根据规范通信运营商必须为公共安全提供应急通信，并做为首要的任务；在IMS/SIP系统中提供该类服务的就是PSAP （Public-Safety Wnswering Point ）
PSAP意为公共安全通道或公共安全接入点，是“基于IMS的应急通信”；它通过即时信息网络进行紧急呼叫，而不是通过CS呼叫。终端可以在注册的网络运营商下进行PSAP调用，也可以不使用USIM (即不在网络中注册)进行 PSAP调用。PSAP的应用涉及到终端和无线网络的参数设置及相关信息；
1. PSAP注册类型 根据注册情况发起PSAP的终端分为以下三种情况
2. 联合注册终端PSAP业务流程
2.1 联合注册终端PSAP业务消息
2.2 消息内容
2.2.1 < System Information Block Type 1 >
BCCH-DL-SCH-Message
message: c1 (0)
c1: systemInformationBlockType1 (1)
systemInformationBlockType1
cellAccessRelatedInfo
plmn-IdentityList: 1 item Item 0 PLMN-IdentityInfo plmn-Identity
mcc: 3 items
Item 0 MCC-MNC-Digit: 0
Item 1 MCC-MNC-Digit: 0
Item 2 MCC-MNC-Digit: 1
mnc: 2 items
Item 0 MCC-MNC-Digit: 0
Item 1 MCC-MNC-Digit: 1
cellReservedForOperatorUse: notReserved (1)
trackingAreaCode: 0001 [bit length 16, 0000 0000 0000 0001 decimal value 1]
cellIdentity: 00000000
cellBarred: notBarred (1)
intraFreqReselection: notAllowed (1)
.... ..0. csg-Indication: False
cellSelectionInfo
q-RxLevMin: -110dBm (-55)
p-Max: 23dBm
freqBandIndicator: 4
schedulingInfoList: 2 items
Item 0
SchedulingInfo
si-Periodicity: rf16 (1)
sib-MappingInfo: 0 items
Item 1
SchedulingInfo
si-Periodicity: rf32 (2)
sib-MappingInfo: 1 item
Item 0
SIB-Type: sibType3 (0)
si-WindowLength: ms20 (5)
systemInfoValueTag: 0
nonCriticalExtension
nonCriticalExtension
ims-EmergencySupport-r9: true (0)
2.2.2 Attach Accept
DL-DCCH-Message
message: c1 (0)
c1: rrcConnectionReconfiguration (4)
criticalExtensions: c1 (0)
c1: rrcConnectionReconfiguration-r8 (0)
dedicatedInfoNASList: 1 item
Item 0
DedicatedInfoNAS: 271166512903074202e0060000f110...
Non-Access-Stratum (NAS)PDU...
Emergency Number List
Element ID: 0x34
Length: 4
Emergency Number Information: 1
Emergency Number Info length: 3
000. .... = Spare bit(s): 0
...0 .... = Mountain Rescue: False
.... 0... = Marine Guard: False
.... .0.. = Fire Brigade: False
.... ..0. = Ambulance: False
.... ...1 = Police: True
Emergency BCD Number: 123
EPS network feature support
Element ID: 0x64
Length: 1
00.. .... = Spare bit(s): 0x00
..0. .... = ESRPS:network does not support use of
...0 0... = CS-LCS:no information about support of
.... .0.. = EPC-LCS: location services via EPC not
.... ..1. = EMC BS: emergency bearer services
in S1 mode supported
.... ...1 = IMS VoPS: IMS voice over PS session
in S1 mode supported
radioResourceConfigDedicated
2.2.3 < PDN Connectivity Request >
UL-DCCH-Message
message: c1 (0)
c1: ulInformationTransfer (9)
。。。。。。
Non-Access-Stratum (NAS)PDU
0010 .... = Security header type: ...
.... 0111 = Protocol discriminator: ...
Message authentication code: 0x3a356704
Sequence number: 6
0000 .... = EPS bearer identity: ...
.... 0010 = Protocol discriminator: ...
