LTE Unleashed - (Work in Progress)

Folks! This is very much work in progress. I wanted to post everything in one single post, but I decided. The below work is still a draft, more like a rough notes, check back soon for more details. All your comments are greatly appreciated. Thanks!

Reference :

3GPP TS 23.401 : General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access

3GPP TS 24.301 : Universal Mobile Telecommunications System (UMTS);Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS);

3GPP TS 24.007 : Digital cellular telecommunications system (Phase 2+);Universal Mobile Telecommunications System (UMTS); LTE; Mobile radio interface signalling layer 3; General Aspects

3GPP TS 36.413 : LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); S1 Application Protocol (S1AP)

3GPP TS 36.300 : LTE;Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2

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During the Initial Attach procedure the Mobile Equipment Identity is obtained from the UE. The MME operator may check the ME Identity with an EIR. At least in roaming situations, the MME should pass the ME Identity to the HSS, and, if a PDN-GW outside of the VPLMN, should pass the ME Identity to the PDN-GW.

Initial Attach Procedure   

3GPP TS 24.301 : NAS signalling:

The non-access stratum (NAS) described in the present document forms the highest stratum of the control plane between UE and MME at the radio interface (reference point "LTE-Uu"; see 3GPP TS 23.401 [10]).

Main functions of the protocols that are part of the NAS are:

• the support of mobility of the user equipment (UE); and
• the support of session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway (PDN GW).

3GPP TS 24_007 v 8.2.0 : Max EBI is 11.

Link between EMM and ESM

• EPS session management messages for the default EPS bearer context activation are transmitted in an information element in the EPS mobility management messages
• The success of the attach procedure is dependent on the success of the default EPS bearer context activation procedure

Avoiding NAS security here

EPS Session Management : ESM

24_301 : EPS Session Management (Chapter 6)

• The main function of the ESM sublayer is to support the EPS bearer context handling in the UE and in the MME
• The ESM comprises procedures for:
  • the activation, deactivation and modification of EPS bearer contexts; and
  • the request for resources (IP connectivity to a PDN or dedicated bearer resources) by the UE.

EPS bearer contexts can remain activated even if the radio and S1 bearers constituting the corresponding EPS bearers between UE and MME are temporarily released.

Default and dedicated EPS bearer contexts can be modified. Dedicated EPS bearer contexts can be released without affecting the default EPS bearer context. When the default EPS bearer context is released, then all dedicated EPS bearer contexts linked to it are released, too

The UE can request the network to allocate, modify or release additional EPS bearer resources. The network decides whether to fulfil a request for additional resources by activating a new dedicated EPS bearer context or modifying an existing dedicated or default EPS bearer context.

Two types of ESM procedures can be distinguished:

1) Procedures related to EPS bearer contexts:

These procedures are initiated by the network and are used for the manipulation of EPS bearer contexts:

- default EPS bearer context activation;

- dedicated EPS bearer context activation;

- EPS bearer context modification;

- EPS bearer context deactivation.

2) Transaction related procedures:

These procedures are initiated by the UE to request for resources, i.e. a new PDN connection or dedicated bearer resources, or to release these resources:

- PDN connectivity procedure;

- PDN disconnect procedure;

- bearer resource allocation procedure;

- bearer resource modification procedure.

When combined with the attach procedure, the PDN connectivity procedure can trigger the network to execute the following transaction related procedure:

- ESM information request procedure.

A successful transaction related procedure initiated by the UE triggers the network to execute one of the procedures related to EPS bearer contexts. The UE treats the start of the procedure related to the EPS bearer context as completion of the transaction related procedure.

When the UE or the network initiates a transaction related procedure, it shall include a valid procedure transaction identity value in the message header and set the EPS bearer identity to "no EPS bearer identity assigned".

Attach request is an EMM message.

Attach request is at Layer 3 in UE. It is send together with RRC parameters indicating the selected network and old GUMMEI.

Few things :-

-- GUTI : Globally unique temporary identity.

-- GUMMEI : Globally unique MME identifier. This consists of PLMN id, MME group Id, and an MME code. MME code is used in the eNB by NAS node selection function to select MME.

Once Attach request reaches eNB, eNB creates enter Initial Context setup function. The communication between eNB and MME is over S1 interface and S1-AP is protocol used.

Initial Context Setup Procedure : 36.300

eNB will send S1-AP initial UE message along with NAS service request, which is nothing but Attach Request. (FFS)

This will trigger MME to send Create session request. SGW will send a session response. Once the session response is recieved MME will send a S1-AP Initial Context Setup Request along with NAS message, which is Attach Accept. This message also contains EPS bearer QoS, EBI and TEID for SGW for user plane.

eNB will send Attach accept message to UE. UE shall store QOs. APN will be provided to UE if it is not aware of.

The UE may provide EPS Bearer QoS parameters to the application handling the traffic flow(s). The application usage of the EPS Bearer QoS is implementation dependent. The UE shall not reject the RRC Connection Reconfiguration on the basis of the EPS Bearer QoS parameters contained in the Session Management Request. UE may recieve an IP address too.

UE sends RRC message to eNB, indicating the completion of procedure.

eNB sends Initial Context Response message which includes TEID of the eNB and address of eNB. In the mean time Ue sends Attach complete with EBI.

After the Attach Accept message and once the UE has obtained a PDN Address, the UE can then send uplink packets towards the eNodeB which will then be tunnelled to the Serving GW and PDN GW.

Upon reception of both, the Initial Context Response message and the Attach Complete message in, the new MME sends a Modify Bearer Request (EPS Bearer Identity, eNodeB address, eNodeB TEID, Handover Indication) message to the Serving GW

3GPP TS 23.401 Annex E : LTE Qos To Pre Rel 8 QoS mapping