关于BVCI,TLLI,BSSGP一些细节的讨论

1)48018上描述,

BVCI: BSSGP Virtual Connections (BVCs) provide communication paths between BSSGP entities 用于识别小区

据我说知,每一个BVCI跟一个Cell联系,可以定位到一个cell.另外,协议上又说

The BVCI value 0000 hex shall be used for the signalling functional entities.

The BVCI value 0001 hex shall be used for the PTM functional entities.

那也就是说,BSSGP根据BVCI的不同来判断,如果是0,则进行信令的处理,如果是非零,则传相应数据到BSC?

那么PCU在与SGSN连时,一般会先静态配置BVCI?

例如,如果一个PCU与一个BSC连接,一个BSC下有20个小区,那就要配置至少21条BVCI了

2)TLLI,协议上说,是Temporary logical link Identity,与TMSI相关.会动态改变

那么,在进行GPRS业务时,一般是手机分配,还是SGSN分配?

3)在BSSGP消息里面,Alignment octets信元是为了4字节对齐,但我不知道,这里的四字节对齐是从哪里开始算起,从BSSGP PDU

BVCI的概念你可以理解为一个小区，一个BVCI就对应一个小区，这个在很多厂家的系统里面是不能设的，是由SGSN分配的，可以通过一些命令查到当前的小区在系统里面对应的BVCI，也可以通过Gb口信令看出来。

关于TLLI：

TMSI和P-TMSI都是由四个字节也就是32个比特组成，所不同的是TMSI是由MSC分配的，而P-TMSI是由SGSN分配的，它们的目的是取代IMSI在空中接口进行传送，这样能起到保密的作用。如果手机在IMSI Attach之后又进行了GPRS Attach，那么手机里面既有TMSI，又有P-TMSI，根据3GPP TS23.003规范，这两者之间的Bit31和Bit30有所区别，如下：

In areas where both MSC-based services and SGSN-based services are provided, some discrimination is needed between the allocation of TMSIs for MSC-based services and the allocation of TMSIs for SGSN-based services. The discrimination shall be done on the 2
most significant bits, with values 00, 01, and 10 being used by the VLR, and 11 being used by the SGSN.

从规范中的描述可以看到，P-TMSI的Bit31和Bit30为11，而TMSI的Bit31和Bit30为00、01或10。

在了解了TMSI和P-TMSI之后，我们再来看一下什么是TLLI以及TLLI与P-TMSI之间的关系。前面讲到的P-TMSI是属于GMM层的一个参数，通常包含在GMM层的一些消息里面，如Attach request、Attach accept、Routing area update request、Routing area update accept等等。而TLLI是属于LLC层（逻辑链路控制层）的一个临时逻辑链路标识符，由四个字节也就是32个比特位组成，是手机和SGSN之间建立逻辑连接时需要用到的，我们接下来看一下TLLI的分类及构成。

根据3GPP规范TS23.003 Chapter 2.6，共有四种类型的TLLI：

1）Local TLLI

A local TLLI is built by a MS which has a valid P-TMSI as follows:

bits 31 down to 30 are set to 1; and

bits 29 down to 0 are set equal to bits 29 to 0 of the P-TMSI.

2）Foreign TLLI

A foreign TLLI is built by a MS which has a valid P-TMSI as follows:

bit 31 is set to 1 and bit 30 is set to 0; and

bits 29 down to 0 are set equal to bits 29 to 0 of the P-TMSI.

3）Random TLLI

A random TLLI is built by an MS as follows:

bits 30 down to 27 are set to 1; and

bits 0 to 26 are chosen randomly.

4）Auxiliary TLLI（辅助TLLI）

An auxiliary TLLI is built by the SGSN as follows:

bit 31 is set to 0;

bits 30 down to 28 are set to 1;

bit 27 is set to 0; and bits 0 to 26 can be assigned independently.

由此，我们可以看到，Local TLLI和Foreign TLLI都是由P-TMSI得出，只是Bit31与Bit30有差别而已，Local TLLI的Bit31和Bit30为11，而Foreign TLLI的Bit31和Bit30为10；Random TLLI的Bit0到Bit26是由手机随机产生的；Auxiliary TLLI（辅助TLLI）我们目前没有用到。

由于Local TLLI的Bit31与Bit30为11，其余Bit29－Bit0均从P-TMSI得出，而P-TMSI的Bit31与Bit30也为11，因此Local TLLI与P-TMSI在数值上完全一样。

那么，在前面提到的四种类型的TLLI中除了Auxiliary TLLI（辅助TLLI）目前没有使用外，其它三种分别在什么情况下使用呢？规范3GPP TS 24.008 Chapter 4.7.1.4.1定义如下：

1）手机具有有效的P-TMSI的情况下

If the MS has stored a valid P-TMSI, the MS shall derive a foreign TLLI from that P-TMSI and shall use it for transmission of the:

- ATTACH REQUEST message of any GPRS combined/non-combined attach procedure; other GMM

messages sent during this procedure shall be transmitted using the same foreign TLLI until the ATTACH ACCEPT message or the ATTACH REJECT message is received; and ROUTING AREA UPDATE REQUEST message of a combined/non-combined RAU procedure if the MS has entered
a new routing area, or if the GPRS update status is not equal to GU1 UPDATED. Other GMM messages sent during this procedure shall be transmitted using the same foreign TLLI, until the ROUTING AREA UPDATE ACCEPT message or the ROUTING AREA UPDATE REJECT message
is received.

After a successful GPRS attach or routing area update procedure, independent whether a new P-TMSI is assigned, if the MS has stored a valid P-TMSI then the MS shall derive a local TLLI from the stored P-TMSI and shall use it for addressing at lower layers.

简单的说明就是如果手机具有有效的P-TMSI，那么手机在发起Attach request和Routing area update request的时候，会使用由这个P-TMSI得出的Foreign TLLI（即把P-TMSI的Bit31和Bit30改为10），而在Attach和路由区更新成功完成之后则使用Local TLLI（和P-TMSI在数值上一样）。

2）手机没有有效的P-TMSI的情况下

When the MS has not stored a valid P-TMSI, i.e. the MS is not attached to GPRS, the MS shall use a randomly selected random TLLI for transmission of the:

- ATTACH REQUEST message of any combined/non-combined GPRS attach procedure.

The same randomly selected random TLLI value shall be used for all message retransmission attempts and for the cell updates within one attach attempt.

Upon receipt of an ATTACH REQUEST message, the network shall assign a P-TMSI to the MS. The network derives a local TLLI from the assigned P-TMSI, and transmits the assigned P-TMSI to the MS.

Upon receipt of the assigned P-TMSI, the MS shall derive the local TLLI from this P-TMSI and shall use it for addressing at lower layers.

如果手机没有有效的P-TMSI（例如第一次开机、上次Attach请求被拒绝等），那么手机在Attach request的时候只能使用IMSI来标识身份，而在LLC层将会使用Random TLLI。在Attach成功完成后，SGSN分配给手机一个P-TMSI，而手机也会从这个P-TMSI中得出Local TLLI。