5G NR Cell Access Control

5G, Call Flow, NAS Signalling, New Radio, NR, RRC Signalling, Tech Fundas

Radio Access barring control refers to a traffic congestion control mechanism to secure and ensure the success of critical communications calls such as emergency calls by restricting connection requests from mobile to base station. The radio access baring can be controlled can be caterogies into following two method:

  1. Access Control Method [Control in Mobile Terminals]
    • Before sending any connection request to base station mobile device shall evaluate broadcast information and identifies the types of call and determines whether a connection request for the call should be barred or not.
  2. RRC Connection Reject Method [Control in Base Station]
    • The base station identifies the type of connection request sent from mobile terminal, and decides whether this request shall be served or rejected with RRCConnectionReject.

   

It’s up-to  the mobile operator to use one or both methods for access baring controls depending on the network connection and traffic conditions.

For 5G NR, 3GPP has been standardizing a series of traffic congestion mechanism to control mobile communication access to network. There are two high level solutions for controlling network access attempts.

  • Cell Reservations and Access Restrictions using broadcast information flags
  • Access Baring with Unified Access Control (UAC) features

Access Barring with Broadcast Information Flags

Cell reservations and access restrictions are based upon a set of flags included within the MIB and SIB, this flag indicates permissions for cell selection and cell reselection.

  • In 5G NR, MIB includes cell barring information with and IE cellBarred  which can be broadcast with a value of ‘barred‘ or ‘notBarred’. This flag allow early detection of the status without requiring the UE to receive and decode SIB 1. If a cell is barred then the UE is not permitted to select or reselect the cell
    • Barring Infrequency Cell: If the MIB indicates that a cell is barred then the UE will also check the intraFreqReselection flag which is also included within the MIB. If this flag is set to ‘notAllowcd’ then the UE is not permitted to reselect another cell on the same frequency. Otherwise, if this flag is set to ‘Allowed’ then the UE is permitted to reselect another cell on the same frequency
    • Barring based on MIB/SIB Decode Failure: A UE also treats a cell as barred if the UE fails to decode the MIB. In this case, the UE is allowed to reselect another cell on the same frequency. Additionally, if the UE fails to decode SIB1, then UE treats a cell as barred. In this case, the UE uses the intraFreqReselection flag from within the MIB to determine whether or not reselection to another cell on the same frequency is allowed. UE treats the cell as barred for up to 300 seconds when it fails to decode the MIB or SIB1
  • 5G NR SIB1 includes an flag cellReservedForOperatorUse can be broadcast with a value of ‘reserved’ or ‘not Reserved’. If a cell is reserved then a UE with an Access Identity of 11 (PLMN Use) or an Access Identity of 15 (PLMN Staff) is allowed to use the cell for selection and reselection, if that UE is within its Home PLMN or an Equivalent Home PLMN. If a cell is reserved then a UE with Access Identity 0, 1, 2, 12, 13 or 14 treats the cell as ‘barred’and not allowed to select or reselect
  • Another flag in SIB1 cellReservedForOtherUse can be broadcast with a value of ‘true’ or can be excluded. When this flag is broadcast by SIB 1 then all UE treat the cell as ‘barred’, i.e. it has the same impact as the cellBarred flag within the MIB. There is a single instance of the flag which is applicable to all PLMN.
    • cellReservedForOtherUse does not have a specific purpose within the release 15 version of the 3GPP specifications but has been introduced to provide forward compatibility with services which may be introduced in the future. For example, Closed Subscriber Group (CSG) cells can be used to show how this flag provides forward compatibility.
    • The release 15 version of the 3GPP specifications docs not support CSG cells for New Radio. If the release 16 version of the specifications introduces support for CSG cells then a release 16 UE will understand the rules for accessing a CSG cell (only subscribers belonging to the CSG are permitted to access the cell). Release 15 UE will not have any knowledge of these rules but the cellReservedForOtherUse flag can be used to ensure that a release 15 UE does not attempt to access the CSG cell

Access Baring with Unified Access Control (UAC)

2G, 3G and 4G networks have used Access Class Barring (ACB) to control access. Access Class is an identifier assigned by network operator to each user to indicate its access priority and it is stored in SIM during provisioning data. Most of the normal SIMs are provisioned with an Access Class within the range o to 9. In addition, a limited group of users may be allocated SIM which have been provisioned with a special Access Class within the range 11 to 15. These special classes is listed below

  • Access Class 10 is for initiating an emergency call
  • Access Class 11 is intended for PLMN Use
  • Access Class 12 is intended for Security Services
  • Access Class 13 is intended for Public Utilities
  • Access Class 14 is intended for Emergency Services
  • Access Class 15 is intended for PLMN Staff

Various Access Class Barring solutions have evolved over time, e.g. barring specific Access Classes within the range 0 to 9; specifying a probability that access is permitted for a UE with Access Class 0 to 9; barring access for specific services and connection establishment causes.

