request进入连接态进行CSFB时，出现service reject消息，被叫连接建立失败，随后被叫进行Attach接入成功后，没有进行CSFB，最后由于被叫CSFB失败导致未接通。 主叫消息：
被叫消息：
解决措施：目前还没有定位出具体原因，需进一步跟进。
? 被叫做TAU更新（5次）
主叫信令（主叫信令到Call Proceeding后，长时间没响应，网络就下发disconnect消息）。
被叫信令（主叫发起寻呼时，被叫正处于TAU更新，导致起呼失败）
解决措施：合理的规划TAU区域，避免TAU边界处于业务繁忙区域。
2、CSFB流程（空闲态下，CSFB呼叫流程） CSFB主叫流程：
UE/ MS eNodeB BSS /RNS MME MSC SGW/PGW SGSN 1a. NAS Extended Service Request 1b. S1 - AP Initial UE Context Request with CS Fallback indicator 1c. S1 - AP Initial UE Context Response message 2. Optional Measurement Report Solicitation 3 a . NACC , . 3b, 3c RRC connection release 4. S1 - AP: S1 UE Context Release Request 5. S1 UE Context Release 6. UE changes RAT then LA Update or RA Update or LAU and RAU 7 a. Suspend (see 23.060) 7b. Suspend Request / Response 8 . Update bearer(s) 9. CM Service Request 10a. Service Reject 10b. Location Area Update 10c. CS MO call If the MSC is changed 9. A/Iu-cs message (with CM Service Request) 11. Routing Area Update or Combined RA/LA Update
CSFB被叫流程:
UE eNodeB MME RNC/BSC MSC VLR HSS G-MSC 1. IAM 2. SRI procedure in TS 23.018 3. IAM 4. Paging Request 5. Paging 6. Paging 7a. Extended Service Request 7a. Service Request 7b. Initial UE Context Setup 8. After the UE context setup the procedure continues as described the active mode mobile terminated call procedures described in Clause 7.3 and 7.4. If the eNodeB knows that PS HO is supported the procedure in clause 7.3 “Mobile Terminating call in Active Mode - PS HO supported” may be applied from step 2, in Clause 7.3, and onwards If the eNodeB knows that PS HO is not supported the procedure in clause 7.4 “Mobile Terminating call in Active Mode – No PS HO support” shall be applied from step 2 in Clause 7.4, and onwards
