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LTE Packet Core Systems – Mobility and QoS
Long Term Evolution (LTE) complements the success of HSPA with higher peak data rates, lower latency and an enhanced broadband experience in high-demand areas. This is accomplished using wide-spectrum bandwidth, OFDMA and SC-FDMA air interfaces, and advanced antenna techniques. These techniques enable high spectral efficiency and superior 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, all-IP-packet architecture, 3GPP™ implemented a new core network, the Evolved Packet Core (EPC). EPC is planned for 3GPP Release 9 and is intended for use by various access networks such as LTE, HSPA/HSPA+ and non-3GPP networks. Evolved Packet System (EPS) includes EPC and a set of access systems such as eUTRAN or UTRAN. EPS is designed from the ground up to support seamless mobility and QoS with minimum latency for IP services.
Developing an all-IP flat architecture
3GPP is evolving wireless networks to become more agile and more simplified. In the user plane of EPS, for example, there are only two types of nodes (base stations and gateways), while current hierarchical networks have four types with a centralized RNC. Another simplification is the separation of the control plane with a separate mobility-management network component. It is worth noting that similar optimizations have been enabled in evolved HSPA network architectures, providing a similarly flat architecture.
A key difference with current networks is that EPC is defined to support only packet-switched traffic. The interfaces are based on the IP protocol. This means all services, including voice, will be delivered over packet connections. Thus, EPS provides savings for operators by using a single-packet network for all services.
Evolved Node-B (eNB)
The noticeable fact is that most of the typical protocols implemented in today’s RNCs are moved to the eNB. The eNB, similar to the Node B functionality in the evolved HSPA architecture, is also responsible for header compression, ciphering and reliable delivery of packets. At the control plane, functions such as access control and radio resource management are also included in the eNB. Advantages of RNC and Node B merging include reduced latency with fewer hops in the media path and distribution of RNC processing load across multiple eNBs.
Serving and PDN Gateway
Between the access network and the PDN (eg, the Internet), gateway interfaces support the differentiation of mobility requirements and QoS flows. EPS defines two logical gateway entities, S-GW and P-GW. The S-GW acts as a local mobility anchor, forwarding and receiving packets to the eNB to which the UE is being served. The P-GW, in turn, interfaces with external PDNs such as the Internet and IMS. It is also responsible for many IP functions such as address allocation, policy enforcement, packet classification and routing, and it provides mobility anchoring for non-3GPP access networks. In practice, both gateways can be implemented as a single physical network element, depending on deployment scenarios and vendor support.
Mobility Management Entity (MME)
The MME is only a signaling entity, so user IP packets do not pass through the MME. Its main function is to manage the mobility of the UE. In addition, the MME also performs authentication and authorization; passive-mode UE tracking and reachability; security negotiations; and NAS signaling. The advantage of a separate network element for signaling is that operators can increase signaling and traffic capacity independently. A similar benefit can be achieved in the direct-tunnel architecture of HSPA Release 7, where the SGSN becomes the signaling entity only.
An important aspect for any all-packet network is a mechanism to guarantee packet flow differentiation based on its QoS requirements. Applications such as video streaming, HTTP, or video telephony have special QoS needs and must receive different services over the network. With EPS, QoS streams 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. Thus, each QoS IP flow (eg VoIP) will be associated with a different EPS bearer and the network can prioritize packets accordingly. The QoS process for packets coming from the Internet is similar to HSPA. When receiving an IP packet, the P-GW classifies the packet based on rule-like parameters 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.
EPS seamless mobility
Seamless mobility is clearly an important consideration for wireless systems. Discontinuous active handoff between eNBs is the first scenario that is commonly considered. However, other scenarios such as handoff to the core network (ie, P-GW, MME), transfer of access technology and passive mobility are also important scenarios covered by EPS.
Seamless active handoff
EPS enables seamless active handoff, supporting VoIP and other real-time IP applications. Since there is no RNC, the interface between eNBs is used to support signaling for handoff preparation. In addition, the S-GW acts as an anchor, switching mobility tunnels between eNBs. The serving eNB maintains the coupling between the mobility tunnel and the radio bearer and also maintains the UE reference 1. In preparation for handoff, the source eNB (eNB 1) sends connection information and UE context to the target eNB (eNB 2). This signaling is triggered by a radio measurement from the UE, which indicates that eNB 2 has a good signal. Once eNB 2 indicates that it is ready to handoff, eNB 1 commands the UE to switch the radio bearer to eNB 2. For eNB handoff to complete, the S-GW must update its records with the new eNB that is serving. UE. For this phase, the MME coordinates a mobility-tunnel switch from eNB 1 to eNB 2. The MME triggers an update on the S-GW based on the signaling received from the eNB 2 indicating that the radio bearer has been successfully transferred.
Functional passive mobility
An additional mobility aspect to consider with new wireless core networks is a mechanism to identify the approximate location of a UE when it is not active. EPS provides an efficient solution for passive mobility management. The basic idea is to connect clusters of ENBs to Tracking 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. Then the MME will page the UE in its most current TA. EPS includes a new concept, which is the ability of a UE to register with multiple TAs simultaneously. This allows the UE to reduce battery consumption during high mobility periods, as it does not need to constantly update its location with the MME. It also reduces registration burden at TA borders.
Heterogeneous network dynamics
LTE is envisioned as a complement to existing HSPA/HSPA+ networks where data demand is high and broadband experiences are enhanced. Therefore, LTE access networks will co-exist with the wider coverage of HSPA/HSPA+ networks, thus requiring robust mechanisms for interoperability. For data interoperability, the EPC will support existing SGSNs and the interface between MME and S-GW, which will allow data handoff. For voice-service continuity, 3GPP is also working on standardizing a voice-call continuity approach that will enable seamless operation between VoIP over LTE and circuit-switched voice over R99.
EPS provides operators with an efficient and robust core network architecture to support all IP services for LTE, HSPA and non-3GPP access networks. Basically, it is a flat architecture that enables simplified network design while still supporting seamless mobility and advanced QoS mechanisms. Many typical RNC functions are embedded in the eNB and the EPS defines the control plane with a separate network element, the MME. QoS logical connections are established between UEs and EPS GWs, providing differentiation of IP flows throughout the network and meeting requirements for low-latency applications. The principles and design are similar to the developed HSPA architecture, providing operators with a smooth migration path to their 3GPP core networks.
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