Authentication in LTE

Once Attach request is received, MME has to authenticate the UE

This Authentication procedure involves,
1.AIR [Authentication Information Request] sent from MME to HSS
2.AIA [Authentication Information Answer] sent from HSS to MME
3.Authentication request sent from MME to UE
4.Authentication response sent from UE to MME

1.Authentication Information Request has username and visited PLMN ID in addition to many other AVPs. These AVPs are used by HSS to generate authentication parameters

2.Authentication Information Answer sends Authentication Info AVP in addition to many other AVPs. This will be used by MME to authenticate UE

3.MME sends AUTN, RAND in authentication request

4.UE uses RAND and generates AUTN and XRES. If AUTN sent in Authentication request matches the one UE generates, UE has successfully authenticated MME. Now UE send Authentication Ans which has
XRES.

MME checks this XRES received from UE against the one received from HSS. If both match, MME has successfully authenticated the UE

In this way mutual authentication happens.

Authentication can also happen during inter-RAT TAU, periodic TAU

LTE DEDICATED BEARER SETUP

DEDICATED BEARER -

UE should have default bearer before dedicated bearer can be created. However, in case of Piggybacked dedicated bearer, default and dedicated bearers are simultaneously activated
UE can create a max of 11 bearers.1 has to be default, rest can be dedicated
Every APN will have 1 default bearer.

EBI varies from 1 to 9

QCI [Qos Class identifier] range 1 to 9
1-4 : Dedicated bearer
5-9 : Default bearer

Default bearer has MBR[Maximum Bit rate], Dedicated bearer has GBR[Guaranteed bit rate]. MBR = GBR for dedicated bearer


Who initiates it ?
Network initiates it. UE can also do it by sending Bearer resource allocation
Dedicated bearer creation can be initiated by PGW based on local config or based on PCRF

Dedicated bearer can be created in one of the following ways -

1.Piggybacked 

Piggybacked dedicated bearer can be done during attach and additional PDN creation
The outcome of dedicated bearer creation depends on whether default bearer activation was successful or not
The dedicated bearer creation request can be sent to UE along with attach accept/activate default bearer request[during additional PDN conn req]
Multiple dedicated bearer creation request can be sent piggybacked
Request is always piggybacked to response
For ex : Create bearer request is piggybacked to create session response
Modify bearer request is piggybacked to create bearer response
Attach accept is always sent first, before activate dedicated bearer request

2.Non-Piggybacked -

If the request is not sent piggybacked, i.e. if it is sent after attach, before attach complete has been sent, MME shall wait for completion of attach and then send out activate dedicated bearer request.
In other words, it would be like a normal non-piggybacked dedicated bearer creation



Reference :
3gpp  23.401
5.4.1 and Annex F