while efficiently managing paging procedures.

Paging GAP Feature in LTE Networks.

The Paging GAP feature is designed to suppress excessive paging, which can negatively impact system performance and capacity. Excessive paging occurs when the MME continuously attempts to reach UEs that fail to respond to pages and have lost contact with the LTE network.

  • MME Paging GAP Feature: This feature allows operators to manage and reduce excessive paging attempts, preventing negative effects on the system’s performance.
  • Paging Suppression: The MME provides the capability to suppress new paging requests for UEs that do not respond after multiple attempts. This is controlled through the Paging Gap Timer.

How It Works

Counting ‘No Page Response’ Failures:

    • The MME tracks the number of consecutive ‘no page response’ failures for each UE.
    • This count increments each time all provisioned page attempts have timed out without reestablishing an S1 connection between the UE and its serving eNodeB.

    Suppression Trigger:

      • When the count of ‘no page response’ failures for a UE reaches three, the MME begins to suppress new paging requests for that UE.
      • Suppression continues for a duration specified by the Paging Gap Timer.

      Paging Gap Timer:

        • The Paging Gap functionality is controlled by setting a Paging Gap Timer parameter on the Timers page at the 5620 SAM.
        • After the Paging Gap Timer interval elapses, the MME resumes paging the UE upon receiving new page requests from the SGW or MSC.

        Repeated Failures and Suppression:

          • If another paging cycle results in a ‘no page response,’ the suppression of paging requests starts again.
          • This cycle continues, with the suppression duration reset according to the Paging Gap Timer after each paging failure.

          Clearing Suppression:

            • The MME clears the ‘no page response’ count and processes new page requests whenever the UE reestablishes an S1 connection with its serving eNodeB.
            • The MME updates the UE’s current location and resumes normal paging operations.

            Benefits

            • System Performance Improvement: By reducing the number of futile paging attempts, the Paging GAP feature helps maintain optimal system performance.
            • Resource Efficiency: It ensures efficient use of network resources, avoiding unnecessary load on the downlink and uplink channels.
            • Enhanced UE Management: It provides a systematic approach to handling UEs that are temporarily out of network reach, improving overall network reliability.

            The Paging GAP feature is a crucial mechanism in LTE networks for managing paging attempts, maintaining system performance, and ensuring efficient use of network resources.

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