问题分析笔记:20200909 终端5G模式下，显示3G，长时间不回4G

08:16:21.843 [0xB0EC] LTE NAS EMM Plain OTA Incoming Message
prot_disc = 7 (0x7) (EPS mobility management messages)
msg_type = 69 (0x45) (Detach request)
lte_emm_msg
emm_detach_request
switch_off = 0 (0x0) (normal detach)
detach_type = 2 (0x2) (re-attach not required)
emm_cause_incl = 1 (0x1)
emm_cause
cause_value = 7 (0x7) (EPS services not allowed)

终端收到7# EPS services not allowed的Detach request消息。

23.122协议中 Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode：

终端将其当前网络的PLMN加入到forbidden PLMNS for GPRS service list中。终端不再接入GPRS服务。而4G只有EPS服务。所以此时终端会选网到23G，在T3245超时前都不返回4G。

24.008协议对T3245的处理:

如果网络返回的错误是#8 “EPS services and non-EPS services not allowed”，那么终端就会无服务，不会到23G。

24.008对forbidden PLMNS for GPRS service lis的描述。