LTE Network: Evolved Packet Core (EPC)

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EPC

Introduction:

The telecommunications industry is in a period of radical change with the advent of mobile broadband services through the rapid availability of Internet on User’s devices. Some of these changes have been enabled by a fundamental shift in the underlying technologies. Now a days, mobile networks are increasingly based on a pure Internet Protocol (IP) network architecture. The Evolved Packet Core (EPC) is the outcome of such a pure IP based-network architecture.

In this article, we’ll provides an introduction of the Evolved Packet Core (EPC) network, its Network Architecture and the Core Network entities that constitute a complete EPC Network.

The EPC is the latest advancement from the 3GPP , and a major technological advancement from earlier cellular network architecture, that includes GSM, GPRS and UMTS (3G) networks. The below Figure 1 illustrates the key technologies that evolve from GSM, GPRS Networks and become a part in the EPC Network.

LTE Network EPC
Figure 1: The evolution of EPC packet-switched network architecture from the GSM, GPRS/UMTS network and the key differences.

EPC Network Architecture:

The Evolved Packet Core (EPC) architecture was first introduced in Release 8 by 3GPP. According to 3GPP recommendations made in Release 8, the EPC architecture was designed on two key principle, i.e:

  • Fewer network nodes will be involved in the handling of the data traffic.
  • Secondly, maximum effort would be made in the design of EPC Network architecture to avoid protocol conversion.
    • Protocol Conversion: (is the process by which the protocol of the sending device are being translated into a different protocol of another device so that compatibility and communication between the sending and receiving devices can be established).

Subsequently, it was also decided to separate the:

  • the user plane (also known as the data layer) and
  • the control plane (also known as the signaling layer)

Thus, enabling the scaling of the network architecture and software processes independent of each other.

The below Figure 2 illustrates the main components of EPC network (MME, S-GW, PDN-GW and HSS) interconnected to LTE (RAN) network and External IP Data Network (Internet)

LTE Network EPC Architecture
Figure 2: The Interconnection between LTE, EPC and External IP Data network.

The four main network elements of EPC network are:

  • MME,
  • The Serving Gateway (Serving GW),
  • The PDN Gateway (PDN GW) and
  • HSS.

The below Figure 3 illustrates the Basic EPC Architecture for LTE Network.

LTE Network EPC
Figure 3: The Basic EPC Network Architecture for LTE Network.

 

MME (Mobility Management Entity)

The MME is a network entity that deals with the control plane. All eNodeB’s in the LTE Network Architecture are connected to at least one MME.

The logical interface between eNodeB and MME is S1-MME interface. The S1-MME logical interface handles all LTE-related control plane signaling information, that includes mobility and security fuctions for devices/UE terminals that are attached over the LTE Radio Access Network or (E-UTRAN).

The main function of the MME is to:

  • Manage all devices/UE terminals that are in idle mode
  • Secondly, is to provide support for Tracking Area Management (TA) for all attach devices, and
  • Lastly, to perform Paging.

All the devices/UE terminals need to first connect to MME before being allowed to request any service on the EPC network.

HSS (Home Subscriber Server)

The HSS is a database that contains user-related and subscriber-related information for querying reasons. Based on the Home Location Register (HLR) and Authentication Centre (AuC) concept, HSS provides a critical role in subscriber-management and user-authorization.

The MME need to rely on the existence of subscriber-related information for all users on the network in order to establish IP connectivity over the LTE RAN.

Therefore, for this main purpose, the MME needs to directly connected with HSS in order to allow ‘registered’ users to access the LTE RAN and to allow them to use the network services. Thus, a physical connectivity is established between MME and HSS.

The S6a provides a logical interface between MME and HSS.

The main functions of the HSS is to:

  • Manage user data/subscriber-related information,
  • Users Authentication and authorizing access for a particular service/feature, and to
  • Support Mobility Management between LTE and other access Networks – for call and session setup (between UE and EPC Network).

Serving GW

The Serving GW (S-GW) deal with the user plane. They transport the IP data traffic between the User Equipment (UE) and the external networks. The Serving GW is the point of interconnect between the radio-side and the EPC.

The S-GW gateway serves the UE by routing the incoming and outgoing IP packets to the external IP Data Networks. It is responsible for “Intra-LTE” mobility (i.e. in case of handover between eNodeBs and Between LTE & other 3GPP accesses).

The main function of the S-GW is to:

  • Terminates the S1-U interafce towards the base stations (eNodeBs),
  • Secondly, to provide the connectivity point for intra-LTE mobility and for mobility between GSM/GPRS/HSPA and LTE,
  • Lastly, to buffers downlink IP Packets destined for terminals that are in Idle mode.

One key point in case of Roaming Users, is that the Serving GW always resides in the visited Newtork and provide support for accounting functions for inter-operator (ASR) charging and billing settlments/information.

The S-GW is logically connected to PDN-GW or P-GW.

PDN GW

The PDN GW is the point of interconnect between the EPC and the external IP networks. These networks are called PDN (Packet Data Network). The PDN GW routes packets to and from the PDNs.

They also performs various different functions such as IP address / IP prefix allocation or policy control and charging fuctions (PCRF).

The main functions of PDN-GW is to:

  • Supports IP address allocation
  • Charging and Packet Filtering
  • and Policy-based control

Most EPC vendors, combine S-GW and P-GW as a one single core hardware in the EPC network. Whereas, in some specific requirements they can work as an independent network element.

Inter-working in EPC Network:

First of all, in the LTE radio network there is at least one eNodeB – the LTE base station. The main functionality of the eNodeB is to provide wireless connections between user devices and the network.

In a reasonably sized commercial cellular network, there may be several thousand eNodeBs in the network; many of these eNodeBs may be interconnected via the X2 interface in order to allow for efficient handovers.

It is mandatory for all eNodeBs to be connected to at least one MME over the S1-MME logical interface. The MME handles all LTE-related control plane signaling, including mobility and security functions for devices and terminals attaching over the LTE RAN. The MME also manages all terminals that are in idle mode, including support for Tracking Area (TA) management and paging.

The MME relies on the existence of subscription-related user data for all users trying to establish IP connectivity over the LTE RAN.

Therefore, for this main purpose, the MME is connected to the HSS  over the S6a interface.

The HSS which is designed to manage user data and related user management logic for users accessing over the LTE RAN perform the authentication and access authorization functions. It also supports, mobility management within LTE as well as between LTE and other access networks.

The user data payload – the IP packets flowing to and from the mobile devices – are handled by two logical nodes called:

  • the Serving Gateway (Serving GW) and
  • the PDN Gateway (PDN GW).

The Serving GW and PDN GW are connected over an interface called either through:

  • S5 (if the user is not roaming, i.e. the user is attached to the home network) or
  • S8 (if the user is roaming, i.e. attached to a visited LTE network).

The Serving GW terminates the S1-U user plane interface towards the base stations (eNodeBs), and constitutes the anchor point for intra-LTE mobility, as well as  for mobility between GSM/GPRS, WCDMA/HSPA and LTE.

The Serving GW also buffers downlink IP packets destined for terminals that happen to be in idle mode. For roaming users, the Serving GW always resides in the visited network, and supports accounting functions for inter-operator charging and billing settlements.

The PDN GW is the point of interconnection to external IP networks through the SGi interface. The PDN GW includes functionality:

  • for IP address allocation,
  • charging, packet filtering, and
  • policy-based control of user-specific IP flows.

The PDN GW also has a key role in supporting QoS for end-user IP services.

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