LTE ENodeB or  Evolved Node B 

LTE eNodeB is the component in LTE Radio Access Network(RAN). The eNodeB full form is Evolved NodeB. The predecessor UTRAN, have NodeB which connects to the core network via RNC and in GSM its similar to BTS. While eNodeB connects to the core network directly over the S1 interface.

In the core network, it connects to the MME and SGW. MME or SGW is similar to the MSC or SGSN in the 3G network. The tutorial will explain the interfaces and functionality of eNodeB in LTE.

eNodeB is a bridge between UE and LTE core networks. Earlier in UTRAN, because of RNC, the packet transfer time was more as compared to eNodeB in LTE. This is an optimization in the LTE network for fast data speed.

eNodeB and S1 interface:

ENodeB connects to the LTE-EPC over the S1 interface. The interface further divides into two subinterfaces, the control plane (S1-C) and user plane(S1-U). S1AP is the protocol between eNodeB and LTE-EPC.

What are the S1AP protocol and S1 data transport?

S1AP protocol is specified in 3GPP TS 36.413 for the control plane (S1-C).  The S1AP protocol stack uses IP layer connectivity between eNodeB and the LTE MME in the core network.  S1 data transport (S1-U is between eNodeB and SGW over IP networks.

S1AP or S1-MME :

S1 Application Part of S1AP does the actual RAB establishment and other control functions. The S1AP protocol uses the services of a reliable SCTP protocol.  SCTP inturns use IP and underlying data link and physical layer.  The following are the functions of S1AP or S1-MME protocol.

  1. E-RAB setup – the procedure is from MME to eNodeB.
  2. E-RAB Modify  – from MME to eNodeB
  3. E-RAB release – this may be initiated from MME or eNodeB
  4. UE-Initial Context setup – MME to eNodeB
  5. UE context release – the could be from MME to eNodeB or vice versa.
  6. UE context Modify – MME to eNodeB
  7. Handover – This is among the MME, New eNodeB, and previous eNodeB. Requires when a subscriber moves to a new eNodeB under the same MME.
  8. PATH switch request – From eNodeB to MME to switch a new GTP tunnel.

S1-U or S1 data transport :

The S1-U interface is between eNodeB and the SGW. S1-U protocol stack uses the GTP V1 protocol over UDP and IP. The protocol specification for S1-U is in 3GPP TS 36.414.   The control plane signaling is done over the S11 interface. The S11 interface uses the GTPV2C protocol. On initial attach, eNodeB set up a session with the PGW via SGW.

In-home network, PGW is the gateway between SGW and external packet networks or the Internet. EnodeB sends the user data to the internet over S1-U protocol and gets the user data from the internet on the same,

What is HeNb?

HeNB is  Home eNodeB. This is a small range tower that connects a mobile device inside the home over a 4G spectrum. It is similar to a femtocell.  On the network side, HeNB connects to the mobile network using a high-speed internet connection. HeNB has all the advantages of a femtocell.

eNodeB protocol stack :

The eNodeB connects one side to the UE over radio Interface and on another side to the MME and SGW over a wired interface.  This section will mention the layers in the eNodeB protocol stack.

UE to eNodeB protocol stack:

ENodeb -> RRC->PDCP->RLC->MAC->PHYSICAL —-Radio Interface –>PHY->MAC->RLC->PDCP->RRC->UE

eNodeB to MME protocol stack:

ENodeB->S1AP->SCTP->IP->MAC —- Ethernet Cables —> MAC->IP->SCTP-> S1AP –> MME

eNodeB to SGW protocol stack:

ENodeB->GTP-U–>UDP->IP->MAC— Ethernet Cable —> MAC->IP->UDP->GTP-U –> SGW 

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