Thursday, 25 August 2016

Admission and congestion control

2 Overview

2.1 Introduction

During admission control, an eNodeB decides whether to accept a guaranteed bit rate (GBR) service request (either new service or handover) based on the physical resource block (PRB) usage and the quality of service (QoS) satisfaction rate of GBR services. During congestion control, the eNodeB controls system load to ensure a high overall QoS satisfaction rate and stable system operation.

2.2 Benefits

Load control maintains system stability by rejecting certain admission requests or releasing some admitted services, while maximizing resource usage. Load control ensures the quality of admitted services by controlling the cell load. It also provides the required QoS for individual radio bearers and maximizes cell capacity.

3 Admission Control

This chapter describes the basic feature TDLBFD-002023 Admission Control.
Admission control is categorized into radio-resource-based admission control and transport-resource-based admission control. This document describes radio-resource-based admission control. For details about transport-resource-based admission control, see Transport Resource Management Feature Parameter Description.
Radio-resource-based admission control is categorized into user admission and service admission.
  • When a piece of user equipment (UE) requests access to a network, that is, when a UE requests setup of a radio resource control (RRC) connection or requests an incoming handover, the E-UTRAN NodeB (eNodeB) performs the user admission procedure.
  • When a UE that has accessed a cell requests a new service, that is, when a UE requests establishment of an E-UTRAN radio access bearer (E-RAB) or requests an incoming handover, the eNodeB performs the service admission procedure.

3.1 User Admission

When a UE requests to access a network, that is, when a UE requests setup of an RRC connection or requests an incoming handover, the eNodeB performs the user admission procedure. Figure 3-1 shows the user admission procedure.
Figure 3-1 User admission procedure
CPU: central processing unit
PUCCH: physical uplink control channel
LBBP: LTE baseband process unit
SRS: sounding reference signal
The user admission procedure is as follows:
  1. The eNodeB checks whether the CPU resource is limited. If the CPU resource is limited, the eNodeB rejects the access request.
  2. The eNodeB checks whether the SRS resource is successfully allocated. The eNodeB performs this step differently based on whether LBBPc or LBBPd is configured. For details about the SRS resource allocation principle, see Physical Channel Resource Management Feature Parameter Description.
  • If the LBBPc is configured, the eNodeB performs this step as follows:
    • If the SRS resource is configured for the cell, the cell supports uplink timing measurement using only the SRS. Therefore, if the SRS resource allocation fails, the uplink timing of the UE may not be accurate and the eNodeB rejects the access request.
    • If the SRS resource is not configured for the cell, the cell supports uplink timing measurement using the demodulation reference signal (DMRS). Even if the SRS resource is not allocated, the eNodeB accepts the access request.
  • The LBBPd supports uplink timing measurement using the DMRS. Even if the SRS resource allocation fails, the uplink timing of the UE is accurate and the eNodeB does not reject the access request.
  1. The eNodeB checks whether the PUCCH resource is successfully allocated. If the PUCCH resource allocation fails, the eNodeB rejects the access request. For details about the PUCCH resource allocation principle, see Physical Channel Resource Management Feature Parameter Description.
  2. The eNodeB checks whether the number of users is limited. If the number of admitted users has reached the maximum or the licensed user number, the eNodeB rejects the access request.
  3. For the user that is admitted by the eNodeB, if the UE requests a service (that is, to establish an E-RAB), the eNodeB performs the service admission procedure. For details, see 3.2 Service Admission.
  4. If the UE's access request is rejected by the eNodeB, the UE can initiate a user preemption procedure. For details, see 3.5.1 User Preemption.
NOTE:
To ensure that emergency-call UEs can preferentially access the network, the eNodeB does not reject the access requests of emergency-call UEs when the number of admitted users has reached the licensed user number. However, the eNodeB rejects the access requests of emergency-call UEs when the number of admitted users has reached the maximum number.

3.2 Service Admission

When a UE that has accessed a cell requests setup of a new service, that is, when a UE requests to establish an E-RAB or requests a handover to the cell, the eNodeB performs the service admission procedure. Figure 3-2 shows the service admission procedure.
Figure 3-2 Service admission procedure

3.3 Non-GBR Service Admission

For a non-GBR service, the admission procedure is as follows:
  • If the SRS or PUCCH resource allocation fails, the eNodeB rejects the access request.
  • If both the SRS resource allocation and the PUCCH resource allocation are successful, the eNodeB accepts the request. In the admission procedure, the eNodeB does not check the UE capabilities or whether the cell is congested, or make the admission decision based on the QoS satisfaction rates.
NOTE:
SRS resource allocation is required only if the SRS resource is configured for the LBBPc.

