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查了一下23.003,考虑到MME情况,NRI长度可以比8短,但是不能比8长。
The NRI shall be coded in bits 23 to 14. An NRI shorter than 10 bits shall be encoded with the most significant bit of the NRI field in bit 23.
NRI在P-TMSI中的位置是bit23-14(考虑MME情况是23-16,8bit)。但是,与MME互联的情况下,NRI长度虽然可以比8短,假如是5,但是后3个bit也不能用于分配。原因请看下面的P-TMSI到GUTI映射规则。
2.8.2.2 Mapping from RAI and P-TMSI to GUTI
2.8.2.2.2 Mapping in the UE
The 8 most significant bits of GERAN/UTRAN <NRI> map to the MME code.
GERAN/UTRAN <P TMSI> maps as follows:
- 6 bits of the GERAN/UTRAN <P TMSI> starting at bit 29 and down to bit 24 are mapped into bit 29 and down to bit 24 of the E UTRAN <M-TMSI>;
- 16 bits of the GERAN/UTRAN <P TMSI> starting at bit 15 and down to bit 0 are mapped into bit 15 and down to bit 0 of the E UTRAN <M-TMSI>.
2.8.2.2.3 Mapping in the new MME
In order to retrieve the UE's information, e.g. the IMSI, from the old SGSN, the new MME extracts the RAI and P-TMSI from the GUTI via the reverse mapping procedure to that specified in subclause 2.8.2.2.2.
容量问题,减掉8个bit的NRI占位,可以分配的值是4,194,304(2^22). 22=6+16或者32-2-8
从上述角度考虑,说NRI长度固定为8也是可以的。因为即使长度不是8, 23-16这段也都不会被map到M-TMSI里。不知道SGSN实现过程是否考虑这点,即使NRI长度不是8也可以不分配其余bit,否则就需要配置NRI长度8了。 |
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