Linkage between the EPS MM and EPS SM

3GPP2 24.301, 4.2 节：

During the EPS attach procedure, the network activates a default EPS bearer context. Additionally, the network can activate one or several dedicated EPS bearer contexts in parallel.

To this purpose the EPS session management messages for the default EPS bearer context activation are transmitted in an information element in the EPS mobility management messages. The UE and the network execute the attach procedure, the default EPS bearer
context activation procedure, and the dedicated EPS bearer context activation procedure in parallel. The UE and network shall complete the combined default EPS bearer context activation procedure and the attach procedure before the dedicated EPS bearer context
activation procedure is completed. The success of the attach procedure is dependent on the success of the default EPS bearer context activation procedure. If the attach procedure fails, then the ESM procedures also fail. Except for the attach procedure and
the service request procedure, during EMM procedures the MME shall suspend the transmission of ESM messages. During the service request procedure the MME may suspend the transmission of ESM messages. Except for the attach procedure, during EMM procedures the
UE shall suspend the transmission of ESM messages.

LTE 将attach procedure 和defualt Bearer activate 绑定在一起， 在attcah 完成的同时， default Bearer 也必然建立。如果default Bearer失败，那么attach 过程也就失败。

虽然MM 和 SM 归属于 两个不同的部分，但是现在将default bearer作为attach 过程中的一个必选部分。

这么做法，主要是为了考虑到随后的decadicate bearer的过程。并且LTE 提出了 always online 的概念，一旦 终端attach完成，终端就与PGW 之间建立了default tunnel。并获得相应的IP。

dedicated bearer需要在default bearer的基础上建立， 那么将default bearer 考虑为attach 的一部分就是顺理成章的事情。

default bearer的建立并没有再选取一条专门的NAS消息进行建立，而是通过在MM 过程中，使用一个NAS的Infomation element，包含于NAS for MM 的消息里 ，来完成建立。

这部分的做法，在GSM ， UMTS 并没有。 在GSM.,UMTS 中，MM,CC 还是基本上两个独立的过程。 Location Update 的过程只是完成了最初的attach过程。随后的Voice Call需要有单独的信令来完成。