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    Overview of the Evolved Packet Core

    The Juniper Networks MobileNext Broadband Gateway, as a Packet Data Network Gateway (P-GW), is a key component of the Long Term Evolution (LTE) architecture’s Evolved Packet Core (EPC). The P-GW faces the IP service and networks, and the Serving Gateway (S-GW) faces the radio network. Together, they provide the user plane from the IP packet network to the Radio Access Network (RAN). However, a few other EPC devices are necessary as well.

    Figure 1 shows the major components and interfaces of the EPC of a mobile network based on LTE standards. The user equipment can attach to only one Mobility Management Entity (MME) and S-GW at a time, but the user equipment can have connectivity to multiple P-GWs.

    Figure 1: Major Components of the Evolved Packet Core

    Major Components of the Evolved Packet

    The major components in the figure are:

    • E-UTRAN—The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) is the radio network portion of the LTE architecture.
    • MME—The Mobility Management Entity (MME) is a device that manages and stores contexts for the user equipment. It generates temporary identifiers for the user equipment, manages the user equipment idle state (so the device is reachable from other devices and services), and distributes paging messages. The MME processes tracking area updates. The MME also manages security and controls bearers (the tunnels from user equipment to service).
    • Serving Gateway (S-GW)—The S-GW handles user-plane handovers for mobility on the radio network side of the EPC and also coordinates P-GW attachments for users. When a user is roaming, at least the S-GW and MME are in the visited public land mobile network (VPLMN), whereas the P-GW can be in the HPLMN (the home routed case) or in the VPLMN (local breakout). In either case, the home network enforces subscriber authentication and polices.
    • Packet Data Network Gateway (P-GW)—The P-GW forms the GTP tunnel endpoint for associated user equipment, allocates IP addresses, and provides support for charging and policy enforcement for service access.
    • Home Subscriber Server (HSS)—The HSS is a user database that stores all subscription-related information about a user. This information supports call (connection) control and session management. The HSS function was performed by the Home Location Register (HLR) in older architectures.
    • Service cloud—These are the services delivered by the Packet Data Network (PDN). This can be the global public Internet or an IP Multimedia Subsystem (IMS) network. IMS networks handle voice over IP (VoIP) calls to and from the user equipment.

    The major interfaces in the figure are:

    • S1—The S1 interface connects both the MME and S-GW to the mobile radio network. Technically, these are the S1-MME and S1-U interface, respectively.
    • S5/S8—The S5 interface connects the P-GW with the local S-GW. When roaming, this is the S8 interface.
    • S6a—The S6a interface connects the MME with the HSS. The interface is the same whether roaming or not.
    • SGi (or Gi)—The SGi interface (“i” for Internet or IP) connects the P-GW to the Internet, IMS, or other IP network (such as a corporate intranet).

    Published: 2011-11-16