3.4 GBR Service Admission

For a GBR service, its QoS has to be guaranteed. The admission procedure is as follows:
  • If the SRS or PUCCH resource allocation fails, the eNodeB rejects the access request.
  • If both the SRS resource allocation and the PUCCH resource allocation are successful, the eNodeB makes the admission decision as follows:
    • For an emergency call, the eNodeB accepts the request.
    • For a non-emergency call, the eNodeB checks the UE capabilities and whether the cell is congested, and then makes the decision based on the QoS satisfaction rates.
NOTE:
SRS resource allocation is required only if the SRS resource is configured for the LBBPc.

3.4.1 UE Capability Evaluation

The UE reports its capabilities to the EPC each time the UE attaches to the network. When the UE needs to update its capabilities, it performs detach and then attach procedures to report the new capabilities to the EPC. If the eNodeB does not know the UE capabilities, it initiates a UE capability transfer procedure to request the UE to report its capabilities to the eNodeB and the EPC. For details, see section 5.6.3 in 3GPP TS 36.331 V10.6.0 (2012-06).
When the UE initiates a new service request, the Mobility Management Entity (MME) sends an INITIAL CONTEXT SETUP REQUEST message to the eNodeB over the S1 interface, indicating the UE capabilities and bearer parameters. For details about UE capabilities, see section 4.1 in 3GPP TS 36.306 V10.6.0 (2012-06). The eNodeB stores the information and checks whether the UE supports the specified bearer If the UE does not support the bearer parameters, the eNodeB rejects the access request. The major concern about the UE capabilities is whether the total rate of various services exceeds the maximum UE capability, indicated by the maximum transport block size for the UE. For details, see section 8.3.1 in 3GPP TS 36.413 V10.6.0 (2012-06).

3.4.2 Cell Congestion Indication

Cell congestion indications are provided by congestion control to indicate that the Uu interface resources are insufficient. There are four types of indications:
  • Uplink Congested and Downlink Not Congested
  • Downlink Congested and Uplink Not Congested
  • Uplink and Downlink Congested
  • Uplink and Downlink Congestion Cleared
If a GBR service access request arrives, the eNodeB rejects the access request (either new services or handovers) regardless of whether congestion occurs in the downlink or uplink.

3.4.3 Admission Based on QoS Satisfaction Rates

The QoS satisfaction rate of GBR services in a cell reflects the QoS conditions of admitted GBR services in the cell. It is used for evaluating cell load and making admission decisions.
In the downlink, radio resources such as physical resource blocks (PRBs) and power are shared by all UEs in a cell. Decreases in the downlink QoS satisfaction rates indicate limited radio resources. The eNodeB performs downlink admission control based only on QoS satisfaction rates.
In the uplink, radio resources may not be fully used because of a limitation on uplink power. The eNodeB performs uplink admission control based on the PRB usage, wait time for uplink scheduling of voice over IP (VoIP) services, and QoS satisfaction rates.
Figure 3-3 and Figure 3-4 show the downlink and uplink admission procedures for GBR services based on QoS satisfaction rates, respectively.
Figure 3-3 Downlink admission procedure for GBR services
Figure 3-4 Uplink admission procedure for GBR services

PRB Usage Monitoring

By monitoring the PRB usage of services with high scheduling priorities in the uplink, the eNodeB learns about the usage of uplink time-frequency resources in the cell and preliminarily evaluates the cell load. In the uplink, services with high scheduling priorities include GBR services and the signaling and services whose scheduling priorities are higher than GBR services, such as retransmissions, signaling radio bearers (SRBs), and scheduling requests (SRs). The Media Access Control (MAC) layer calculates the PRB usage using the following formula:
After filtering, the eNodeB compares the calculated PRB usage with the uplink PRB usage thresholds CellRacThd.UlRbHighThd and CellRacThd.UlRbLowThd and then determines the PRB usage status. Using both the upper and lower thresholds prevents a possible ping-pong effect.

Evaluation of the Wait Time for Uplink Scheduling of VoIP Services

The wait time starts when a VoIP service requests uplink resources and stops when the VoIP service is allocated uplink resources. eNodeBs regard wait time longer than 100 ms to be too long.

