ESM(EPS会话管理)概念及流程

hycl5410

zglaojiang 发表于 2012-7-11 23:22
请问下为什么SAE里面新的MME去找旧的MME时通过OLD GUTI解析，为什么不是OLD TAI。

好问题！
这个就要看GUTI的结构来分析为什么了。
TS 23.003 r8以上

<GUTI> = <GUMMEI><M-TMSI>,

      where <GUMMEI> = <MCC><MNC><MME Identifier>

      and <MME Identifier> = <MME Group ID><MME Code>

简单说就是，GUTI带了old MME groupID和MME ID。所以可以通过GUTI直接找到old MME。而P-TMSI是无法直接找到old SGSN的，所以要加RAI才可以解析出old SGSN来。

以下是UE侧 GUTI和RAI+P-TMSI的映射关系，用于2G/3G和LET之间跨SGSN/MME切换。

The mapping of the GUTI shall be done to the combination of RAI of GERAN / UTRAN and the P‑TMSI:

E‑UTRAN <MCC> maps to GERAN/UTRAN <MCC>

E‑UTRAN <MNC> maps to GERAN/UTRAN <MNC>

E‑UTRAN <MME Group ID> maps to GERAN/UTRAN <LAC>

E‑UTRAN <MME Code> maps to GERAN/UTRAN <RAC> and is also copied into the 8 most significant bits of the NRI field within the P‑TMSI;

E‑UTRAN <M-TMSI> maps as follows:

-     6 bits of the E‑UTRAN <M-TMSI> starting at bit 29 and down to bit 24 are mapped into bit 29 and down to bit 24 of the GERAN/UTRAN <P‑TMSI>;

-     16 bits of the E‑UTRAN <M-TMSI> starting at bit 15 and down to bit 0 are mapped into bit 15 and down to bit 0 of the GERAN/UTRAN <P‑TMSI>;

-     and the remaining 8 bits of the E‑UTRAN <M-TMSI> are mapped into the 8 MBS bits of the <P-TMSI signature> field.

For UTRAN, the 10-bit long NRI bits are masked out from the P-TMSI and also supplied to the RAN node as IDNNS (Intra Domain NAS Node Selector). However, the RAN configured NRI length should not exceed 8 bits.

SGSN/MME侧的映射关系也可以在23.003中找到，这里就不再赘述。