ABSTRACT
Long-Term Evolution (LTE) complements the success of HSPA with higher peak data rates, lower latency, and an improved broadband experience in high-demand areas. This is achieved through the use of broader spectrum bandwidths, OFDMA and SC-FDMA air interfaces, and advanced antenna techniques. These techniques enable high spectral efficiency and excellent user experience for a wide range of converged IP services. To take full advantage of these broadband access networks and enable the coexistence of multiple technologies through an efficient IP packet architecture, 3GPP™ implemented a new core network, the Evolved Packet Core (EPC). EPC is planned for 3GPP version 9 and is designed to be used by various access networks such as LTE, HSPA/HSPA+ and non-3GPP networks. The Evolved Packet System (EPS) which includes the EPC and a set of access systems such as eUTRAN or UTRAN. EPS has been designed from the ground up to support seamless mobility and QoS with minimal latency for IP services.
EVOLVING ALL-IP FLAT ARCHITECTURE
3GPP is evolving wireless networks to make them flatter and more simplified. In the EPS user plane, for example, there are only two types of nodes (base stations and gateways), while in current hierarchical networks there are four types, including a centralized RNC. Another simplification is the separation of the control plane, with a separate mobility management network element. It is worth noting that similar optimizations are enabled in the evolved HSPA network architecture, providing a similarly flattened architecture.
A key difference from today’s networks is that the EPC is defined to only support packet switched traffic. The interfaces are based on IP protocols. This means that all services will be delivered over packet connections, including voice. Therefore, EPS provides savings to operators by using a single package network for all services.
EVOLVED NODE-B (eNB)
A notable fact is that most of the typical protocols implemented in the current RNC are ported to the eNB. The eNB, similar to the Node B functionality in the evolved HSPA architecture, is also responsible for header compression, encryption, and reliable packet delivery. At the control plane, functions such as admission control and radio resource management are also incorporated into the eNB. The benefits of merging the RNC and Node B include reduced latency with fewer hops in the media path and spreading the processing load of the RNC across multiple eNBs.
SERVICE GATEWAYS AND PDNs
Between the access network and the PDNs (eg the Internet), the gateways support the interfaces, mobility needs, and differentiation of QoS flows. EPS defines two logical gateway entities, the S-GW and the P-GW. The S-GW acts as a local mobility anchor, forwarding and receiving packets to and from the eNB where the UE is served. The P-GW, in turn, interacts with external PDNs, such as the Internet and IMS. It is also responsible for various IP functions such as address assignment, policy enforcement, packet routing and classification, and provides mobility anchoring for non-3GPP access networks. In practice, both gateways can be deployed as one physical network element, depending on deployment scenarios and vendor support.
MOBILITY MANAGEMENT ENTITY (MME)
The MME is a signaling-only entity, so the user’s IP packets do not go through the MME. Its main function is to manage the mobility of the EU. In addition, the MME also performs authentication and authorization; UE tracking and accessibility in idle mode; security negotiations; and NAS signage. An advantage of a separate network element for signaling is that operators can increase signaling and traffic capacity independently. A similar benefit can also be achieved in the HSPA Release 7 forward tunnel architecture, where the SGSN becomes a signaling-only entity.
QoS EFFICIENT
An important aspect for any packet network is a mechanism to ensure differentiation of packet flows based on their QoS requirements. Applications such as video streaming, HTTP, or video telephony have special QoS needs and must receive a differentiated service over the network. With EPS, QoS flows called EPS bearers are established between the UE and the P-GW. Each EPS bearer is associated with a QoS profile and is composed of a radio bearer and a mobility tunnel. Therefore, each QoS IP flow (eg VoIP) will be associated with a different EPS bearer, and the network can prioritize packets accordingly. The QoS procedure for packets arriving from the Internet is similar to that of HSPA. Upon receiving an IP packet, the P-GW performs packet classification based on parameters such as the rules received from the PCRF and sends it through the appropriate mobility tunnel. Depending on the mobility tunnel, the eNB can assign packets to the appropriate radio QoS bearer.
EPS ENDLESS MOBILITY
Seamless mobility is clearly a key consideration for wireless systems. Seamless active handover via eNB is the first scenario that is typically considered. However, other scenarios such as handovers through core networks (ie P-GW, MME), transfer of access technologies, and idle mobility are also important scenarios covered by EPS.
SEAMLESS ACTIVE TRANSFERS
EPS allows continuous active handoffs, compatible with VoIP and other IP applications in real time. Since there is no RNC, an interface between the eNBs is used to support signaling for handover preparation. Furthermore, the S-GW behaves like an anchor, switching mobility tunnels between eNBs. A serving eNB maintains the coupling between the mobility tunnels and the radio bearers, and also maintains the UE1 context. In preparation for the handover, the source eNB (eNB 1) sends the binding information and the UE context to the destination eNB (eNB 2). This signaling is triggered by a radio measurement of the UE, indicating that the eNB 2 has a better signal. Once eNB 2 indicates that it is ready to perform the handover, eNB 1 commands the UE to change the radio bearer to eNB 2. For the eNB handover to complete, the S-GW must update its registers with the new eNB. that is serving the eNB. EU. For this phase, MME coordinates the mobility tunnel switch from eNB 1 to eNB 2. MME triggers the update at the S-GW, based on signaling received from eNB 2 indicating that the radio bearer was transferred successfully.
EFFICIENT IDLE MOBILITY
An additional mobility aspect to consider with a new wireless core network is the mechanism to identify the approximate location of the UE when it is not active. EPS provides an efficient solution for idle mobility management. The basic idea is to associate a group of eNBs in monitoring areas (TAs). The MME tracks which TA the UE is in, and if the UE moves to a different TA, the UE updates the MME with its new TA. When the EPS GW receives data for an idle UE, it will buffer the packets and query the MME for the location of the UE. The MME will then page the UE at its most current TA. EPS includes a new concept, which is the ability of a UE to be registered to multiple TAs simultaneously. This allows the UE to minimize battery consumption during periods of high mobility, since it does not need to constantly update its location with the MME. It also minimizes the registration burden on TA limits.
MOBILITY IN HETEROGENOUS NETWORK
LTE is envisioned as a complement to existing HSPA/HSPA+ networks in places with high data demand and an enhanced broadband experience. Therefore, LTE access networks will coexist with the wide coverage of HSPA/HSPA+ networks, so robust mechanisms will be required to interoperate. For data interoperability, EPC will support interfaces between existing SGSNs and the MME and S-GW, allowing data transfers. For voice service continuity, 3GPP is also working on standardizing a voice call continuity approach that will enable transparent operation between VoIP over LTE and circuit-switched voice over R99.
RECOMMENDATIONS
EPS provides operators with an efficient and robust core network architecture to support all IP services for LTE, HSPA and non-3GPP access networks. Fundamentally, it is a flattened architecture that allows for simplified network design while supporting seamless mobility and advanced QoS mechanisms. Many of the typical RNC functions are incorporated into the eNB, and the EPS defines a control plane with a separate network element, the MME. QoS logical connections are established between the UE and the EPS GW, providing differentiation of IP flows across the network and meeting the requirements for low latency applications. The principles and design are similar to the evolved HSPA architecture, providing operators with a seamless migration path for their 3GPP core networks.