Evaluation of QoS Satisfaction Rates

The QoS class identifier (QCI) of a service reflects the QoS requirements of that service, and each QCI corresponds to a unique set of QoS parameters. For details, see section 6.1.7 in 3GPP TS 23.203 V10.7.0 (2012-06). This protocol describes the characteristics of different services and the standards for evaluating their QoS satisfaction rates in the LTE QoS mechanism.
In the downlink, QoS satisfaction rates are evaluated based on the logical channels corresponding to the QCIs. In the uplink, however, QoS satisfaction rates are evaluated based on logical channel groups. A logical channel group is defined as a group of radio bearers with similar QoS requirements. For details about logical channel groups, see section 5.4.5 in 3GPP TS 36.321 V10.5.0 (2012-03). The reason for the difference between the uplink and the downlink is that the eNodeB cannot estimate the amount of buffered data to be transmitted through each logical channel on the UE side. The evaluation method for the uplink helps reduce signaling load and the number of Buffer Status Reports (BSRs).
The uplink and downlink QoS satisfaction rates of GBR services are calculated per QCI, and admission thresholds are also set per QCI to achieve admission differentiation between QCIs.

Evaluation of Downlink QoS Satisfaction Rates

Services with QCI 1 are VoIP services. Their downlink QoS satisfaction rate is represented by the average downlink satisfaction rate of VoIP services in a cell, that is, the ratio of the sum of the downlink satisfaction rates of all VoIP services in a cell to the total number of VoIP services in a cell.
Services with QCIs 2 to 4 are GBR services. Their downlink QoS satisfaction rate is represented by the ratio of the scheduled data volume of these services to the total data volume of these services to be transmitted.
NOTE:
Section 6.1.7 in 3GPP TS 23.203 V11.6.0 (2012-06) provides an example mapping between QCIs and service types but does not provide definitions of the mapping. Huawei implements the mapping according to the examples, where QCI 1 represents VoIP services.

Evaluation of Uplink QoS Satisfaction Rates

Services with QCI 1 are VoIP services. Their uplink QoS satisfaction rate is represented by the average uplink satisfaction rate of VoIP services in a cell, that is, the ratio of the sum of the uplink satisfaction rates of all VoIP services in a cell to the total number of VoIP services in a cell.
Services with QCIs 2 to 4 are GBR services. Their uplink QoS satisfaction rate is evaluated for each logical channel group. The QoS satisfaction rate is represented by the ratio of the transmitted data and the total amount of data to be transmitted. A higher ratio indicates a higher QoS satisfaction rate.
QCIs 1 to 4 can be mapped to logical channel groups. For details about the mapping, see Scheduling Feature Parameter Description. If QCIs 2 to 4 are mapped to logical channel group 2, the uplink QoS satisfaction rates for services with QCIs 2 to 4 are represented by the satisfaction rate for logical channel group 2.