Procedure transaction identity: 24
NAS EPS session management messages:
PDN connectivity request (0xd0)
0010 .... = PDN type: IPv6 (2)
.... 0100 = Request type: emergency (4)
Protocol Configuration Options
< INVITE >
INVITE urn:service:sos SIP/2.0
Supported: timer,100rel,precondition
Allow: INVITE,ACK,OPTIONS,CANCEL,BYE,UPDATE,INFO,REFER,NOTIFY,MESSAGE,PRACK
User-Agent: testIMS /OK2 5.1.1
Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";video
P-Access-Network-Info: 3GPP-E-UTRAN-FDD;utran-cell-id-3gpp=0010100010000000
Session-Expires: 1800;refresher=uac
Min-SE: 90
Content-Type: application/sdp
Accept: application/sdp,application/3gpp-ims+xml
Route: <sip:[2001:0:0:3::2]:5060;lr>
P-Preferred-Identity: <sip:001010123456789@ims.mnc01.mcc001.3gppnetwork.org>
From: <sip:001010123456789@ims.mnc01.mcc001.3gppnetwork.org>;tag=414344133
To: "123" <urn:service:sos>
Call-ID: 1800855744@2001:0:0:3::1
CSeq: 1 INVITE
Max-Forwards: 70
Contact: <sip:001010123456789@[2001:0:0:3::1]:5060;transport=UDP>;
+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";video;
+g.3gpp.mid-call;+g.3gpp.srvcc-alerting;+g.3gpp.ps2cs-srvcc-orig-pre-alerting;
+sip.instance="<urn:gsma:imei:35609506-513511-0>"
Via: SIP/2.0/TCP [2001:0:0:3::1]:5060;branch=z9hG4bK2071720670smg;transport=TCP;rport
Content-Length: 780
v=0
o=SAMSUNG-IMS-UE 1234567 0 IN IP6 2001:0:0:3::1
s=SS VOIP
c=IN IP6 2001:0:0:3::1
b=AS:49
b=RS:0
b=RR:0
t=0 0
m=audio 1234 RTP/AVP 114 113 102 115 105 101
b=AS:49
b=RS:0
b=RR:0
a=rtpmap:114 AMR-WB/16000/1
a=fmtp:114 mode-change-capability=2;max-red=0
a=rtpmap:113 AMR-WB/16000/1
a=fmtp:113 octet-align=1;mode-change-capability=2;max-red=0
a=rtpmap:102 AMR/8000/1
a=fmtp:102 mode-change-capability=2;max-red=0
a=rtpmap:115 AMR/8000/1
a=fmtp:115 octet-align=1;mode-change-capability=2;max-red=0
a=rtpmap:105 telephone-event/16000
a=fmtp:105 0-15
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos optional remote sendrecv
a=sendrecv
a=ptime:20
a=maxptime:240
2.2.4 < 183 Session Progresss >
Call-ID: 1800855744@2001:0:0:3::1
CSeq: 1 INVITE
Record-Route: <sip:[2001:0:0:3::2]:5060;lr>
Require: precondition
Require: 100rel
Supported: precondition
Supported: 100rel
RSeq: 1
User-Agent: Test-PSAP/fcefab8
Allow: INVITE, ACK, CANCEL, BYE, PRACK, UPDATE
P-Asserted-Identity: <tel:123;phone-context=test-cscf.com>
Content-Type: application/sdp
Content-Length: 804
Contact: <sip:[2001:0:0:3::2]:50101;transport=tcp>;+g.3gpp.icsi-ref="urn:urn-7:3gpp-service.ims.icsi.mmtel";video
Privacy: none
v=0
o=test 301 60302 IN IP6 2001:0:0:3::2
s=-
i=A VOIP Session
c=IN IP6 2001:0:0:3::2
t=0 0
m=audio 60000 RTP/AVP 114 113 102 115 105 101
b=AS:49
b=RS:0
b=RR:0
a=rtpmap:114 AMR-WB/16000/1
a=fmtp:114 mode-change-capability=2;max-red=0
a=rtpmap:113 AMR-WB/16000/1
a=fmtp:113 octet-align=1;mode-change-capability=2;max-red=0
a=rtpmap:102 AMR/8000/1
a=fmtp:102 mode-change-capability=2;max-red=0
a=rtpmap:115 AMR/8000/1
a=fmtp:115 octet-align=1;mode-change-capability=2;max-red=0
a=rtpmap:105 telephone-event/16000
a=fmtp:105 0-15
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
a=sendrecv
a=ptime:20
a=maxptime:240
a=conf:qos remote sendrecv
a=rtcp:60001
3. 无USIM卡终端PSAP业务流程
终端由于无USIM卡，在发起PSAP业务请求时，网络对其注册和PDN建立使用IMEI进行标识；
原文来自：http://www.sharetechnote.com
整理：kangguoying20201006
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