Wednesday, August 19, 2009

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


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


Manish said...


It is a power pack article with all the essential procedure of LTE with thier ref. Anyone can understand that to write everything concisely needs more efforts that you have put.

Keep it up

Regards and Keep Smiling

Santosh said...

Thanks Mate!

I never realised it was going to be so tough to put everything together. One hour with 3GPP specs and you will already feel dizzy. I am still trying fish more out of the specs and make the post little meaningful.

vkkstyle said...

good post,

its really good information tht u have shared with this blog.Keep posting

Sanjay said...

Hi Santhosh,
Can you tell me the difference between EMM messages and ESM messages? Also what is the purpose of PTI (procedure transaction identity) in ESM messages?

Santosh said...

Hi Sanjay

For EMM and ESM refer to this.

PTI is procedure transaction ID. This is used to uniquely identify a procedure. For eg, UE initiated Dedicated bearer creation/deletion. Look at 23.401 for more procedures.


Girish V said...

Hi Santosh,
I have a doubt in Service Request Procedure in LTE. In the SR message there is no IE which describes about the Service Type (i.e. data or signaling), however in the legacy system (legacy 3gpp) it was available. Why?

Santosh said...

Hi Girish

I dont think there is a need to mention whether Service request procedure is initiated to do data or signalling later. I believe Service request is used to move UE from IDLE to connected mode.

Not aware of 3G though.

Thanks, Santosh

Girish V said...

Hi Santosh
Thanks for your reply. I understand that the SR procedure is used by the UE to move from IDLE to CONNECTED, but doesn't the service type make any difference at the core network? May be it doesn't make any difference... why don't we use any identity in a service request message? how is the network going to identify the UE? The spec says SR message is a non-standard L3 message. What was the reason behind defining SR message to be a non-standard L3 message?

Santosh said...

Hi Girish

I am not sure why Service request is called non standard L3 message. But I think we send S-TMSI in service request so that network can identify UE.

buyukkoc said...

you have a great site...there is a slight confusion of what the dedicated bearer deactivation would result: on the one hand, deactivating the dedicated bearer the traffic on it can be captured by "catch all" and place on default bearer, on the other hand this is not one would want..If you define a TFT for default bearer (which is optional I think) one can explicitly define what goes there and dedicated traffic deactivation would not result any new traffic on default..Are there mechanisms for this, what document? Best regards
Cagatay Buyukkoc

Santosh said...

Hi there

Interesting! Here is what I think!

Typically we want to classify the traffic and assign a particular QoS to it based on the subscription. However you will still need sommething called as "pass all" where you can send any data without caring much about the QoS. Now it really doesnt make sense to define a TFT for "pass all" tunnel (default bearer)as TFT is for picking out a particular kind of traffic.

As far as I know networks will support dedicated bearer and they wont be deactivated soon enough so that traffic will have to flow on default bearer. Even if that is the case than what is the harm in running traffic on default bearer. I believe its more of a implementation specific issue.

Cheers, Santosh

buyukkoc said...

Thanks Santosh,
i agree with first paragraph but not with second..dedicated traffic SHOULD NOT be put on default as it goes against the grain of being dedicated and hence have a particular QoS rules...My question was more on the mechanics of how this is done, which docs cover this case...there are so many docs and some details are buried...


Santosh said...

Hi Cagatay

Dedicated bearers are explained in 3GPP TS 23.401. Concept of dedicated bearers has come up to meet the SLA's. Mechanism lies in TFT and QoS values. The above spec talks of it. Let me know if you need anything else.

Cheers, Santosh

Anil Rout said...

Can you please clarify me, what will happen (what should be the response by UE)if the network sends QCI value as GBR(suppose 1)in Activate Default EPS bearer context request ???



Santosh Kumar Dornal said...

Ideally network shouldnt be doing it. But if network does, then UE may reject the request.