LTE Attach Procedure

At last I found what I needed. It was always there since beginning but I never found time to read it. So I went through the spec TS 23.401 (GPRS enhancements for EUTRAN) and I found all the call flows required. I started with the initial on, EUTRAN Initial Attach procedure. Now my interests lie in GTP based interfaces (I am avoiding PMIP here) and only on IP side. So I eliminate all the radio signals, S1- MME request response and what I arrive at is the below. A simple diagram depicting initial attach procedure of UE.

  

A UE needs to register with the network to receive services that require registration. This registration is described as network attachment. The always on IP connectivity for UE of the EPS is enabled by establishing a default EPS bearer during Network attachment. The attach procedure may trigger one or multiple dedicated bearer establishment procedures to establish dedicated EPS bearer for that UE. During the attach procedure UE may request for an IP address allocation.

Now lets look at the request/responses and the IE;s that are passed in them. As I said, I am not much of radio guy I will directly jump into

3. Create Default Bearer Request: MME ----> S-GW

MME selects a S-GW and allocate an EPS bearer Identity for default bearer associated with UE. Then it sends Create Default Bearer Request with following IE;s.

• IMSI
• MSISDN
• MME TEID for Control Plane
• PDN GW address
• PDN Address
• APN
• RAT Type
• Default EPS bearer QoS
• PDN Type
• APN-AMBR
• EPS Bearer Identity (EBI)
• Protocol COnfiguration Options
• Handover Indication
• ME identity
• User Location Information (ECGI)
• MS Info Change Reporting Support Indication
• Selection Mode
• Charging Characteristics
• Trace Reference
• Trace Type
• Trigger ID
• OMC identity
• Maximum APN Restriction
• Dual Address Bearer Flag
• Protocol over S5/S8.

Subscribed APN AMBR for the APN is provided in this message. MSISDN is included if the attach type indicates handover. Selection Mode indicates that a subscribed APN was selected. Charging characteristics indicates which kind of charging the bearer context is liable for.

4. Create Default Bearer Request : S-GW -------->PDN-GW

The S-GW creates a new entry in its EPS bearer table and sends a create default bearer request message to PDN GW indicated by the PDN GW address received in the previous step. This message contains

• IMSI
• MSISDN
• APN
• S-GW address for User Plane
• S-GW TEID of the User Plane
• S-GW TEID of the Control plane
• RAT Type
• Default EPS bearer QoS
• PDN Type
• PDN Address
• Subscribed APN-AMBR
• EPS bearer identity
• Protocol Configuration Options
• Handover Indication
• ME Identity
• USer Location Information (ECGI)
• MS Info change Reporting SUpport Indication
• Selection Mode
• Charging Characteristics
• Trace Reference
• Trace Type
• Trigger ID
• OMC Identity
• Max APN Restriction
• Dual Address Bearer Flag.

5. Create Default Bearer Response : PDN-GW ------> S-GW

The P-GW creates a new entry in its EPS bearer context table and generates a charging ID. THe new entry allows the P-GW to route user plane PDU's between the S-GW and the packet data network and to start charging. The PDN-GW returns a create default bearer response with following IE's

• PDN-GW address for the user plane
• PDN GW TEID of the user plane
• PDN GW TEID of the control plane
• PDN Type
• PDN address
• EPS Bearer Identity
• EPS bearer QoS
• Protocol Configuration Options
• Charging ID
• Prohibit payload compression
• APN Restriction
• Cause
• MS Info Change Reporting Action
• APN AMBR

6. Create Default Bearer Response : S-GW --------> MME

S-GW returns a create default bearer response to MME with following IE's.

• PDN Type
• PDN Address
• S-GW address for User Plane
• S-GW TEID for User Plane
• S-GW TEID for control plane
• EPS bearer Identity
• EPS bearer QoS
• PDN GW addresses and TEID's at the PDN GW for uplink traffic
• Protocol configuration options
• Charging ID
• Prohibit payload compression
• APN restriction
• Cause
• MS Info change Reporting Action
• APN-AMBR

These are the Default bearer request and response messages. Watch out more messages.

