In this tutorial we shall see what is attach request and the Information Elements (IE) present in Attach Request.
Attach Request is a NAS layer message procedure.
It will be initiated by UE to MME. As eNB will not have NAS protocol, the Attach request will be included inside RRC Connection Setup Complete message.
“dedicatedInfoNAS” is the Information Element inside RRC Connection Setup Complete.
Then this message will be forwarded to MME by eNB.
The IE available in Attach Request are:
1. EPS Attach Type
2. EPS mobile Identity
3. ESM message container
4. Additional Update Type
5. Voice domain preference and UE’s usage setting
6. Last Visited Registered TAI
7. UE network capability
1. EPS Attach Type: It is used to indicate the type of the requested attach. It can be EPS attach or combined EPS and non EPS services (EPS/IMSI attach) (Non EPS services: CS fallback, SMS)or it can be EPS Emergency attach.
2. EPS mobile Identity:
It can be GUTI or IMSI or IMEI.
If the UE has a GUTI value it will include it.
If it do not have a valid GUTI, UE shall include IMSI.
During emergency attach, IMEI will be included.
3. ESM message container:
This IE is included to carry a single ESM message.
It can have EPS bearer identity, Procedure transaction identity, PDN connectivity request, PDN type, Request type etc.
4. Additional Update Type:
The UE shall include this IE if the UE requests “SMS only”.
5. Voice domain preference and UE’s usage setting:
This IE shall be included if the UE supports CS fallback and SMS over SGs, or if the UE is configured to support IMS voice, but does not support 1xCS fallback.
6. Last Visited Registered TAI:
This IE shall be included if the UE holds a valid last visited registered TAI.
7. UE network capability:
The UE shall include this IE, unless the UE performs a periodic tracking area updating procedure.
Reference: 3GPP TS 24.301