LTE attach procedure is the procedure in which the UE registers to the network, and creates the EPS Bearer between the UE and the PGW, in order to be able to send and receive data, to and from the PDN.
In order to well understand this topic, you need to be familiar with,
LTE Network Architecture
After the LTE Attach Procedure ends, the following should be accomplished,
- The MME assigns the UE a GUTI.
- An EPS Bearer should be created between the UE and the PGW.
GUTI, is a temporary identifier that the MME assigns to the UE during the attach procedure. And, the UE uses this GUTI to identify itself to the network.
LTE Attach Signaling
- The UE sends 'Attach request' to the MME. This includes the GUTI of the UE received from the last attach, and the APN that the UE would like to connect to.
- The MME performs authentication and security for the UE, in order to authenticate the UE, and to secure the messages exchanged between the MME and the UE.
- The MME sends 'update location request' to the HSS, this includes the MME ID of the MME. This is done because the HSS should always know the MME ID of the MME that currently serves the UE.
- The HSS replies with 'Update location response. This includes the subscription information of the UE.
- Now the MME starts to create the EPS Bearer between the UE and the PGW by sending 'create session request' to the SGW. This includes the APN that the UE would like to connect to.
- The SGW sends 'create session request' to the PGW including the APN the UE would like to connect to.
- The PGW assigns an IP address and a QoS profile for this EPS Bearer, and replies with 'create session response', including the IP address and the QoS that the PGW assigned to the Bearer.
- The SGW sends 'create session response' to the MME including the IP address and the QoS that the PGW assigned to the Bearer.
- The MME sends 'initial context setup request' to the eNodeB, and this message includes ' Attach Accept' message that the eNodeB should forward to the UE. The 'Attach Accept' ,message includes the IP address and the QoS of the EPS Bearer.
- The eNodeB establishes the Radio Bearer with the UE, and replies the MME with 'initial context setup response'
- And the UE replies the MME with 'Attach Complete'
- Finally the MME performs 'modify bearer' with the PGW, in order to complete the exchange of the EPS Bearer parameters.