We’ve seen that for each terminal exchanging signalingwith the base station, there’s an S1-AP connectionwhich enables a direct exchange of signalingbetween the terminal and the MME.我们已经看到，对于与基站交换信令的每个终端，存在S1-AP连接，其使得能够在终端和MME之间直接交换信令。
How is the S1-AP connection established?如何建立S1-AP连接？
That is the question we’re going to answer in this video.这是我们将在本视频中回答的问题。
We saw, in week three, the random-access mechanismand verification of the RNTI.我们在第三周看到了RNTI的随机访问机制和验证。
Let’s presume, at this moment, that both RNTIs are confirmed.现在让我们假设两个RNTI都得到确认。
Let’s take the example of anattachment to the network.我们以网络附件为例。
The message sent by the mobile terminal is anEMM Attach Request, which will be put inan RRC message called RRC Connection Setup Complete.移动终端发送的消息是EMM附着请求，其将被置于称为RRC连接建立完成的RRC消息中。
This message is associated to an RNTIand an LCID.该消息与RNTI和LCID相关联。
The eNodeB will choose the identity of the local S1-AP connection, for example, ID1 and send anS1-AP Initial UE Message with the value ID1.eNodeB将选择本地S1-AP连接的标识，例如ID1，并发送值为ID1的S1-AP初始UE消息。
Due to the encapsulation mechanism, thismessage contains the EMM Attach Request ,this message issued by the terminal.由于封装机制，该消息包含EMM Attach Request，该消息由终端发出。
The MME will store the receivedvalue, choose its own connectionidentity and memorize the linkbetween the two IDs.MME将存储接收的值，选择其自己的连接标识并记住两个ID之间的链接。
The MME then sends back a message called S1-AP DownlinkNAS Transport (we’ll cover the term NAS in the next video) with the two connection identitiesand the response to the Attach Requestwhich here is an authentication request toverify that the terminal actually has the correct secret key.然后，MME发回一条称为S1-AP下行链路NAS传输的消息（我们将在下一个视频中覆盖术语NAS），其中包含两个连接标识和对附着请求的响应，这是一个验证终端的验证请求 实际上有正确的密钥。
What the eNodeB does is: store ID 2(chosen by the MME) and also the link between ID 1 and ID 2.eNodeB的作用是：存储ID 2（由MME选择）以及ID 1和ID 2之间的链接。
Of course, it will also retransmit the message to the terminal.当然，它也会将消息重新发送到终端。
For all the following messages, we will proceed in the same way.对于以下所有消息，我们将以相同的方式进行。
A response from the terminal, forexample, an Authentication Response,is placed in an RRC message, arrivesat the eNodeB and the eNodeBadds ID1 and ID2, since itnow knows the connection identities associated with that terminal.来自终端的响应（例如，认证响应）被置于RRC消息中，到达eNodeB并且eNodeB添加ID1和ID2，因为它现在知道与该终端相关联的连接标识。
Because these connectionidentities are numbered over three or four bytes, avery large number of simultaneous connections are possible.因为这些连接标识的编号超过三个或四个字节，所以可以进行大量的同时连接。
The MME can manage these simultaneous connections withoutcaring about the precise identity of theGUTI, which can vary from time to time for a terminal.MME可以管理这些同时连接，而无需关心GUTI的精确身份，GUTI可以随时间变化。
Note that S1-AP also has other functions.请注意，S1-AP还具有其他功能。
For instance, It is used for the exchange of generalconfiguration messages between the eNodeB and the MMEbut its most important function isto enable the exchange of messages between the MME andthe eNodeB about a certain terminal.例如，它用于在eNodeB和MME之间交换一般配置消息，但其最重要的功能是使MME和eNodeB之间能够在某个终端之间交换消息。
All these exchangeslinked to a certain terminal are also linked to a connection identifier.链接到某个终端的所有这些交换也链接到连接标识符。
转载于:https://www.cnblogs.com/sec875/articles/10020874.html
