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 with the use of wider spectrum bandwidths, OFDMA and SC-FDMA air interfaces, and advanced antenna techniques. These techniques enable high spectral efficiency and an excellent user experience for a wide range of converged IP services. Take full advantage of these broadband access networks and allow the coexistence of multiple technologies through an efficient architecture of IP packets, 3GPP ?? implemented a new core network, the Evolved Packet Core (EPC). EPC is planned for 3GPP Release 9 and is intended to be used by various access networks such as LTE, HSPA / HSPA + and non-3GPP networks. The Evolved Packet System (EPS) included the EPC and a set of access systems such as eUTRAN or UTRAN. EPS has been designed from the ground up to support seamless QoS and mobility with minimal latency for IP services.
EVOLUTION OF ALL IP FLAT ARCHITECTURE
3GPP is evolving wireless networks to be flatter and more simplified. In the EPS user plane, for example, there are only two types of nodes (base stations and gateways), whereas in today’s 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 an equally flat 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 for operators by using a single packet network for all services.
EVOLVED NODE B (eNB)
A notable fact is that most of the typical protocols implemented in the current RNC are carried over 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. On the control plane, functions such as admission control and radio resource management are also incorporated into the eNB. The benefits of merging RNC and Node B include reduced latency with fewer hops in the media path and distribution of RNC processing load across multiple eNBs.
SERVICE GATEWAYS AND PDN
Between the access network and PDNs (eg Internet), gateways support 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 being 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 classification, and routing, and provides mobility anchor for non-3GPP access networks. In practice, both gateways can be implemented as a physical network element, depending on deployment scenarios and vendor support.
MOBILITY MANAGEMENT ENTITY (MME)
The MME is a signaling-only entity, so user IP packets do not go through the MME. Its main function is to manage EU mobility. In addition, the MME also performs authentication and authorization; UE tracking and accessibility in idle mode; security negotiations; and NAS signaling. An advantage of a separate network element for signaling is that operators can independently increase signaling and traffic capacity. A similar benefit can also be achieved in HSPA version 7 direct tunnel architecture, where the SGSN becomes a signaling-only entity.
EFFICIENT QoS
An important aspect for any packet network is a mechanism to ensure the 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 HSPA. Upon receiving an IP packet, the P-GW performs a packet classification based on parameters such as the rules received from the PCRF and sends it through the appropriate mobility tunnel. Based on the mobility tunnel, the eNB can map packets to the appropriate radio QoS bearer.
SEAMLESS MOBILITY EPS
Perfect mobility is clearly a key consideration for wireless systems. Seamless active handover through eNB is the first scenario that one usually considers. However, other scenarios such as transfers across core networks (i.e. P-GW, MME), transfer of access technologies, and idle mobility are also important scenarios covered by EPS.
SEAMLESS ACTIVE BROCHURES
EPS enables seamless, active transfers, supporting VoIP and other IP applications in real time. Since there are no RNCs, an interface between the eNBs is used to support signaling for handover preparation. Furthermore, the S-GW behaves like an anchor, switching mobility tunnels through eNB. A serving eNB maintains the coupling between mobility tunnels and radio bearers, and also maintains the context of UE1. In preparation for handover, the originating eNB (eNB 1) sends the coupling information and context of the UE to the destination eNB (eNB 2). This signaling is triggered by a radio measurement of the UE, indicating that eNB 2 has a better signal. Once the eNB 2 signals that it is ready to do the handover, the eNB 1 instructs the UE to change the radio bearer to eNB 2. For the eNB handover to complete, the S-GW must update its records with the new eNB that is serving the UE. For this phase, MME coordinates the mobility tunnel switching from eNB 1 to eNB 2. MME triggers the update in the S-GW, based on the signaling received from eNB 2 indicating that the radio bearer was successfully transferred.
EFFICIENT MOBILITY IN IDLE
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 the management of inactive mobility. The basic idea is to associate a group of eNBs in monitoring areas (TA). 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 send a message to the UE on its most current TA. EPS includes a new concept, which is the ability for a UE to register with multiple TAs simultaneously. This allows the UE to minimize battery consumption during periods of high mobility, as it does not need to constantly update its location with the MME. It also minimizes the logging load at TA limits.
HETEROGENIC NETWORK MOBILITY
LTE is envisioned as a complement to current HSPA / HSPA + networks in locations that have high demand for data and an enhanced broadband experience. Therefore, LTE access networks will coexist with the broad coverage of HSPA / HSPA + networks, which will require robust mechanisms to interoperate. For data interoperability, EPC will support interfaces between existing SGSNs and the MME and S-GW, allowing data transfer. For voice service continuity, 3GPP is also working on standardizing a voice call continuity approach that will allow seamless operation between VoIP over LTE and circuit-switched voice over R99.
CONCLUSIONS
EPS provides operators with an efficient and robust core network architecture to support all IP services for LTE, HSPA and non-3GPP access networks. Essentially, it is a flat architecture that enables simplified network design while supporting seamless mobility and advanced QoS mechanisms. Many of the typical RNC functions are built into the eNB, and the EPS defines a control plane with a separate network element, the MME. The logical QoS connections are established between the UE and the EPS GW, which provides differentiation of IP flows throughout the network and meets the requirements of low latency applications. The principles and design are similar to the evolved HSPA architecture, providing operators with a smooth migration path for their 3GPP core networks.