Admission Decision Based on QoS Satisfaction Rates

The admission threshold for handovers is lower than that for new services. The eNodeB defines four handover thresholds QcixHoThd (x = 1–4) for the QCIs. Based on the handover thresholds, service differentiation can be achieved by setting admission offsets for new gold-, silver-, and bronze-level services, based on the mapping between ARP values and service priorities. The admission offsets are NewGoldServiceOffset, NewSilverServiceOffset, and NewCopperServiceOffset. These offset values apply to both the uplink and downlink.
The eNodeB categorizes services based on their ARP values. The ARP values are mapped to three service priorities: gold, silver, and bronze. The mapping can be adjusted by setting GoldServiceArpThd (the ARP threshold for gold-level services) or SilverServiceArpThd (the ARP threshold for silver-level services). The ARP values beyond the ranges defined by these two parameters correspond to bronze-level services. A small ARP value indicates a high priority level. Table 3-1 shows a typical mapping between the ARP values and the service priorities.
Table 3-1 Typical mapping between the ARP values and the service priorities
ARP Value
Service Priority
1–5
Gold
6–10
Silver
11–15
Bronze
For details about ARPs, see section 4.7.3 in 3GPP TS 23.401 V10.8.0 (2012-06).
For GBR services whose QCIs range from 1 to 4, the admission thresholds for handovers and new services corresponding to a QCI are as follows:
  • The admission threshold for handovers is QcixHoThd.
  • The admission threshold for new gold-level services is QcixHoThd plus NewGoldServiceOffset.
  • The admission threshold for new silver-level services is QcixHoThd plus NewSilverServiceOffset.
  • The admission threshold for new bronze-level services is QcixHoThd plus NewCopperServiceOffset.
The relationship between these thresholds is as follows:
QcixHoThd QcixHoThd + NewGoldServiceOffset QcixHoThd + NewSilverServiceOffset QcixHoThd + NewCopperServiceOffset ≤ 100%
Assume that SQCI = x (x = 1–4) represents the QoS satisfaction rate for QCI x. The following rules apply to downlink admission evaluation:
  • For SQCI = xQcixHoThd + NewCopperServiceOffset
    If the QoS satisfaction rates of all QCIs except QCI x are higher than the corresponding handover admission thresholds, then the eNodeB accepts requests for new gold-, silver-, and bronze-level services and handovers with QCI x.
    If the QoS satisfaction rate of any QCI is lower than the corresponding handover admission threshold, then the eNodeB rejects requests for new gold-, silver-, and bronze-level services with QCI x but accepts requests for incoming handovers with QCI x.
  • For QcixHoThd + NewCopperServiceOffset > SQCI = x QcixHoThd + NewSilverServiceOffset
    If the QoS satisfaction rates of all QCIs except QCI x are higher than the corresponding handover admission thresholds, then the eNodeB accepts requests for new gold- and silver-level services and handovers with QCI x but rejects requests for new bronze-level services with QCI x.
    If the QoS satisfaction rate of any QCI is lower than the corresponding handover admission threshold, then the eNodeB rejects requests for new gold-, silver-, and bronze-level services with QCI x but accepts requests for incoming handovers with QCI x.
  • For QcixHoThd + NewSilverServiceOffset > SQCI = xQcixHoThd + NewGoldServiceOffset
    If the QoS satisfaction rates of all QCIs except QCI x are higher than the corresponding handover admission thresholds, then the eNodeB accepts requests for new gold-level services and handovers with QCI x but rejects requests for new silver- and bronze-level services with QCI x.
    If the QoS satisfaction rate of any QCI is lower than the corresponding handover admission threshold, then the eNodeB rejects requests for new gold-, silver-, and bronze-level services with QCI x but accepts requests for incoming handovers with QCI x.
  • For QcixHoThd + NewGoldServiceOffset > SQCI = xQcixHoThd
    The eNodeB rejects requests for new gold-, silver-, bronze-level services with QCI x but accepts requests for handovers with QCI x.
  • For QcixHoThd > SQCI = x
    The eNodeB rejects requests for new services and handovers with QCI x.
The rules for uplink admission evaluation are similar to the rules for downlink admission evaluation. The difference is that the eNodeB evaluates the QoS satisfaction rates of logical channel groups when admitting GBR services in the uplink. For details, see Evaluation of Uplink QoS Satisfaction Rates

3.5 Preemption

This section describes radio resource preemption in the optional feature TDLOFD-00102901 Radio/transport Resource Pre-emption. For details about transport resource preemption, see Transport Resource Management Feature Parameter Description.

3.5.1 User Preemption

When an emergency-call UE accesses the network, it preempts the resources of non-emergency-call UEs with low ARPs if the SRS or PUCCH resource allocation fails.
When a UE with a high ARP or an emergency-call UE requests access to the network, the eNodeB rejects the access request because the number of users is limited. If the IntraOpUeNumPreemptSwitch(IntraOpUeNumPreemptSwitch) option of the UeNumPreemptSwitch parameter is selected, this UE preempts the resources of non-emergency-call UEs with low ARPs.
NOTE:
In the user-number-based preemption, the eNodeB establishes a temporary RRC connection for any UE requesting access to the eNodeB in order to obtain the UE's ARP attributes.
In the user-number-based preemption, the resources for the following types of low-ARP UEs are preempted in descending order of priority:
  • Low-ARP UEs in the out-of-synchronization state performing non-GBR services
  • Low-ARP UEs in the out-of-synchronization state performing GBR services
  • Low-ARP UEs in the synchronized state performing non-GBR services
  • Low-ARP UEs in the synchronized state performing GBR services
If the preemption succeeds and redirection is enabled, the eNodeB performs a redirection for the UE whose resource is preempted. If the preemption request is rejected and redirection is enabled, the eNodeB performs a redirection for the UE that requests a preemption. For details about redirection procedure, see 3.6 Redirection.

