LTE Initial Attach 的过程以及相关消息的具体解析 （三）

这段开始描述 MME, SGW, PGW 之间建立Tunnel 的过程。主要使用Create Session Req/Resp for default Bearer 和 Create Bearer Request/Resp for dedicated Bearer setup.

so, 1st one is MME send to SGW the Create Session Req, the message sample is :

1. The tunnel Endpoiint Identifier value is 0 , as the C-TEID for Uplink doesn't assign by the SGW.

2. PDN TYPE is : IPV6 , ask the PGW to assign the ipv6 for the UE.

The more important things are t he Fully Qualified Tunnel Endporint Identifier (F-TEID), PCO, and the Bearer Context.





There are two F-TEIDs, One is the Control Plane of MME with IP address , another one is for the PGW.( Which is selected by the MME accroding to the APN name by DNS).

SGW will do the Create Session REQ/RESP towards the IP of PGW. so that the TEID is "0" for the PGW, Need to wait the PGW to assign the value.

About the PCO (Ptorocol Configuration Options )

here is a answer that :http://lteuniversity.com/ask_the_expert/f/59/t/2422.aspx

The purpose of the Protocol Configuration Options (PCO) is to provide additional optional information about the destination network that the UE is connecting to. For example, when a UE connects to the Internet it
will be given an IP address and it will be given a PCO that includes the default gateway IP address, the DNS server address, etc. All of these additional pieces of information are given in the PCO.

Give to examples:

for the IMS accsss, the UE need to get the PCSCF address.

for the internet access, the UE need to get the DNS service ip address.



the first one , the "IP address allocation via NAS signalling" , i think that the UE ip will get be LTE, don't the use the DHCP to get from the external network.

for the Bearer Context: MME only assign a EBI, others value need the PGW to assign ( by the Policy and Charging rule from PCRF)



2nd one is the Create Session Req from SGW to PGW, most of message header and value are same as the 1st one sent from MME to SGW.

Except: The TEID AND IP for Control Plane are changed to SGW,

And in the Bearer Context: there is the TEID and IP of SGW towards PGW for User-Plane.





到这个地步，MME 和SGW 已经给出了下行信令面通道会使用到的TEID 和IP，并且SGW 也给出了下行通道会使用到的用户面的TEID和IP。

随后PGW 就会分配相应的QOS，信令面和用户面，但是这个qos 和charging rule 也收到PCRF 的控制。。。。扔到第4节描述下PCRF的流程，再回来写PGW的。