Difference between attach and service request message ?

Question

Does UE send APN information while registering with network using the "Attach request" message ?
How MME come to know that UE should attach to APN "XYZ" ?

Answer 1

Both the messages come under LTE NAS protocol but the objective of both messages are quite different.
Attach request message is sent by UE to MME when it want to register with network for example: when an UE switch on and a lte cell is available in that particular location.
Service request or Extended service request message is sent from UE to MME when UE want to send/receive data and UE is already registered with network.
I think you got the answer. If you have still any doubt please ask.

Answer 2

Difference between attach and service request message is -
1. when UE is in EMM-DeRegistered state and want to move in EMM-Registered state then UE needs to perform attach procedure.
2. In EMM-Registered state UE can be in ECM-connected, RRC-connected state or ECM-Idle, RRC-Idle state. If UE is in ECM-idle, RRC-Idle state during EMM-Registered state and UE has data to send in UL/getting paging for MT data then UE sends service request to move in ECM-connected,RRC-connected mode.

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best answer

Answer 3

On behalf of Bart (https://www.facebook.com/groups/LTE.Professionals/ )
Long story short Attach is to obtain the IP where Service Request happens after attach when UE goes out from emm-idle to establish NAS