3.5.2 Service Preemption

Introduction

If no PUCCH resource is allocated to a newly-admitted emergency call, the emergency call preempts non-emergency calls with low APR priorities.
When a GBR service with a high ARP requests service preemption, it preempts GBR services with low ARPs if the request is rejected because the cell is congested or the admission based on the QoS satisfaction rates fails. Service preemption is triggered if one of the conditions shown in Figure 3-3 or Figure 3-4 is met.
Service preemption is enabled by selecting the PreemptionSwitch(PreemptionSwitch) check box under the RacAlgoSwitch parameter. Note that SRBs, IP multimedia subsystem (IMS) signaling, and emergency calls cannot be preempted.
A service can preempt other services only if its ARP information element (IE) Pre-emption Capability is "may trigger pre-emption". If a service is not capable of preemption, it cannot be admitted. If service A is capable of preemption, it can preempt a service B that meets all of the following conditions:
  • Service B belongs to the same service type (GBR or non-GBR) as service A.
  • The value of the ARP IE Pre-emption Vulnerability of service B is "pre-emptable".
  • The value of the ARP IE Priority Level of service B is greater than that of service A.
  • If service B is a GBR service, the resources allocated to it are greater than or equal to the resources required by GBR service A.
Preemption fails if any one of the preceding conditions is not met. When a new service fails to preempt other services during initial access, the UE is redirected if redirection is enabled. For details about redirection, see 3.6 Redirection.
If a handover service fails to preempt other services, relevant operations are performed based on the actual situation. For details about operations, see Mobility Management in Connected Mode Feature Parameter Description.
NOTE:
Services whose ARP is 15 can neither preempt other services nor be preempted. For details, see section 9.2.1.60 in 3GPP TS 36.413 V10.6.0 (2012-06).

Procedure for Preempting a GBR Service

During the preemption procedure, the eNodeB first groups GBR services with low priorities and then selects services that can be preempted from this group. The procedure is as follows:
  1. The eNodeB first selects releasable GBR services, and then groups the services with the lowest priorities among them.
    • A releasable GBR service must meet the following conditions:
      -The value of the ARP IE pre-emption vulnerability of the service is pre-emptable.
      -The service is not an emergency call.
      The service rate is not 0.
    • The eNodeB sorts these GBR services based on the following rule: The service with the largest value of the ARP IE "priority level" has the lowest priority and ranks first. If two services have the same "priority level" value, the service that occupies more PRBs ranks ahead of the other.
  2. The eNodeB selects services to be preempted.
    • The eNodeB estimates the number of PRBs required by a preempting service based on the average spectral efficiency of the cell.
    • The eNodeB calculates the number of PRBs released from the group of GBR services with low priorities. The calculation stops after the number of released PRBs meets the requirements of the preempting service. The preemption fails if no service can be preempted or the number of PRBs released from ten preempted services does not meet the requirements of the preempting service.
    • If the GBR to be preempted is the only service on a UE, the UE can be redirected when redirection is enabled. For details about redirection, see 3.6 Redirection.

3.6 Redirection

A UE that fails to access a cell can be redirected to another cell (even an inter-RAT cell). This section describes the eNodeB operations related to redirection.
To release a UE, the eNodeB sends an RRC Connection Release message to the UE. This message includes information about the target frequency that the UE will be redirected to. Redirection is controlled by LoadBasedSwitch under the RedirectSwitch parameter.
The eNodeB selects the target frequency as follows:
  • If a subscriber profile ID (SPID) has been specified for the UE, the eNodeB determines the UE-capable RATs and frequency bands and checks for the mapping between the frequencies and priorities in the associated SPID configuration. The eNodeB filters the frequencies defined in the SPID configuration based on the UE capabilities and then includes the remaining frequencies in the message in descending order of priority. For details about dedicated priorities and SPIDs, see Idle Mode Management Feature Parameter Description and Flexible User Steering Feature Parameter Description, respectively.
  • If an SPID has not been specified for the UE or frequency priorities have not been set for the SPID of the UE, the eNodeB determines the UE-capable RATs and frequency bands and selects the UE-capable inter-RAT neighboring frequency with the highest reselection priority specified by the CellReselPriority parameter. The eNodeB then delivers the information about the selected frequency to the UE.
    If no reselection priority is specified by the CellReselPriority parameter, the eNodeB determines the UE-capable RATs and frequency bands and selects the UE-capable inter-RAT neighboring frequency with the highest reselection priority specified by the CellReselPriority parameter. The eNodeB then delivers the information about the selected frequency to the UE.

