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Saturday, November 19, 2011

System Architecture Evolution

Functional split between RAN and CN

  • For WCDMA/HSPA, the philosophy behind the functional split between RAN and CN is to hide all the radio interface functionality from the CN meaning that, any radio access technology can be used with the same CN
  • The LTE RAN builds on the same philosophy as WCDMA/HSPA with an added key design feature, to minimize the number of nodes.


  • In HSPA, the node B handles all physical layer functions except for macro-diversity which is handled by the RNC
  • Serving and drift RNC is one way of handling a terminal that has moved to a cell that is under another RNC. Another way is SRNS relocation.
  • In addition to macro-diversity, security functions is another reason for keeping the RNC since the large number of nodeB’s and the sometimes hard-to-protect locations they are used in is considered to make them unsafe for hosting sensitive functionality.


  • For LTE, it was decided that the gains of keeping the RNC does not motivate the increased complexity and so, it was removed along with macro-diversity.
  • The e-nodeB is connected to the CN using the S1 interface. The e-nodeB’s are interconnected using the X2 interface which is mainly used for connected mode mobility.

Evolved Packet Core(EPC)

The nodes for the Evolved Packet Core is:                                                                                  

  • Mobility Management Entity(MME): this is the control plane node
  • Serving Gateway: this is the user plane node that connects the EPC to the LTE RAN
  • Packet Data Network Gateway(PDN Gateway): this is the user plane node that connects the EPC to the Internet
·         S1 flex enables a more robust network. If one of the EPC nodes becomes unavailable another one can cover in its place
·         EPC does not only connect to 3GPP RAN’s. In particular, WIFI, WIMAX and CDMA2000/EV-DO access support is planned.

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