EPS Bearers -- Short Notes

I havent read anything for while, work is keeping me busy. Today i got a chance to get back to 3GPP Spec TS 23401 and I started reading the QoS in EPS bearers. Now LTE being all IP with no circuit swtching, the operation is little different from UMTS. In UMTS/GSM networks the phone will first get an IP address to do packet data. But in LTE its always IP. Lest see what EPS bearer is.

EPS bearer uniquely identifies traffic flow that recieves a common QoS treatment between a UE and a PDN GW.
Packet filters signalled in the NAS procedure are associated with unique packet filter identifier on per PDN connection basis. The EPS bearer Traffic Flow Template (TFT) is set of all packet fileters associated with EPS bearer.

One EPS bearer is estabalished when UE connects to PDN and that remains lifetime of that PDN connection to provide UE with always on IP connnectivity to PDN. THis is default bearer. Any additional bearer that is established to the same PDN is referred as dedicated bearer.

Up Link Traffic Flow Template (UL TFT) : - Set of uplink packet filters in TFT
Downlink Traffic Flow Template (DL TFT) : - Set of downlink packet filters in TFT
Every dedicted EPS bearer is associated with TFT.

More to follow, please correct me if I am wrong and comments are always welcome

Notes :

- S11 interface : MME and S-GW

- S5/S8 interface : S-GW and PDN

- Default and dedicated bearer

- Uplink TFT and Downlink TFT

- The decision to establish or modify a dedicated bearer can only be taken by the EPC and the bearer level QoS parameter values are always assigned by the EPC.

- No QoS negotiation between E-UTRAN and EPC during dedicated bearer establishment/modification is supported.

Will we support two TFT's? Both UL and DL TFT?

EPS Mobility Management and Connection Management

I was reading the spec 3GPP TS 23.401 v8.4.1 (GPRS Enhancements for E-UTRAN) today. Below is what i could make out about EPS Mobility Management (EMM) and EPS Connection Mangement (ECM).
Two states of EMM (EMM-Registered and EMM-Deregistered) and two states of ECM (ECM-IDLE and ECM-Connected) were described in the document. EMM states describe the mobility management statest that result from the mobility management procudeures e.g Attache and Tracking area update procedures.
ECM states describe the signallin gconnectivity between the UE and EPC. Definition of EMM States: EMM-De-registered : In the EMM‑DEREGISTERED state, the EMM context in MME holds no valid location or routeing information for the UE. The UE is not reachable by a MME, as the UE location is not known. EMM-Registered : The UE enters the EMM-REGISTERED state by a successful registration with an Attach procedure to either E-UTRAN or GERAN/UTRAN. The MME enters the EMM-REGISTERED state by a successful Tracking Area Update procedure for a UE selecting an E-UTRAN cell from GERAN/UTRAN or by an Attach procedure via E-UTRAN. In the EMM-REGISTERED state, the UE can receive services that require registration in the EPS.The UE location is known in the MME to at least an accuracy of the tracking area list allocated to that UE (excluding some abnormal cases).
In the EMM-REGISTERED state, the UE shall:
- always have at least one active PDN connection;
- setup the EPS security context.
Definitions of ECM States: ECM Idle : A UE is in ECM-IDLE state when no NAS signalling connection between UE and network exists. In ECM-IDLE state, a UE performs cell selection/reselection according to TS 36.304 [34] and PLMN selection according to TS 23.122 [10]. There exists no UE context in E-UTRAN for the UE in the ECM-IDLE state. There is no S1_MME and no S1_U connection for the UE in the ECM-IDLE state. ECM Connected : The UE location is known in the MME with an accuracy of a serving eNodeB ID. The mobility of UE is handled by the handover procedure.
The UE performs the tracking area update procedure when the TAI in the EMM system information is not in the list of TA's that the UE registered with the network, or when the UE handovers to an E‑UTRAN cell and the UE's TIN indicates "P-TMSI". For a UE in the ECM-CONNECTED state, there exists a signalling connection between the UE and the MME. The signalling connection is made up of two parts: an RRC connection and an S1_MME connection.

EMM State Model in UE

EMM State Model in MME

ECM State Model in UE

ECM State Model in MME