4 Congestion Control

Congestion control reduces congestion caused by an insufficiency of radio resources or transport resources. This document describes only the former. For details about the latter, see Transport Resource Management Feature Parameter Description.
Congestion can be prevented in most cases if admission control is performed. However, congestion may occur in the following cases:
  • The services are diverse and the data rates of certain services vary significantly. Variations in the data volume inevitably affect the cell load.
  • The radio conditions vary because of user mobility. The same service at the same data rate may require different radio resources (such as PRBs and power) on different occasions.
In these two cases, the cell load varies and the quality of admitted services gets affected, even if the number of users in the cell does not change. The congestion control algorithm is required to handle possible congestion.
Figure 4-1 shows the congestion control process. Congestion control is enabled if the DlLdcSwitch(dlLdcSwitch) or UlLdcSwitch(ulLdcSwitch) check box under the CellAlgoSwitch.RacAlgoSwitch parameter is selected.
Figure 4-1 Congestion control process
When the cell is congested, the eNodeB releases the GBR services with low priorities first to make some resources available. The release reduces system load and helps ensure the quality of other admitted services.
To ensure continuity and stability of emergency calls, the eNodeB does not release emergency calls.
This chapter describes the basic feature TDLBFD-002024 Congestion Control.

4.1 Load Status Evaluation

The eNodeB checks the load status by monitoring the PRB usage, QoS satisfaction rate, and downlink transmit power. The cell status can be either of the following:
  • Congested state
    The cell is regarded as congested if the QoS satisfaction rate of one or more QCIs is lower than the relevant congestion threshold and the uplink or downlink PRB usage is high, or if the downlink QoS satisfaction rate of one or more QCIs is lower than the relevant congestion threshold and the downlink transmit power is limited.
    When the cell is congested, the congestion control algorithm instructs the admission control algorithm to reject all access requests and, in addition, takes actions to reduce cell load. When the QoS satisfaction rates of QCIs 1, 2, 3, and 4 all become higher than the sum of the values of related congestion threshold and CongRelOffset, the cell returns to the normal state. The congestion thresholds are specified by QcixCongThd (x = 1~4), that is, by the Qci1CongThd, Qci2CongThd, Qci3CongThd, and Qci4CongThd parameters.
    For each QCI, the sum of QcixCongThd (x = 1~4) and  CongRelOffset must be lower than the corresponding QcixHoThd (x = 1~4).
  • Normal state
    The cell is regarded as normal if the QoS satisfaction rates of QCIs 1, 2, 3, and 4 are all higher than the corresponding value of QcixCongThd (x = 1~4).

4.2 Release of Low-Priority GBR Services

If a cell is congested, congestion control selects a service that ranks the first in the group of admitted low-priority GBR services and releases the selected service. If the only GBR service running on the UE on which the selected service is running is released and redirection is enabled, the eNodeB redirects the UE to another frequency or RAT, increasing the access success rate of the UE. For details about redirection, see 3.6 Redirection.
After the GBR service is released, the eNodeB checks whether the QoS satisfaction rates of GBR services are restored. If the QoS satisfaction rates of GBR services are not restored, the eNodeB performs the GBR service release procedure again, until the congestion is relieved. 


System Capacity

The congestion control algorithm maximizes system capacity while preferentially satisfying the QoS requirements of users with high ARP priorities. When a cell becomes congested, the congestion control algorithm informs the admission control algorithm. Then, the admission control algorithm starts to reject access requests.

Network Performance

The congestion control algorithm increases the service drop rate because congestion-control-triggered releases are regarded as service drops. Counters are provided to indicate congestion-triggered service drops


When a network becomes congested with an increasing number of users and higher QoS requirements, eNodeBs need to perform radio resource management so that the QoS requirements of ongoing services can be fulfilled and differentiated services can be provided.
When radio bearers cannot be set up because of radio resource congestion, activate admission control to relieve congestion and provide service-priority-based access for services.
When congestion increases so that QoS requirements cannot be fulfilled, activate congestion control to enable low-priority service release

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