LTE Initial Attach Call Flow ( Part 1 )

In this case we will look initial attach procedure of an unknown UE to a LTE network for the first time.

1)  Radio Link synchronization:
In order for the UE to connect to eNB it needs to establish a radio connection with eNB. UE selects the eNB through PLMN selection and cell search procedure. After an RRC signalling connection the UE can then communicate with the eNB. UE is in EMM-De-registered state at this point of time.

2) ECM connection establishment:
UE sends an attach Request (IMSI, UE capability) message to MME via eNB through NAS layer.In order for the attach request message to  be delivered to MME, ECM connection between UE and MME is required.At the end of ECM connection, RRC connection between UE and eNB  and S1 signalling  connection between eNB and MME is required.NAS messages are send as RRC messages when passing through RRC connection and SI AP when passing through S1 signalling collection.

3) RRC Connection Establishment:
An RRC connection is established between the RRC layers of UE and eNB.
The procedure of RRC connection between UE and eNB is as follows.

UE--->eNB [ RRC connection request ]
UE sends an 'RRC connection request' to eNB  ( Establishment Cause=“Mobile Originating Signalling”). The message sent by UE is  delivered to eNB through SRB 0 and  CCCH ( Common control channel )
SRB 0 is used by all UE's in a cell.

eNB --> UE [ RCC connection setup ]
The eNB allocates SRB1 to UE by sending an RRC connection setup message, which is delivered through SRB 0 and CCCH.Now the uplink/downlink resources of the UE are controlled by eNB.UE can now use the resources allocated to UE by eNB through SRB configuration.

UE --> eNB [ RRC Connection Setup Complete ]
The UE notifies the eNB that the RRC connection setup is completed by sending it an RRC Connection Setup Complete message through SRB 1 and DCCH (Dedicated Control Channel).The attach request message that was delivered to the NAS layer is sent to eNB along with RRC connection setup complete message.

4) S1 Signalling connection establishment:

Control messages between eNB and MME are sent Over S1-MME interface as embedded in S1 AP messages. S1-AP messages are delivered through S1 signalling connections dedicatedly for each user.S1 Signalling connections are defined by an ID pair ( eNB UE  S1 AP ID , MME UE S1 AP ID). These are allocated by eNB and MME for identifying UE's .The eNB allocates an eNB UE S1 AP ID for establishment of S1 signalling connection and sends the MME an attach request as embedded in  an ' initial UE message'. The Attach Request message is delivered as embedded in the NAS-PDU field of the Initial UE Message. The Initial UE Message consists of the following information elements:
Initial UE Message (eNB UE S1AP ID, NAS-PDU, TAI, ECGI, RRC Establishment Cause) 

  • eNB UE S1AP ID: ID identifying UEs in an eNB over S1-MME interface (Uplink) 
  • NAS-PDU: a NAS message (Attach Request) 
  • TAI: shows the TA a UE is located in 
  • ECGI: shows the cell a UE is located in 
  • RRC Establishment Cause = mo-Signalling: indicates the signalling was generated by a UE

Now MME received the UE intial context message and assigns an  MME SI AP UE ID. After the allocation the signalling connection between the eNB and MME is established. MME S1 AP UE ID is used later when MME identifies UE over S1-MME interface.

5) IMSI Acquisition 
The NAS layer of the MME acquires the IMSI of  the UE from the intial message and also determines the network capability of the UE and the security algorithms it supports.Now the MME performs authentication and NAS security keys setup procedures for secured delivery of NAS messages.

An Overview Till Now :

  • UE Has sent an RRC connection request.
  • eNB Receives RRC connection request and sends UE RRC connection setup message and allocates resources to UE
  • UE sends an 'Attach Request' along with RRC connection complete message to eNB 
  • eNB initiates S1 Signalling connection  between eNB and MME for the UE 
  • MME recives Attach request from UE
  • MME knows UE IMSI and network capability.

The next will be Available here soon

Please Refer to the List of other topics in LTE tutorials


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