4G LTE Network Elements - Telecommunications Blog

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Friday, December 31, 2010

4G LTE Network Elements

We will speak about the LTE Network Elements .
LTE network comprises of two main segments.
       1. LTE EUTRAN
       2. LTE Evolved Packet Core.


EUTRAN consists of eNB.

    EUTRAN is responsible for complete radio management in LTE. When UE comes up eNB is responsible for Radio Resource Management, i.e it shall do the radio bearer control, radio admission control, allocation of uplink and downlink to UE etc. When a packet from UE arrives to eNB, eNB shall compress the IP header and encrypt the data stream. It is also responsible for adding a GTP-U header to the payload and sending it to the SGW. Before the data is actually transmitted the control plane has to be established. eNB is responsible for choosing a MME using MME selection function.
    As the eNB is only entity on radio side, the whole QoS is taken care by it. It shall mark the packets in uplink, i.e Diffserv based on QCI, and also schedule the data.

    Other functionalities include scheduling and transmission of paging messages, broadcast messages, and bearer level rate enforcements based on UE-AMBR and MBR etc.

LTE Evolved Packet Core (EPC)

    LTE EPC comprises of MME, SGW and PGW.

MME: - Mobility Management Entity

     MME is a control entity, which means it’s completely responsible for all the control plane operations. All the NAS signaling originates at UE and terminates in MME. MME does tracking area list management, selection of PGW/SGW and also selection of other MME during handovers.

     It is the first contact point for the 2G and 3G networks. MME is also responsible for SGSN selection during LTE to 2G/3G handovers.

      The UE is also authenticated by MME. All signaling traffic flow through MME so the same can lawfully intercepted. MME is also responsible for bearer management functions including establishment of dedicated bearers.

SGW: - Serving Gateway

     Serving gateway terminates the interface towards EUTRAN. For each UE associated with EPS, at a given point of time, there is a single Serving GW.

     SGW acts a local mobility anchor for inter eNB handovers. It also acts a mobility anchor for inter 3GPP mobility.

     SGW is responsible for packet routeing and forwarding, buffering the downlink packets and lawful interception. As eNB is responsible for uplink packet marking, SGW is responsible for downlink packet marking. One way to do this is mark the Diffserv field is IP packet based on QCI field.
      If the S5/S8 interface is PMIP based then SGW acts a Mobility Anchor Gateway (MAG). All the MAG responsibilities can be assigned to SGW. It is also responsible for assigning a GRE key that can be used by PGW in downlink. For a MAG of non trusted 3GPP access SGW acts as LMA.

PGW: - PDN Gateway

      PGW terminates SGi interface towards the PDN.

     PGW is responsible for all the IP packet based operations such as deep packet inspection, UE IP address allocation, Transport level packet marking in uplink and downlink, accounting etc. PGW contacts PCRF to determine the QoS for bearers. It is also responsible for UL and DL rate enforcement based on APN-AMBR. It is synonymous to GGSN of pre release 8 networks.
      For PMIP based S5/S8 interface PGW acts as LMA. It is responsible for assigning IP
address to UE, and also GRE Key to SGW that should be used in uplink.