Mobile Packet Core

The UE makes an 'Attach Procedure' in order to register to the network.

The main identifier for the UE is called the 'IMSI'.

When the UE makes an 'Attach' for the first time, it will send its IMSI to the network.

At the end of the Attach procedure, the network will send a temporary identifier to the UE. This temporary identifier is called the 'P-TMSI'.

When the UE makes an 'Attach' to the network in the future, the UE will send its P-TMSI to the network instead of its IMSI.

This is because for security reasons, the UE shouldn't frequently send its IMSI to the network.

Attach Procedure signaling

Below graph shows the Attach procedure signaling,

1. The UE sends 'Attach Request' to the SGSN.

• If this is the fist 'Attach' for the UE, The UE will send its IMSI in this message. If it is not the first Attach, the UE will send the P-TMSI assigned to it from previous Attach.

2. Identification and Authentication

• The SGSN identifies the UE and authenticates it along with the HLR.

3. 'Update Location' and 'insert subscriber data'

• 'Update Location' means that the SGSN sends its address to the HLR
• This is done because the HLR should always know which SGSN is currently serving the UE.
• 'Insert Subscriber data' means that the HLR sends the subscription information for this UE to the SGSN.
• The SGSN will store this data in its memory in a table called the 'MM context'. 

4. Attach Accept

• The SGSN sends 'Attach accept' to the UE including the P-TMSI assigned to it.

5. Attach Complete

• The UE will store the P-TMSI in the MM context exist in its memory, and reply with 'Attach complete'.

Summary

After the Attach Procedure succeeds, the following should be accomplished.

• An 'MM context' is established on the UE, and the SGSN.
• The SGSN authenticated the UE.
• The HLR knows the address of the SGSN that serves the UE.
• The SGSN received the subscription information of the UE, and stored it in its MM context.
• The UE received the P-TMSI and stored it in its MM context.