3GPP release 15 specifications introduces the concept of Unified Access Control (UAC) which is based upon the use of’ Access Identities’ and ‘Access Categories’. A UE associates itself with an Access Identity based upon the available rules. Below table shows a mapping of Access Identifies

Access Identity UE Configuration
0  UE is not configured with any parameters from this table
1  UE is configured for Multimedia Priority Service [MPS]
2  UE is configured for Mission Critical Service [MCS]
3 -10  Reserved
11  UE has Access Class 11  [PLMN Use]
12  UE has Access Class 12  [Security Services]
13  UE has Access Class 13  [Public Utilities]
14  UE has Access Class 14  [Emergency Services]
15  UE has Access Class 15  [PLMN Staff]

From the table is can be seen Access Identities 11 to 15 are mapped directly from Access Classes 11 to 15. Access Identities 1 and 2 are linked to subscribers permitted to use the Multimedia Priority Service (MPS) and Mission Critical Service (MCS)  services. Access Identity 0 is linked to all other cases. In general, Access Identity 0 will capture Access Classes 0 to 9.

The NAS: Registration Accept message includes an ‘MPS Indicator‘ which can be set to ‘1’ when providing permission to use the MPS service, i.e. this flag can be used to allocate Access Identity 1. Similarly, the same field includes an ‘MCS Indicator’ which can be set to’ 1′ when providing permission to use the MCS service, i.e. this nag can be used to allocate Access Identity 2. Alternatively, the file ‘EFUAC AIC’ available in USIM can be used to provision Access Identity 1 or 2.

A UE connection attempt links to a specific Access Category using the rule shown in table below. If a connection attempt matches multiple rules, then Access Category belonging to the lowest rule number is selected.

Rule Access Category Type of Access
1 0 [MT Access ]
  • Paging
  • Notification over non-3-GPP Access 0ver (MT Access)
  • 5G MM Connection Management procedure
2 2 [ Emergency ]      Emergency
3 32 to 64 [Operator Defined]      Operator Defined Access Categories
4 1 [Delay Tolerant]      Delay Tolerant Service
5 4 [MO Multimedia Telephony Voice]       Mobile Originated Multimedia Telephony Voice
6 5 [MO Multimedia Telephony Video]       Mobile Originated Multimedia Telephony Video
7 6 [MO SMS & SMSoIP]       Mobile Originated Multimedia SMS and SMS over IP
8 3 [MO Signalling]       UE NAS initiated 5G MM procedure
9 7 [MO Data]
  • UE NAS initiated 5GMM Connection Management procedure
  • UE NAS initiated 5GMM NAS Transport procedure
10 7 [MO Data]       Uplink User Plane Packet transfer For PDU Session

UE with Access Category 0 are not subject to Access Barring because the network is responsible for managing mobile terminating calls or Page UE, i.e. the network should not page the UE if the connection is not allowed. Access Categories 32 to 63 can be configured by the operator. The NAS: Registration Accept message provided by the AMF can include the ‘Operator Defined Access Category Definitions’ field which specifies a set of mapping rules.

Access Control Checks based upon Access Identity and Access Category are required when a UE initiates the transition from RRC Idle mode to RRC Connected mode. Access Control Checks arc also required for UE in RRC Connected and RRC Inactive modes when specific events occur, e.g. a mobile originated voice or video call is initiated, a mobile originated SMS is initiated, an uplink NAS message is used to setup or modify a PDU Session, uplink data transfer which requires the use of a suspended PDU Session.

The uac-Barringlnfo within SIB 1 provides the parameters to determine when an Access Barring check is required. An access barring check involves the UE generating a uniformly distributed random number between 0 and 1. If the random number is less than the value of uac-BarringFactor, then the access attempt is permitted. Otherwise, the access attempt is barred. Configuring a value of ‘0’ for uac-BarringFactor means that all checks will lead to barred access attempts. If an access attempt is barred, the UE generates a second uniformly distributed random number between 0 and 1. The value of T390 is then set equal to (0.7 + 0.6 *rand) *uac-Barring Time seconds, where ‘rand’ is the random number. T390 defines the duration that the UE treats the cell as barred for the corresponding Access Category