UAC is a set of mechanism (algorithm) to determine whether to allow the UE for a specific services or state changes. UAC is a pretty complicated algorithm that get many factors involved at each point of decision making process. Overall concept similar to LTE Access Control, but I think the related parameters are more dispersed here and there... causing more confusion -:).  In LTE, Access control is mainy controlled by RRC(SIBs) but it seems NAS has more control ever Access Control in NR seems.

• Factors_for_UAC_mechanism
• Triggers for Checking UAC
• RRC Information for UAC
• USIM Information for UAC
• How to determine Access Identity ?
• What kind of service can be provided for a access category ?
• Mapping between access categories/access identities and RRC establishment cause

Factors for UAC mechanism

The rough categories of factors affecting UAC is illustarated below and main framework is specified in 24.501-4.5, but you may refer to additional specification like 38.331 and USIM specification for further details.

Triggers for Checking UAC

Triggers for UAC check is specified in 24.501-4.5.1 and can be summarized as follows.

RRC Information for UAC

SIB1 ::=        SEQUENCE {

    cellSelectionInfo                   SEQUENCE {

        q-RxLevMin                          Q-RxLevMin,

        q-RxLevMinOffset                    INTEGER (1..8)      OPTIONAL,   -- Need R

        q-RxLevMinSUL                       Q-RxLevMin          OPTIONAL,   -- Need R

        q-QualMin                           Q-QualMin           OPTIONAL,   -- Need R

        q-QualMinOffset                     INTEGER (1..8)      OPTIONAL    -- Need R

    }    OPTIONAL,   -- Need S

    cellAccessRelatedInfo               CellAccessRelatedInfo,

    connEstFailureControl               ConnEstFailureControl   OPTIONAL,   -- Need R

    si-SchedulingInfo                   SI-SchedulingInfo       OPTIONAL,   -- Need R

    servingCellConfigCommon             ServingCellConfigCommonSIB    OPTIONAL,   -- Need R

    ims-EmergencySupport                ENUMERATED {true}             OPTIONAL,   -- Need R

    eCallOverIMS-Support                ENUMERATED {true}             OPTIONAL,   -- Cond Absent

    ue-TimersAndConstants               UE-TimersAndConstants         OPTIONAL,   -- Need R

    uac-BarringInfo                     SEQUENCE {

        uac-BarringForCommon                UAC-BarringPerCatList     OPTIONAL,   -- Need S

        uac-BarringPerPLMN-List             UAC-BarringPerPLMN-List   OPTIONAL,   -- Need S

        uac-BarringInfoSetList              UAC-BarringInfoSetList,

        uac-AccessCategory1-SelectionAssistanceInfo CHOICE {

            plmnCommon                      UAC-AccessCategory1-SelectionAssistanceInfo,

            individualPLMNList              SEQUENCE (SIZE (2..maxPLMN))

                                               OF UAC-AccessCategory1-SelectionAssistanceInfo

        }   OPTIONAL

    }     OPTIONAL,   -- Need R

    useFullResumeID                     ENUMERATED {true}    OPTIONAL,   -- Need N

    lateNonCriticalExtension            OCTET STRING         OPTIONAL,

    nonCriticalExtension                SEQUENCE{}           OPTIONAL

}

UAC-AccessCategory1-SelectionAssistanceInfo ::= ENUMERATED {a, b, c}

UAC-BarringInfoSetList ::= SEQUENCE (SIZE(1..maxBarringInfoSet)) OF UAC-BarringInfoSet

UAC-BarringInfoSet ::= SEQUENCE {

    uac-BarringFactor              ENUMERATED {p00, p05, p10, p15, p20, p25, p30, p40,

                                               p50, p60, p70, p75, p80, p85, p90, p95},

    uac-BarringTime                ENUMERATED {s4, s8, s16, s32, s64, s128, s256, s512},

    uac-BarringForAccessIdentity   BIT STRING (SIZE(7))

}

RRCSetupRequest ::= SEQUENCE {

   rrcSetupRequest                        RRCSetupRequest-IEs

}

RRCSetupRequest-IEs ::= SEQUENCE {

    ue-Identity                          InitialUE-Identity,

    establishmentCause                   EstablishmentCause,

    spare                                BIT STRING (SIZE (1))

}

InitialUE-Identity ::= CHOICE {

   ng-5G-S-TMSI-Part1                    BIT STRING (SIZE (39)),

   randomValue                           BIT STRING (SIZE (39))

}

EstablishmentCause ::= ENUMERATED {

   emergency,

   highPriorityAccess,

   mt-Access,

   mo-Signalling,

   mo-Data,

   mo-VoiceCall,

   mo-VideoCall,

   mo-SMS,

   mps-PriorityAccess,

   mcs-PriorityAccess,

   spare6, spare5, spare4, spare3, spare2, spare1}

USIM Information for UAC

EFUAC_AIC (UAC Access Identities Configuration) : 31.102-4.4.11.7

EFUAC_ACC (Access Control Class) : 31.102-4.2.15

How to determine Access Identity ?

Basically Access Identity is determined by a few parameters in UICC as described in the following table.

< 24.501-Table 4.5.2.1: Access identities >

What kind of service can be provided for a access category ?

< 24.501-Table 4.5.2.2: Mapping table for access categories >

Mapping between access categories/access identities and RRC establishment cause

< 24.501 - Table 4.5.6.1: Mapping table for access identities/access categories and RRC establishment cause when establishing N1 NAS signalling connection via NR connected to 5GCN >

< 24.501 - Table 4.5.6.2: Mapping table for access identities/access categories and RRC establishment cause when establishing N1 NAS signalling connection via E-UTRA connected to 5GCN >

Reference

[1]