Apart of LTE contains EPC, which is a new system design based on the all-IP mobile core network architecture. EPC forms a converged framework based on real-time and non-real-time packet-based services. EPC is specified by 3GPP Release 8 standards.
The EPC is responsible for providing mobile core functionality, which used to be two separate sub-domains in previous mobile generations (2G, 3G). These two sub-domains are: Circuit-Switched (CS) (i.e., supporting voice) and Packet-Switched (PS) (i.e., supporting data). These two distinct sub-domains are used for individual mobile voice and data switching and processing under a unified single IP mobile domain. LTE offers an end-to-end IP-based architecture, which covers from the mobile handsets and other terminal devices offering embedded IP capabilities on top of LTE base stations. The LTE base station is an IP-based device often called Evolved NodeB.
LTE's EPC is an essential functional entity offering end-to-end IP service, as well as allowing the introduction and creation of new business models, including partnerships and revenue sharing with application and third-party content providers. EPC promotes the enablement of new applications and new innovative services.
EPC addresses those fundamental requirements of LTE that deal with media-rich and advanced real-time services offering enhanced Quality of Experience (QoE). Network performance can be improved using EPC. This is done by separating data and control planes by using a flat IP architecture that is able to reduce the hierarchy delay among mobile data elements. For instance, the data path traversing from eNodeB passes only through EPC gateways.
The introduction of the all-IP network architectural EPC in the design of mobile networks has caused various degrees of implications on the following mechanisms:
- All-IP mobile services, such as; IP-based voice, data and video communications.
- New mobile architecture interworking with previous mobile generations (2G/3G)
- Network scalability, which is required by all core elements to address any changes in the bandwidth and the number of user-terminal direct connections.
- Availability and reliability offered by each elements ensuring service continuity.
To address various service and network requirements, the EPC is designed in such a way to change the existing mobile network paradigms.
The following subcomponents are part of the EPC architecture:
- MME (Mobility Management Entity): In LTE, the MME is a key control-node for the access network method. MME is responsible for paging procedure including retransmissions and the UE (User Equipment) tracking idle mode. It is also responsible for match an appropriate SGW for a UE at the time of intra-LTE handover that involves the Core Network (CN) node relocation and at the initial attach time. MME is further involved in the user (by interacting with the HSS) authentication and the bearer activation/deactivation procedures. The MME can terminate the non-Access Stratum (NAS) signaling can generate and allocate temporary identities for UEs. It also checks for the authorization of the UE for camping on the service provider's Public Land Mobile Network (PLMN) and enforcing UE roaming restrictions. The MME handles the security key management and is the termination point in the network for ciphering/integrity protection for NAS signaling. The MME also offers lawful signaling and LTE/2G/3G mobility control plane functions with the S3 interface that terminates at the MME from SGSN. The S6a interface is also terminated by MME towards the home HSS for UEs roaming purposes. The following interfaces were considered for MME:
- S3: This interface enables bearer and user exchange information for inter 3GPP access network mobility between SGSNs.
- S6a: This interface enables subscription and authentication data transfer for authenticating and authorizing user access.
- PGW (PDN Gateway): From the UE to the external packet data networks, PDN Gateway provides connectivity by providing the point of entry and exit for the UE's traffic. More than one PGWs may provide simultaneous connectivity for a UE providing multiple PDN access. The PGW performs packet filtering for each user, policy enforcement, lawful Interception, charging support, and packet screening. The PGW may act as the anchor for mobility between non-3GPP and 3GPP technologies, which is nother key role of PGW. These technologies include: 3GPP2 (CDMA IX and EvDO), WiMAX and etc.
- SGW (Serving Gateway): User data packets are routed and forwarded by the SGW. During inter-eNodeB handovers the SGW acts as a mobility anchor for the user plane. It also acts as an anchor for mobility among LTE and other 3GPP technologies, where it can terminate the S4 interface and relay the traffic among PGW and 2G/3G systems. The SGW terminates the DL data path for the idle state of UEs and when DL data arrives for the UE, it triggers paging. The SGW also stores and manages UE contexts, such as parameters of network internal routing information and the IP bearer service. In case of lawful interception, it also performs replication of the user traffic.
- PCRF (Policy and Charging Rules Function): Though PGW, SGW, and MME were introduced in 3GPP Release 8, PCRF was part of 3GPP Release 7 introduction. Architectures using PCRF have not so far been widely adopted by standards, however the interoperability of PCRF's with the EPC gateways and the MME is essential for the operation of the LTE and mandated in Release 8.
