LTE Call Flow explained

The call flow in the LTE network is unique among mobile communication standards and represents the signaling and sessions established across the network.

The LTE call flow navigates over the elements of the network going through certain steps in order to complete its end-to-end signaling from the user equipment (UE) all the way to the rest of the network components. 

Further, we will discuss the following topics:

LTE Call Flow explained 1

The main components of the LTE network

  • User equipments
    Mobile phones, tablets, IoT etc.
  • LTE eNodeB
  • Core netwwork elements(MME, HSS, SGW and PGW)
    MME, HSS, SGW and PGW

These elements communicate with each other through interfaces, such as S1 MME, S6a and others to set up signaling and session. 

The communication starts with eNodeB and the UE requesting Radio Resource Connection (RRC LTE).  It goes all the way to the core network components that exchange some messages with each other. At the end of the call flow the UE establishes a successful connection to the network through the GPRS Tunnelling Protocol (GTP) and the buffered data will be passed to the UE LTE call flow architecture.

LTE call flow architecture

  • Radio Access Network (RAN)

    eNodeB is the RAN of the LTE call flow. It is a part of the E-UTRAN radio access network and is the component that allows UEs to connect to the LTE network.
  • Evolved Packet Core (EPC)

    The elements of the Core Network that are part of this process are:

    › Mobility Management Entity (MME)
    › Home Subscriber Server (HSS)
    › Serving Gateway (SGW)
    › Packet Data Network Gateway (PGW)

The LTE Call Flow process

  • 1.
    Beacon (MIB, SIB)
    Master Informations Block (MIBs) and System Information Block (SIBs) elements allow UE to find and sync itself to network.
  • 2.
    Random Access Preamble (RAP)
    Is the first message from the UE to eNB, to achieve uplink synchronization in order to obtain the resource for the third message.
  • 3.
    Random Access Preamble Response (RAP Response)
    Random Access Preamble response allows the UE to send further messages.
  • 4.
    Radio Resource Connection (RRC LTE)
    The UE uses UL-SCH allocation message to eNodeB which contains UE identity (typically S-TMSI: MMEC+M-TMSI) and the establishment cause for the RRC connection.
  • 5.
    Radio Resource Connection Setup (RRC Setup)
    eNodeB sends DL-SCH message to the UE in order to create the signaling radio bearer (SRB) . The message contains: configuration parameters for uplink RLC, UL-SCH, Power Head Room (PHR) and Uplink Power Control.
  • 6.
    PDN Connectivity
    The UE sends a message to eNodeB which contains: RRC has been completed, Initiate the Attach procedure as Non-access stratum (NAS) payload and PDN Connectivity request.
  • 7.
    Attach PDN request
    eNodeB will send its first message to the core network passing the attach request to the MME. This message is sent via S1AP interface and it contains the initial UE message which includes: the PDN Connectivity Request , the Tracking Area Identify (TAI) and E-UTRAN Cell Global Identifier (ECGI).
  • 8.
    Authentication request and info
    The MME will reach the HSS will send the scurity tuple to the MME containing K-ASME, AUTN, XRES and RAND.
  • 9.
    Authentication response
    The UE sends the Autch response value which was computed from the key K, AUTN and RAND.
  • 10.
    Security Mode Complete
    Security mode command MME sends the encryption and integrity protection algorithms and key selection identifier (KSI-ASME). The UE response message back to the MME with NAS ciphering and integrity protection.
  • 11.
    Location update request
    Acknowledgment message sent from HSS to MME that contains PDN subscription contexts (EPS subscribed QoS profile and the subscribed APN-AMBR).
  • 12.
    Session request
    Create session request message from the Mobility Management Entity (MME) to the Serving Gateway(SGW) to create a GTP tunnel.
  • 13.
    Default Bearer Request
    Serving Gateway (SGW) will send this request to Packet Data Network Gateway (PGW), to create a new entry in its EPS bearer context table and generates a Charging Id.
  • 14.
    Default bearer response
    The default bearer response from the PGW to the SGW will contain PDN GW User Plane address, PDN GW TEIDs User and Control Plane, EPS Bearer Identity and QoS. On the other hand, PGW will also send Downlink data that will be buffered in the SGW for now.
  • 15.
    Session response
    Acknowledgment message from SGW to the MME that indicates the establishment of GPRS Tunneling Protocol for control (GTP-C) tunnle.
  • 16.
    Initial context setup request
    MME will send eNB initial context setup message containing S1 interface context setup request, NAS attachement accept and activate default bearer request.
  • 17.
    RRC security mode
    The eNodeB will reach the UE with RRC security mode message with the AS integrity protection and encryption algorithms and START parameters. The UE sends to eNodeB acknowledge message that uses the newly activated keys to encrypt and integrity protection.
  • 18.
    RRC reconfig
    eNodeB will send RRC reconfig to activate the default radio bearer.
  • 19.
    RRC complete
    The next thing happening in the LTE call flow is that the UE will send acknowledgment message and attach RRC LTE complete (EPC Bearer Identity, NAS sequence number, NAS-MAC).
  • 20.
    Data flowing
    Now, the UE has successfully established a connection to the network and the buffered data will be passed to the UE in the Data Radio bearer.

Test and enable full LTE mobile networks

  • ✔ Is an eNodeB for test that provides the full functionality of an LTE network on your desktop.

    ✔ It is intended for mobile network operators labs that need LTE test equipment for security tests. It can be used as a callbox for IoT testing, M2M application developement and manufacturing, mobile phone vendors and researchers.
  • ✔ MiniCore is a core network which is mostly used for testing, but can also act as the core network part of a mobile network for under 500 users.

    ✔ It combines our YateHSS/HLR, YateUCN and YateSMSC in a small package. It can be used a second core network for testing. In a laboratory it can be paired with the LTE LabKit for a full LTE/GSM network.

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