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Paging procedure in LTE

In Long-Term Evolution (LTE) cellular networks, paging is used to alert a specific user equipment (UE) that there is data waiting for it on the network. Paging is typically used when the UE is in an idle state, which means that it is not currently communicating with the network.

When the network has data to send to a particular UE, it will initiate a paging procedure by sending a paging message to all cells that the UE is registered with. The message includes the identity of the UE and the location of the cell where the UE is expected to be located.

The UE monitors the paging channel of the cell it is currently camped on and will respond to the paging message if it recognizes its identity. If the UE does not respond within a certain time frame, the network will assume that the UE is not available and will retry the paging procedure in other cells where the UE is registered.

The paging procedure is an important component of the LTE network’s signalling overhead, and efforts are made to minimize its impact on network resources. Techniques such as paging group optimization and paging based on location information can be used to reduce the number of cells that need to be paged, resulting in a more efficient use of network resources.

Why Paging is required?

  • Battery Life: When a UE is in an idle state, it periodically wakes up to check for any new data that may be waiting for it. This process consumes power and can drain the UE’s battery life. Paging reduces the need for the UE to continuously monitor the network by allowing the network to notify the UE only when data is waiting for it. This can help to extend the UE’s battery life.
  • Network Efficiency: Paging allows the network to reduce the amount of signalling required to deliver data to the UE. Instead of constantly communicating with the network, the UE only needs to be notified when data is available. This can help to reduce signalling overhead and improve network efficiency.
  • Network Coverage: Paging allows the network to deliver data to a UE even when it is not actively communicating with the network. This can help to improve network coverage and ensure that data can be delivered to the UE regardless of its current state.
  • User Experience: Paging helps to ensure that the UE receives important information or notifications in a timely manner. This can be critical for services such as emergency alerts, where delays can have serious consequences.
  • Network Congestion: When a large number of UEs are actively communicating with the network, paging can help to reduce network congestion by reducing the amount of signalling required to deliver data to idle UEs.
  • Roaming: When a UE roams between different cells or networks, paging allows the network to locate the UE and deliver data to it. This can be important for services such as voice calls, where the UE must be reachable at all times.
  • Security: Paging can be used as a security feature in LTE networks. For example, the network can page a UE to verify its presence in a specific location and prevent unauthorized access.

Paging Parameters in SIB2

The paging parameters are defined in System Information Block Type 2 (SIB2), which is broadcasted by the network periodically to all UEs in the coverage area. The following are the main paging parameters in SIB2:

  • Paging Cycle: The paging cycle defines the time interval between successive paging occasions. It is expressed in terms of a number of radio frames, with values ranging from 32 to 4096 frames. The paging cycle determines how often the network checks for new data for a particular UE and initiates a paging procedure.
  • Paging Frame: The paging frame specifies the radio frame within a paging cycle on which the network sends a paging message. The paging frame can be set to any value from 0 to 1023, and different UEs can have different paging frames to avoid collisions.
  • Paging Subframe: The paging subframe specifies the subframe within a paging frame on which the network sends a paging message. It can be set to one of four possible values: 0, 2, 4, or 6. The choice of paging subframe depends on the type of paging message and other network-specific factors.
  • Paging Group ID: The paging group ID is used to identify a group of UEs that share the same paging cycle, paging frame, and paging subframe. Paging group IDs are used to reduce the amount of paging overhead by minimizing the number of UEs that need to be paged.
  • Paging DRX (Discontinuous Reception): The paging DRX parameter specifies how often the UE should wake up to check for paging messages. It is expressed in terms of a number of radio frames, with values ranging from 32 to 5120 frames. Longer DRX cycles can help to reduce power consumption but may also result in longer delays in receiving paging messages.

Paging initiated by entities

In LTE networks, paging can be initiated by the UE or the eNodeB, depending on the situation. UE-initiated paging occurs when the UE needs to send data or establish a connection with the network, while eNodeB-initiated paging occurs when the network needs to deliver data to an idle UE.

