Solution Integrator Engineer

LTE SAE(System Architecture Evolution) consists UE, eNodeB and EPC(evolved packet core). Various interfaces are designed between these entities which include Uu between UE and eNodeB, X2 between two eNodeB, S1 between EPC and eNodeB. eNodeB has functionalities of both RNC and NodeB as per previous UMTS architecture. LTE is completely IP based network.
The basic architecture contains the following network elements.
1. LTE EUTRAN (Evolved Universal Terrestrial Radio) : It is a radio access network standard meant to be a replacement of the UMTS, HSDPA and HSUPA . Unlike HSPA, LTE's E-UTRA is an entirely new air interface system. It provides higher data rates, lower latency and is optimized for packet data. EUTRAN (Evolved Universal Terrestrial Radio) consists of eNB (Base station). EUTRAN is responsible for complete radio management in LTE. 
2. LTE Evolved Packet Core : The LTE EPC architecture consists of MME, SGW, PGW, HSS and PCRF.

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.
LTE Call Flow Chart
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.

Within an LTE network, LTE Interfaces connect the various components to or within the core.
Interfaces allow the MME, SGW and PGW to cooperate with other network elements (e.g. HSS or PCRF).
Each one of them is built in a standard way described by 3GPP. Each interface described here is taken from the 23.401 3GPP documentation.
These LTE interfaces are used within either between eNodeBs and the core, or within the Evolved Packet Core (EPC)

Each eNodeB (LTE radio base station) is connected to one and only one MME. The MME selects S-GW and PDN-GW, based on location and APN.

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