EPC in SAE or Core network behind LTE

CN (SAE called EPC) is responsible for the overall monitoring of the UE and the establishment media. Main logical nodes of the EPC are:

  • PDN Gateway (P-GW)
  • Serving GateWay (S-GW)
  • Mobility Management Entity (MME)
  • Evolved Serving Mobile Location Centre (E-SMLC)

In addition to these nodes, EPC also includes other logical nodes and functions like a gateway mobile location center (GMLC), subscriber server (HSS) and policy control and charging rules function (PCRF). Since EPS only provides the bearer a certain QoS, control multimedia applications such as VoIP is provided by IMS, which is considered to be outside the EPS itself. When a user is roaming outside their home country network, P-GW member, GMLC and the IMS domain can be located in any home network or the visited network.


The PCRF is responsible for the control of policy decision-making and control functionality based load flow control function of the implementation of the Policy (PCEF) that resides in the P-GW. The PCRF provides authorized QoS (QoS class identifier and bit rates) that decides how it will treat a given data flow in the PCEF and ensures that this is in accordance with the user’s subscription profile.


GMLC contains the functions necessary to support location services (LCS). After the authorization, it sends positioning requests to the MME, and receives final location estimates.

Home Subscriber Server (HSS):

HSS provides users SAE subscription data, such as EPS-subscribed QoS profile, and any access restrictions for roaming. It also contains information about PDNS, the user can connect. This may be in the form of access point name (APN), or address PDN (indicating subscribed IP address (es)). In addition, HSS comprises dynamic information such as the identity of the MME that the user is currently connected or can integrate zaregistrirovany.HSS Authentication Center (AUC), which generates vectors for authentication and security keys.


P-GW is responsible for allocating IP-address for the UE, as well as QoS and on the basis of the flow of charge in accordance with the rules of PCRF.P-GW is responsible for filtering the downlink user IP packet to various QoS based on the carrier. This is done based on patterns of traffic (TFT). P-GW performs QoS enforcement for guaranteed bit rate (GBR) media. It also serves as the anchor for mobility interworking with non-3GPP technologies such as WiMAX and CDMA2000 networks.


All user IP packets transmitted via S-GW, which serves as a local mobility anchor for a data carrier, when the UE moves between the eNodeBs. It also stores information about the media, when the UE is in the standby state (also known as EPS control compounds IDLE (ECM-IDLE) and temporarily buffers the downlink data while the MME triggers paging of the UE for the carrier recovery.

Moreover, S-GW makes some administrative functions in the visited network, such as the collection of information for charging (for example, the amount of data transmitted or received by the user) and lawful intercept. It also serves as the anchor for mobility interworking with other 3GPP technologies such as GPRS3 and UMTS4.

MME: The MME is a node that processes control signaling between the UE and the CN. Operating protocols between the EU and the CN protocols are known as non-access stratum (NAS).

Basic functions supported MME subdivided as follows:

  • Functions related to vehicle control. This includes the establishment, maintenance and release carriers and processed by the session management layer NAS protocol.
  • The functions associated with connection management. This involves establishing a connection between the network and security and UE, and processed using a compound or a mobility management layer in the layer NAS protocol.
  • Functions associated with the interworking with other networks. This includes the transmission of voice calls in traditional networks.

E-SMLC: E-SMLC manages the overall coordination and planning of the resources needed to find the location of UE, which is attached to the E-UTRAN. It also calculates the final location based on the assessment it receives, and he estimates the UE speed and accuracy achieved.

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