【求助】关于CSN.1语法问题

zxr1001

CSN.1语法中H,L代表什么含义？
P.S 谁能给小弟一份比较详尽的CSN.1的资料啊，最好有例子分析的那种。24.007的附录实在没看明白

44.018 10.5.2.37b
关于SI13消息结构的CSN.1描述如下
< SI 13 Rest Octets > ::=
{ L | H
< BCCH_CHANGE_MARK : bit (3) >
< SI_CHANGE_FIELD : bit (4) >
{ 0 | 1 < SI13_CHANGE_MARK : bit (2) >
< GPRS Mobile Allocation : GPRS Mobile Allocation IE > } -- Defined in 3GPP TS 44.060
{ 0 -- PBCCH not present in cell (This shall always be indicated. See sub-clause 1.8) :
< RAC : bit (8) >
< SPGC_CCCH_SUP : bit >
< PRIORITY_ACCESS_THR : bit (3) >
< NETWORK_CONTROL_ORDER : bit (2) >
< GPRS Cell Options : GPRS Cell Options IE > -- Defined in 3GPP TS 44.060
< GPRS Power Control Parameters : GPRS Power Control Parameters IE >
| 1 -- PBCCH present in cell (This shall never be indicated. See sub-clause 1.8) :
< PSI1_REPEAT_PERIOD : bit (4) >
< PBCCH Description : PBCCH Description struct >
}
{ null | L -- Receiver compatible with earlier release
| H -- Additions in release 99 :
< SGSNR : bit >
{ null | L -- Receiver compatible with earlier release
| H -- Additions in release Rel-4:
< SI_STATUS_IND : bit >
{ null | L -- Receiver compatible with earlier release
| H -- Additions in release Rel-6:
{ 0 | 1 < LB_MS_TXPWR_MAX_CCH : bit (5) > }
< SI2n_SUPPORT : bit(2) >
}
}
}
}
< spare padding > ;

请教其中的H,L是代表什么含义？解码时怎么确定到对应的分支上？
3Q