  • eNodeB Initiated Paging
  • MME Initiated Paging

reference picture – http://www.techtrained.com

eNodeB Initiated Paging

eNodeB-initiated paging is used when the network has data waiting for an idle UE that has not been in contact with the network for a specified period of time. This is known as the idle mode signaling reduction (ISR) period, which is typically set to several hours or more. If the UE remains idle beyond this period, the eNodeB will initiate a paging procedure to deliver the data to the UE.

  • The eNodeB-initiated paging procedure consists of the following steps:
  • The eNodeB sends a paging message over the paging channel (PCH) to notify the UE that data is waiting for it.
  • The UE wakes up and monitors the PCH for a paging message during the paging occasion specified in SIB2. If the UE receives a paging message, it responds to the eNodeB with a random-access preamble to establish a connection.
  • Once the connection is established, the eNodeB delivers the data to the UE and the UE can proceed with the requested service.

The eNodeB-initiated paging procedure is designed to minimize the impact on network resources and reduce signaling overhead by only paging idle UEs when necessary. The paging parameters in SIB2 play a critical role in optimizing the performance of the paging procedure and ensuring that UEs receive paging messages in a timely manner.

MME Initiated Paging

In addition to eNodeB-initiated paging, LTE networks also support MME (Mobility Management Entity)-initiated paging. MME-initiated paging is used when the network needs to deliver data to a UE that is not currently registered with an eNodeB or is in an idle state that is beyond the scope of the eNodeB-initiated paging procedure.

The MME-initiated paging procedure consists of the following steps:

  • The MME sends a paging request message to all eNodeBs that may be serving the UE, requesting them to initiate a paging procedure.
  • The eNodeBs transmit the paging message to the UE over the PCH, similar to the eNodeB-initiated paging procedure.
  • The UE responds to the paging message by establishing a connection with the network and receiving the requested data.

MME-initiated paging is used in situations where the UE is not registered with an eNodeB or has moved to a new location outside the scope of the eNodeB’s coverage area. This can occur when the UE is in a low-power state or has been turned off for an extended period of time.

The MME-initiated paging procedure is more resource-intensive than eNodeB-initiated paging, as it requires the MME to send a paging request to all eNodeBs that may be serving the UE. However, it is necessary in situations where eNodeB-initiated paging is not possible, such as when the UE is not registered with an eNodeB or has moved out of the eNodeB’s coverage area.

Paging Cycle

  • The paging cycle defines the interval between consecutive paging occasions, and it can range from 16 to 2560 radio frames, with the default value being 256 radio frames (i.e., 5.12 seconds). The value of the paging cycle is chosen based on the trade-off between the paging latency and the paging overhead. Commonly used value for Paging cycles is 128. It means 128 radio frames. (128 radio frames=1280 msec=1.28 seconds)
  • It means UE will wake up after every 1.28 seconds even in Idle Mode to see if there is Paging information for the UE or not.

Paging Frame and Paging Occasion

In LTE networks, paging frames and paging occasions are used to optimize the paging procedure and reduce the impact on network resources.

A paging frame is a radio frame in which the UE (User Equipment) monitors the paging channel (PCH) for paging messages. The paging frame is specified in the System Information Block Type 2 (SIB2) and is typically set to a value that aligns with the radio frame boundary of the UE’s cell.

A paging occasion is a specific subframe within a paging frame in which the network searches for an idle UE to deliver data to. Instead, it wakes up in a specific subframe either subframe 0, 4, 5 or 9 within a radio frame. These specific subframes within a Paging Frame when UE wakes up are called as Pagi.ng Occasions (POs).

The paging occasion is determined by the combination of the paging cycle and the radio frame number (RFN) of the cell. The paging occasion is used to minimize the signalling overhead by limiting the number of subframes that the network searches for idle UEs.

