Unified Access Control（通用接入控制）
上周将NR重选的时候，在SIB2中定义了UAC，当时由于时间问题，没有展开来进行解释，所以考虑补充一个专题文档。
UAC是UE接入控制或状态改变的机制集合。
在LTE中，我们就经常会碰到需要接入控制的时候，比如某个小区超载了，大量UE接入到一个小区，导致UE间相互干扰严重，无法成功解码。在4G中，接入控制主要有两种类型：
● UE初始接入请求的时候，网络下发拒绝消息；
● 自己阻止UE尝试初始接入（也就是CellBar）
Case1：SIB1配置bar，所有UE都不能接入，含紧急呼叫
Case2：SIB2配备Bar，只针对特定服务特定的UE禁止接入
在LTE中，接入控制主要在RRC层进行，但是，在NR中，不仅在RRC层进行控制，还可以在NAS层进行控制。具体如下：
valid from是什么意思
Factors
Description
RRC
∙ uac-BarringInfo in SIB1
∙ RRC State (Idle, Connected,Inactive)
NAS
∙ 见下文 
USIM
∙ EFUAC_AIC, EFACC
UAC触发
根据24.501 4.5.1总结如下：
Trigger
Description
NAS 状态改变
∙ 5GMM-IDLE --> 5GMM-CONNECTED
服务请求 (in 5GMM-CONNECTED)
∙ MO-MMTEL-voice-call-started indication
∙ MO-MMTEL-video-call-started indication
∙ MO-SMSoIP-attempt-started
∙ MO SMS over NAS
∙ UL NAS TRANSPORT message for PDU session establishment (except for the purpose of IDLE -> CONNECTED NAS State change)
∙ UL NAS TRANSPORT message for PDU session modification (except for the purpose of IDLE -> CONNECTED NAS State change)
∙ Request to re-establish the user-plane resources for an existing PDU session
∙ Notification for an uplink user data packet to be sent for a PDU session with suspended user-plane resources
SIM卡信息相关的UAC
EFUAC_AIC (UAC Access Identities Configuration)，31.102 定义如下：
b8
b7
b6
b5
b4
b3
b2
b1
b1=0: UE is not configured for Multimedia Priority Service in the HPLMN,EHPLMN or a visited PLMN of the home country.
b1=1: UE is configured for Multimedia Priority Service in the HPLMN,EHPLMN or a visited PLMN of the home country.
b2=0: UE is not configured for Mission Critical Services in the HPLMN,EHPLMN or a visited PLMN of the home country.
b2=1: UE is configured for Mission Critical Services in the HPLMN,EHPLMN or a visited PLMN of the home country.
RFU
EFACC (Access Control Class)
Byte 1:
b8
b7
b6
b5
b4
b3
b2
b1
15
14
13
12
11
10
09
08
Number of the ACC (except for bit b3)
Byte 2:
b8
b7
b6
b5
b4
b3
b2
b1
07
06
05
04
03
02
01
00
Number of the ACC
Identifier: '6F78'
Structure: transparent
Mandatory
SFI: '06'
File size: 2 bytes
Update activity: low
Access Conditions:
    READ    PIN
    UPDATE    ADM
    DEACTIVATE    ADM
    ACTIVATE    ADM
Bytes
Description
M/O
Length
1 to 2
Access control classes
M
2 bytes
如何获取接入标识符？
Access identities
Access Identity number
UE configuration
0
UE is not configured with any parameters from this table
1 (NOTE 1)
UE is configured for multimedia priority service (MPS).
2 (NOTE 2)
UE is configured for mission critical service (MCS).
3-10
Reserved for future use
11 (NOTE 3)
Access Class 11 is configured in the UE.
12 (NOTE 3)
Access Class 12 is configured in the UE.
13 (NOTE 3)
Access Class 13 is configured in the UE.
14 (NOTE 3)
Access Class 14 is configured in the UE.
15 (NOTE 3)
Access Class 15 is configured in the UE.
NOTE 1:    Access identity 1 is valid when:
- the USIM file EFUAC_AIC indicates the UE is configured for access identity 1 and the selected PLMN, if a new PLMN is selected, or RPLMN is the HPLMN (if the EHPLMN list is not present or is empty) or EHPLMN (if the EHPLMN list is present), or a visited PLMN of the home country (see the definition of home country in 3GPP TS 24.301 [15]); or
- the UE receives the 5GS network feature support IE with the MPS indicator bit set to "Access identity 1 valid" from the RPLMN as described in subclause 5.5.1.2.4 and subclause 5.5.1.3.4.
NOTE 2:    Access identity 2 is used by UEs configured for MCS and is valid when:
- the USIM file EFUAC_AIC indicates the UE is configured for access identity 2 and the selected PLMN, if a new PLMN is selected, or RPLMN is the HPLMN (if the EHPLMN list is not present or is empty) or EHPLMN (if the EHPLMN list is present), or a visited PLMN of the home country (see 3GPP TS 23.122 [5]); or
- the UE receives the 5GS network feature support IE with the MCS indicator bit set to "Access identity 2 valid" from the RPLMN as described in subclause 5.5.1.2.4 and subclause 5.5.1.3.4.
NOTE 3:    Access identities 11 and 15 are valid in HPLMN (if the EHPLMN list is not present or is empty) or EHPLMN (if the EHPLMN list is present). Access Identities 12, 13 and 14 are valid in HPLMN and visited PLMNs of home country only (see the definition of home country in 3GPP TS 24.301 [15]).
那么，接入等级能够提供哪些服务呢？

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