The paging cycle determines the interval between consecutive paging occasions, as I explained in my previous answer. The RFN of the cell is a counter that increments with every radio frame, and it is used to determine the subframes that correspond to the paging occasion.

picture reference – http://www.techtrained.com

The paging occasion is calculated using the following formula:

SFN mod T = (T/N) x (UE_ID mod N)

Where,

T = DRX cycle length in radio frames

N = Min(T,nB)

nB is broadcast within SIB2 and can have values of {4T,2T,T,T/2,T/4,T/8,T/16,T/32} N can have values of {T,T/2,T/4,T/8,T/16,T/32}

UE_ID = IMSI mod 1024

The formula to compute paging occasion is extracted from a look-up table which is indexed using:

Ns = Max (1, nB/T )

i_s = Floor(UE_ID / N) mod Ns

PAGING message

What is T3413?

T3413 is a supervision timer for the paging procedure.

T3413 is a timer used in LTE networks that determines the duration for which a UE (User Equipment) stays in an idle state before it reattaches to the network. This timer is used by the MME (Mobility Management Entity) to release resources associated with the UE when it is in an idle state for an extended period.

The T3413 timer is started when the UE enters the idle state, either after it has completed an RRC (Radio Resource Control) connection or when it is switched on. The value of the timer is specified by the network and can vary depending on the specific network configuration. The default value for T3413 is 10 hours.

If the UE receives a paging message during the T3413 timer duration, the timer is reset and the UE remains in an idle state. However, if the T3413 timer expires without any paging message being received, the UE initiates a location update procedure and reattaches to the network.

The T3413 timer is important in ensuring efficient use of network resources by releasing resources associated with idle UEs. It also ensures that UEs reattach to the network in a timely manner and remain reachable by the network. The specific value of the T3413 timer is determined by the network operator and can be adjusted based on factors such as network traffic and user behaviour.

how paging takes place in LTE networks:

  • Paging is used by the MME to initiate a connection setup procedure for a specific UE that is not reachable at the time.
  • The MME sends a Paging Message to all eNodeBs in the TA (Tracking Area) or equivalent location area to which the UE is registered.
  • The Paging Message contains one or more Paging Records, each of which specifies the TAI (Tracking Area Identity) or equivalent location area, the paging identity, the CN (Core Network) domain, the Paging DRX (Discontinuous Reception) cycle, and the UE identity of the target UE.
  • The eNodeBs in the TA or equivalent location area receive the Paging Message and forward it to all UEs that are registered in their cell and match the criteria specified in the Paging Records.
  • The UE monitors the PCH (Paging Channel) periodically according to its DRX cycle, and if it receives a paging message that matches its UE identity, it responds with a Service Request message.
  • The MME receives the Service Request message, initiates a connection setup procedure, and establishes a connection with the UE.
  • If the UE does not respond to the paging message within a certain time period, the MME considers the UE as unreachable and may take further action such as initiating a location update procedure or releasing the signalling connection.

Some of the common Paging Message IEs and their definitions are as follows:

  • Procedure Code: This IE indicates the type of procedure being initiated by the message. In the case of a Paging Message, the procedure code is “paging”.
  • Criticality: This IE indicates the level of importance of the IE. It can be set to “reject” if the IE is mandatory and missing or if the value is not acceptable.
  • PagingUEIdentity: This IE contains the UE Identity (UE ID) that the MME is paging. The UE ID is typically the TMSI (Temporary Mobile Subscriber Identity) or the IMSI (International Mobile Subscriber Identity) of the target UE.
  • PagingDRX: This IE specifies the DRX (Discontinuous Reception) cycle that the UE is using, which determines how often the UE monitors the Paging Channel (PCH) for paging messages.
  • CN Domain: This IE specifies the CN (Core Network) domain to which the paging message applies, which can be either “PS” (Packet Switched) or “CS” (Circuit Switched).
  • Paging Priority: This IE specifies the priority of the paging message, which can be used by the UE to determine whether to respond to the paging message or not.
  • Paging Record: This IE contains information about the paging message, such as the TAI (Tracking Area Identity) or equivalent location area to which the paging message applies, the PLMN (Public Land Mobile Network) identity, and the paging identity.

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